BENDIX BA-921 COMPRESSOR-STD-CLOSED ROOM User guide

DESCRIPTION
Thefunctionoftheaircompressoristoprovideandmaintain
airunder pressureto operatedevices inair brakesystems.
The Bendix®BA-921®compressor is a single-cylinder
reciprocating compressor with a rated displacement of
15.8 cubic feet per minute at 1250 RPM.
The compressor consists of a water-cooled cylinder
head assembly and an integral air-cooled crankcase
assembly.
The cylinder head assembly is made up of the cylinder
head, cooling plate and valve plate assembly and uses
two sealing gaskets. Depending on the application, the
cylinder head and cooling plate may be aluminum or cast
iron. The cylinder head contains air and water ports as
well as an unloader assembly. A cooling plate is located
betweenthe cylinderhead andvalve plateassemblies and
assists in cooling.
The valve plate assembly consists of brazed steel plates
which have valve openings and passages for air and
engine coolant to flow into and out of the cylinder head.
SD-01-690
BENDIX®BA-921®COMPRESSOR: STANDARD AND CLOSED ROOM
FIGURE 2 - BENDIX®BA-921®COMPRESSOR: STANDARD AND CLOSED ROOM VERSIONS
FIGURE 1 - BENDIX®BA-921®COMPRESSOR
Safety
Valve
Valve Plate
Assembly
Crankcase
Cylinder
Head
Cooling
Plate
Standard:
Aluminum
Cylinder Head
With 4
Bolts
Standard:
Cast Iron
Cylinder Head
With 4
Bolts
Closed Room:
Aluminum
Cylinder Head
With 6
Bolts
"STANDARD" AND "CLOSED ROOM" VERSIONS
This service data sheet covers two versions of the Bendix®BA-921®
compressor. The first version was originally released in 2002 and will
bereferredtoasthe“Standard”compressorinthisServiceDatasheet.
The Standard compressor was offered on CaterpillarACERT Medium
and Heavy duty engines, and the DDC S60 EGR (2002-06) engine.
These installations required an externally-mounted inlet check valve
(ICV)ontheair inlet side ofthecompressor. Dependingonwhether the
air induction system was naturally aspirated or turbocharged dictated
whether or not additional hardware was required along with the ICV
(See Figure 6, page 3 and Section 1 of "Air Induction", on page 6).
Thesecondversion was originallyreleasedin 2007 and willbereferred
to as the “Closed Room” compressor in this service data sheet.
This compressor is only permitted to be naturally aspirated – use of
engine turbocharger as an air source is not allowed. This compressor
eliminates the need for an externally mounted inlet check valve (ICV)
on the air inlet side of the compressor (See Figure 6, page 3).
Refer to Figure 2 below to see the visual differences between the
two BA-921®compressor heads. Other differences between the two
versions will be referenced throughout this Service Data sheet.
(STANDARD VERSION SHOWN)

2
The compressor's discharge valves are part of the valve
plate assembly. The inlet reed valve/gasket is installed
between the valve plate assembly and the top of the
crankcase.
The cast iron crankcase houses the piston assembly,
connecting rod, crankshaft and related bearings.
All Bendix®BA-921®compressors are equipped with a
safety valve to protect the compressor head in the event
of, for example, a discharge line blockage downstream
of the compressor. Excessive air pressure will cause the
safety valve to unseat, release air pressure and give an
audible alert to the operator. The safety valve is installed
in the cylinder head safety valve port, directly connected
to the cylinder head discharge port.
Thecrankcasecover located atthebottom of thecrankcase
is stamped with information identifying the compressor
model, customer piece number, Bendix piece number and
serial number. See Figures 1 and 4.
OPERATION
The compressor is driven by the vehicle's engine and
functions continuously while the engine is in operation.
Actual compression of air is controlled by the compressor
unloading mechanism operating in conjunction with a
governor.
Compressor Model,
Customer Piece Number,
Bendix Piece Number and
Serial Number shown here
FIGURE 4 - CRANKCASE BASE COVER
FIGURE 3 - BENDIX®BA-921®COMPRESSOR (CUT-AWAY) (STANDARD AND CLOSED ROOM)
Unloader
Piston
Piston
Rod
Coolant Ports
(3 total)
Piston
Air
Intake
Unloader
Cover
Discharge
Reed
Valves (2)
Crankshaft
Cooling
Plate
Crankcase
Cover
Splash
Shield
Unloader
Piston
Piston
Rod
Coolant Ports
(3 total)
Piston
Unloader
Cover
Discharge
Reed
Valves (2)
Crankshaft
Cooling
Plate
Crankcase
Cover
Closed RoomStandard
FIGURE 5 - TYPICAL COMPRESSOR DRIVE FLANGES
Detroit
Diesel S60
Caterpillar
HD Acert
Engines
Caterpillar
C7, C9
Engines

3
AIR INTAKE (LOADED)
Just as the piston begins the down stroke, (a position
known as top dead center, or TDC), the vacuum created
in the cylinder bore above the piston causes the inlet
reed valve to flex open. Atmospheric air (in naturally
aspirated applications) or pressurized air (in turbocharged
applications)flows throughthe openinlet valveand fillsthe
cylinder bore above the piston. See Figure 7A.
AIR COMPRESSION (LOADED)
When the piston reaches the bottom of the stroke, (a
position known as bottom dead center, or BDC), the inlet
reed valve closes. Air above the piston is trapped by the
closed inlet reed valve and is compressed as the piston
moves upwards. When air in the cylinder bore reaches
a pressure greater than that of the system pressure, the
discharge reed valves open and allow air to flow into the
discharge line and air brake system.
At the same time air flows into the hollow center of the
unloaderpistonthrough anopening in theend of thepiston.
Compressedairactson theinteriorsurfacesof theunloader
pistonand, alongwith theunloader pistonspring, holdsthe
unloader piston in the down position, against its seat on
the valve plate. See Figure 7B.
FIGURE 6 - STANDARD AND CLOSED ROOM BA-921®
COMPRESSOR CHARGING SYSTEMS
Compressor
Supply Reservoir
Governor
Reservoir
Port
Governor
Unloader Port Air Dryer
Standard Compressors use an Inlet Check Valve
(ICV), or alternately, an ICV plus a reservoir,
in the air inlet line depending on the application
Compressor
Supply Reservoir
Governor
Reservoir
Port
Governor
Unloader Port Air Dryer
Closed Room Compressor Typical System
FIGURE 7B - OPERATION - LOADED (COMPRESSION)
(SIMILAR FOR ALL COMPRESSORS - STANDARD
COMPRESSOR SHOWN)
Piston Moving Up
Air Inlet
Port
Inlet
Valve
Closed
Air
Discharge
Port
Discharge
Valve
Open
Unloader
Piston
Down &
Seated
Valve
Plate
Unloader
Port
Cooling
Plate
FIGURE 7A - OPERATION - LOADED (INTAKE). (SIMILAR
FOR ALL COMPRESSORS - STANDARD COMPRESSOR
SHOWN)
Piston Moving Down
Air Inlet
Port
Inlet
Valve
Open
Air
Discharge
Port
Discharge
Valve
Closed
Unloader
Piston
Down &
Seated
Valve
Plate
Unloader
Port
Cooling
Plate

4
NON-COMPRESSION OF AIR (UNLOADED)
Section 1: For Standard Compressor. See Figure 8.
When air pressure in the supply reservoir reaches the
cutout setting of the governor, the governor delivers
system air to the compressor unloader port. Air entering
the unloader port acts on the unloader piston causing the
piston to move upwards, away from its seat on the valve
plate assembly. When the unloader piston is unseated
an air passageway is opened between the cylinder bore
and the air inlet cavity in the cylinder head.
As the piston moves from bottom dead center (BDC) to
top dead center (TDC) air in the cylinder bore flows past
the unseated unloader piston, into the cylinder head inlet
cavityand outthe inletport. To preventthe airfrom flowing
back into the engine air induction system, an inlet check
valve(ICV) isinstalled upstreamof the aircompressor inlet
port. The location of the device and the way it is plumbed
into the compressor air induction system is unique to the
specific engine and the type of air induction (naturally
aspirated or boosted air) the compressor uses. These air
induction systems will be explained in further detail in the
“Air Induction” section on page 4. On the piston down
stroke(TDC toBDC) airflows inthe reversedirection, from
the inlet cavity past the unseated unloader piston and inlet
reed valve, and into the cylinder bore.
Section 2: For Closed Room Compressor. See Figure 9.
When air pressure in the supply reservoir reaches the
cutout setting of the governor, the governor delivers
system air to the compressor unloader port. Air entering
the unloader port acts on the unloader piston causing
the piston to move away from its seat on the valve plate
assembly. When the unloader piston is unseated, an air
passageway is opened between the cylinder bore and a
secondary compartment or “closed room” in the interior of
the cylinder head.
Asthe pistonmoves frombottom dead center (BDC) totop
dead center (TDC) air in the cylinder bore flows past the
unseatedunloader piston, intothe “closedroom”.The size
of the closed room is sufficient to accept the compressed
air provided by the compressor piston without creating
excessive air pressure in the “closed room”. On the piston
downstroke(TDC toBDC)air flowsin thereversedirection,
from the “closed room” past the unseated unloader piston
and inlet reed valve, and into the cylinder bore. Note: For
optimumperformance, itis recommendedthat theair dryer
is equipped with “turbo cut-off”.
FIGURE 8 - OPERATION - UNLOADED (STANDARD)
Air From
Governor
Unloader
Port
Air Inlet Port
Valve
Plate
Air in Pistons Shuttles Back and Forth from the
Piston to the Cylinder Head and Inlet Port During
Unloaded Mode
Unloader
Piston Up &
Unseated
Cooling
Plate
FIGURE 9 - OPERATION - UNLOADED (CLOSED ROOM)
Air From
Governor
Unloader
Port
Air in Pistons Shuttles Back and Forth from the
Piston to the Closed Room
Unloader
Piston Up &
Unseated
Closed
Room

5
LUBRICATION
The vehicle's engine provides a continuous supply of oil
to the compressor. Oil is routed from the engine to the
compressor's oil inlet. An oil passage in the crankshaft
routespressurizedoil totheprecision sleevemainbearings
and to the connecting rod bearings. Spray lubrication of
the cylinder bores, connecting rod wrist pin bushings, and
ball type main bearings is obtained as oil is forced out
around the crankshaft journals by engine oil pressure.
Oil then falls to the bottom of the compressor crankcase
and is returned to the engine through drain holes in the
compressor mounting flange.
Standard Compressor Lubrication for Caterpillar C11
and C13 engine applications
Bendix®BA-921®compressor - for Caterpillar C11 and
C13 engine installations only - use an "oil jet" that sprays
oil under the piston for purposes of cooling. This oil jet
is part of a special crankcase cover that is used only on
the BA-921®compressor for CAT C11 and C13 engine
installations (see Figure 17).
Thisdesign slightlyalters theflow ofoil for lubrication. The
oil supply line from the engine is directly connected to the
back side of the special crankcase over. The oil flows in
parallelthrough a passageway in thecrankcase coverand
through the oil jet to spray oil under pressure up onto the
underneath of the piston for cooling. At the same time,
oil flows out of the opposite end of the special crankcase
cover, through a fitting and a metal tube and second fitting
into the oil supply port of the compressor. At this point oil
flows in a similar manner as in the first paragraph of this
section.
COOLING
Bendix®BA-921®compressors are cooled by air flowing
throughthe enginecompartmentas itpasses thecompres-
sor's cast-in cooling fins and by the flow of engine coolant
throughthe cylinder head. Coolantsupplied bythe engine
cooling system passes through connecting lines into the
cylinder head and passes through internal passages in
the cylinder head, cooling plate and valve plate assembly
and returns to the engine. Figures 10 and 11 illustrate the
variousapproved coolant flowconnections. Propercooling
isimportant inminimizing dischargeair temperatures- see
the tabulated technical data on page 18 of this manual for
specific requirements.
FIGURE 10 - STANDARD BENDIX®BA-921®COMPRESSOR
CYLINDER HEAD
Discharge
Safety Valve
Discharge
Port
Inlet
Port
Unloader
Cover
Coolant In or Out
Head
Bolt (4)
CYLINDER HEAD PORT IDENTIFICATION
The cylinder head connection ports are identified with cast
in numerals as follows:
AIR IN 0
Compressed AIR OUT 2
Coolant IN or OUT 9
Governor Control 4
Governor
Connection
Coolant In or Out
(One or other not used)
FIGURE 11 - CLOSED ROOM BENDIX®BA-921®
COMPRESSOR CYLINDER HEAD
CYLINDER HEAD PORT IDENTIFICATION
The cylinder head connection ports are identified with cast
in numerals as follows:
AIR IN 0
Compressed AIR OUT 2
Coolant IN or OUT 9
Governor Control 4
Discharge
Safety Valve
Discharge
Port
Inlet
Port
Unloader
Cover
Coolant In or Out
Head
Bolt (6)
Governor
Connection
Coolant In
or Out
(One or other not
used)

6
AIR INDUCTION
Section 1: For Standard Compressors.
GENERAL
TheStandardBendix®BA-921®aircompressor canbeused
bothwith air inductionsystems that arenaturally aspirated
(atmospheric air) and pressurized (turbocharged). The
following section covers Caterpillar and Detroit Diesel
engine air induction arrangements. See Figure 5, for
typical flanges used.
CATERPILLAR
Caterpillar HD ACERT engines (C11, C13, C15 and C18)
andMDACERTengines(C7andC9)are typicallyequipped
with Bendix®BA-921®compressors. These engines
providepressurized (turbocharged)air tothecompressor's
inlet port. Caterpillar recommends the use of an inlet
check valve in the air induction system to prevent the air
from the compressor being forced back into the engine
air induction system when the compressor is operating in
the "unloaded" condition (not building air). Because the
compressorinductionsystemis turbocharged,anadditional
air volume is required between the compressor inlet port
andthe inletcheck valve toprevent excessiveair pressure
at the compressor inlet in the unloaded mode. Figures 12
and13show examplesofthedifferentairinduction systems
used by Caterpillar to perform this function.
CATERPILLAR C7/C9 ENGINES
TheBendix®BA-921®compressorintheC7/C9 airinduction
system (see Figure 12) receives its air from the engine's
intake manifold (turbocharged). During the pumping
condition (loaded mode), the air flows from the engine
intake manifold through the inlet check valve and inlet
line to the compressor inlet port. During the non-pumping
condition (unloaded mode), the compressor cylinder
pushesair backout ofthe inletport tothe inletcheck valve.
The ICV prevents the air from traveling beyond this point.
Becausethe airis boosted(under pressure), it is important
thatthe compressorinletline isofsufficient length,strength
and volume to minimize the build-up of air pressure in the
inlet system. The air shuttles back and forth between the
compressor cylinder bore and the ICV during this phase
of the compressor operation.
FIGURE 12 - EXAMPLE OF CATERPILLAR (ACERT
ENGINE) C7/C9 COMPRESSOR AIR INDUCTION SYSTEM
(TURBOCHARGED)
Inl
e
t
P
o
r
t
Inlet Check Valve
I
n
le
t
Li
n
e
A
ir
S
uppl
y
F
rom
E
n
gi
n
e
FIGURE 13 - EXAMPLE CATERPILLAR (HD ACERT
ENGINE) C11/C13/C15/C18 COMPRESSOR AIR INDUCTION
SYSTEM (TURBOCHARGED)
I
n
le
t
Po
r
t
Expansion Tank
Inlet Check Valve
Air Suppl
y
From En
g
in
e
I
n
le
t
Li
n
e
FIGURE 14 - EXAMPLE OF DETROIT DIESEL (EGR) S60
COMPRESSOR AIR INDUCTION SYSTEM (NATURALLY
ASPIRATED)
Inl
e
t
P
o
r
t
Inlet
Check
Valve
A
F
r

7
CATERPILLAR C11, C13, C15 AND C18 ENGINES
The Bendix®BA-921®compressor in the C11, C13, C15,
and C18 air induction systems (see Figure 13) receives its
air from the engine's intake manifold (turbocharged). During
the pumping condition (loaded mode), the air flows from
the engine intake manifold through the inlet check valve,
expansion tank and inlet line to the compressor inlet port.
During the non-pumping condition (unloaded mode), the
compressor cylinder pushes air back out of the inlet port into
the expansion tank. The ICV (at the end of the expansion
tank) prevents the air from traveling beyond this point.
Because the air is boosted (under pressure), it is important
that the compressor inlet line is of sufficient length, strength
and volume to minimize the build-up of air pressure in the
inlet system. The air shuttles back and forth between the
compressor cylinder bore and the expansion tank during this
phase of the compressor operation.
DETROIT DIESEL
The Detroit Diesel Series 60 (EGR) engine is equipped with
theBendix®BA-921®compressorand usesnaturallyaspirated
air induction system. Detroit Diesel recommends the use of
an inlet check valve in the air induction system to prevent the
airfrom thecompressor cylinder borefrom beingforced back
into the engine air induction system when the compressor is
in the unloaded mode (non-pumping condition). A flexible
high-pressure hose is installed between the inlet check
valve and the compressor inlet fitting. This hose can be of
various lengths to accommodate the distance between the
compressor and inlet check valve. See Figure 14.
During operation, non-pressurized air from the engine's air
source is routed to the compressor from a point between the
engineair filterandthe non-pressure sideof theturbocharger.
When the compressor is building air (loaded mode), the air
flowsfrom theengineintaketube,throughthe inletcheckvalve
into the inlet port of the compressor. When the compressor
is not building air (unloaded mode), the compressor pushes
the air back out the compressor during the cylinder upstroke
towards the inlet check valve. The ICV prevents the air from
traveling beyond this point. The air shuttles back and forth
between the compressor cylinder bore and the ICV during
this phase of the compressor operation.
AIR INDUCTION
Section 2: For Closed Room Compressors.
Bendix Closed Room BA-921®air compressors are
only permitted to be naturally aspirated – use of engine
turbocharger as an air source is not allowed. See Figure 6
onpage 3for anexampleof anaturally aspiratedair induction
system.
NOTE: DO NOT install an inlet check valve (ICV) on air
induction systems where a closed room compressor is used.
PREVENTATIVE MAINTENANCE
Regularly scheduled maintenance is the single most
important factor in maintaining the air brake charging
system. Refer to TableAin the Troubleshooting section
on page 21, for a guide to various considerations that
must be given to maintenance of the compressor and
other related charging system components.
Important Note: Review the warranty policy before
performing any intrusive maintenance procedures.
An extended warranty may be voided if intrusive
maintenance is performed during this period.
EVERY 6 MONTHS, 1800 OPERATING HOURS
OR AFTER EACH 50,000, MILES WHICHEVER
OCCURS FIRST, PERFORM THE FOLLOWING
INSPECTIONS AND TESTS.
AIR INDUCTION
The Bendix®BA-921®compressor is designed for
connection to the vacuum side of the engine’s air
induction system and the pressure side (turbocharged)
of the engine’s air induction system.
Asupply of clean air is one of the single most important
factors in compressor preventive maintenance. Since
theair supplyfor BA-921®compressorand engine is the
engine air cleaner, periodic maintenance of the engine
air filter is necessary.
Inspect the compressor air induction system each time
engine air cleaner maintenance is performed.
1. Inspect the intake hose adapters for physical
damage. Makecertain tocheck theadapters atboth
ends of the intake hose or tubing.
2. Inspect the intake hose clamps and tighten them if
needed.
3. Inspect the intake hose or line for signs of drying,
cracking, chafing and ruptures and replace if
necessary.
4. Verify that the compressor inlet fitting is tight (check
torque).
5. Any metal tubes should also be tight (torqued
properly) to the mating fitting. Inspect the metal
tubes for any cracks or breaks and replace if
necessary.
6. If an expansion tank is present (turbocharged air
induction systems only), inspect for any cracks and
replace if necessary.
COMPRESSOR COOLING
Inspect the compressor discharge port, inlet cavity and
discharge line for evidence of restrictions and carbon
buildup. Ifmorethan 1/16"ofcarbonis found,thoroughly
cleanorreplacethe affectedparts. In somecase,carbon

8
buildup indicates inadequate cooling. Closely inspect the
compressorcooling system. Checkallcompressor coolant
linesforkinks andrestrictionsto flow. Minimumcoolant line
size is 3/8" I.D. Check coolant lines for internal clogging
from rust scale. If coolant lines appear suspicious, check
the coolant flow and compare to the tabulated technical
data present in the back of this manual. Carefully inspect
the air induction system for restrictions.
LUBRICATION
Caterpillar Engine Installations.
Check the external oil supply line for kinks, bends, or
restrictions to flow. Supply lines must be a minimum of
3/16” I.D. Refer to the tabulated technical data in the back
of this manual for oil pressure minimum values.
Check the exterior of the compressor for the presence of
oilseepage andrefer totheTROUBLESHOOTINGsection
for appropriate tests and corrective action.
Detroit Diesel S60 Installations
On Detroit Diesel Series 60 engine installations, the
compressor utilizes an internal oil feed design. Check
the exterior of the compressor for the presence of oil
seepage and refer to the TROUBLESHOOTING section
for appropriate tests and corrective action. Refer to the
tabulated technical data in the back of this manual for oil
pressure minimum values.
OIL PASSING
All reciprocating compressors pass a minimal amount of
oil. Air dyers will remove the majority of oil before it can
enter the air brake system. For particularly oil sensitive
systems, the Bendix®PuraGuard®system can be use in
conjunction with a Bendix®air dryer.
If compressor oil passing is suspected, refer to the
TROUBLESHOOTING section (starting on page A-1)
for the symptoms and corrective action to be taken. In
addition, Bendix has developed the “Bendix Air System
Inspection Cup” or BASIC™kit to help substantiate
suspectedexcessive oilpassing. The steps tobe followed
when using the BASIC™kit are presented in APPENDIX
B, on page A-16.
COMPRESSOR DRIVE
Checkfornoisy compressoroperation,whichcould indicate
excessive drive component wear. Adjust and/or replace
as necessary. Check all compressor mounting bolts and
retighten evenly if necessary. Check for leakage and
proper unloader mechanism operation. Repair or replace
parts as necessary.
COMPRESSOR UNLOADER & GOVERNOR
Test and inspect the compressor and governor unloader
system for proper operation and pressure setting.
1. Checkforleakageat theunloaderport. Replaceleaking
or worn o-rings.
2. Make certain the unloader system lines are connected
as illustrated in Figure 6.
3. Cyclethe compressorthroughthe loadedandunloaded
cycle several times. Make certain that the governor
cuts-in (compressor resumes compressing air) at a
minimum of 105 psi (cut-out should be approximately
15 - 20 psi greater than cut-in pressure). Adjust or
replace the governor as required.
4. Note that the compressor cycles to the loaded and
unloaded conditions promptly. If prompt action is not
noted, repair or replace the governor and/or repair the
compressor unloader.
IMPORTANT NOTE
Replacement air governors must have a minimum cut-in
pressure of 100 psi. The cut-in pressure is the lowest
system pressure registered in the gauges before the
compressor resumes compressing air.
Compressorswith no signalline totheunloader portshould
haveavent cap(e.g. Bendixpartnumber 222797)installed
in the port. Under no circumstances should the port be
plugged or left open.
SERVICE TESTS
GENERAL
Thefollowingcompressoroperating andleakagetestsneed
notbe performed ona regularbasis. Thesetests shouldbe
performedwhenit issuspectedthat leakageissubstantially
affecting compressor buildup performance, or when it is
suspected that the compressor is “cycling” between the
loaded (pumping) and unloaded (non-pumping) modes
due to unloader leakage.

9
FIGURE 15 – BA-921®STANDARD COMPRESSOR EXPLODED VIEW
28 Crankcase
Cover
21 Crankcase
13 Cylinder Head
15 Cooling
Plate 16 Valve Plate
Assembly
4 Unloader Cover
22 Crankshaft
2 Unloader Cover Cap Screws (2)
10 Unloader Piston
6 Unloader Balance Piston
14 Head Gasket
(2)
9 O-Ring
5 Unloader Cover Gasket
27 Bottom Cover
Gasket
12 ST-4™Safety Valve
8 Spring
18 Piston
1 Head Cap Screws (4)
(include washers)
7 O-Ring
17 Inlet Reed Valve/
Gasket 19 Connecting
Rod
20 Bearing
Sleeve
29 Cap
Screws (4)
23 Rear Bearing
24 O-Ring
25 Rear End
Cover
26 Cap
Screws (4)
11 O-Ring
3 Unloader Cover Splash Shield
Item Qty. Description
1 4 Head Cap Screws - (Kit 4)
2 2 Unloader Cover Cap Screws - (Kit 4)
3 1 Unloader Cover Splash Shield - (Kit 4)
4 1 Unloader Cover - (Kits 2 & 4)
5 1 Unloader Cap Gasket - (Kits 2 & 4)
6 1 Unloader Balance Piston - (Kits 2 & 4)
7 1 O-Ring - (Kits 2 & 4)
8 1 Spring - (Kits 2 & 4)
9 1 O-Ring - (Kits 2 & 4)
10 1 Unloader Piston - (Kits 2 & 4)
11 1 O-Ring - (Kits 2 & 4)
12 1 ST-4™Safety Valve - (Kit 4)
13 1 Cylinder Head - (Kit 4)
14 2 Head Gaskets - (Kits 1 & 4)
15 1 Cooling Plate - (Kit 4)
16 1 Valve Plate Assembly - (Kit 4)
17 1 Inlet Reed Valve/Gasket - (Kits 1 & 4)
18 1 Piston
19 1 Connecting Rod
20 1 Sleeve Bearing
21 1 Crankcase
22 1 Crankshaft
Kit Notes:
Kit 1: Cylinder Head Gasket
Kit (5008558)
Kit 2: Unloader Kit (5008557)
Kit 3: Governor Adapter Kit
(5008561)
Kit 4: Cylinder Head
Assembly Kit (K023600,
K023601)
Other:
Crankcase Compressor Seal
Kit (5008559)
Unique Engine Seal Kits
are available, for Cat C7/C9
Engine,CatC11/C13/C15/C18
Engines, and DDC Series 60
Engines
Item Qty. Description
23 1 Rear Bearing
24 1 O-Ring
25 1 Rear End Cover
26 4 Cap Screws
27 1 Bottom Cover Gasket
28 1 Crankcase Cover
29 4 Cap Screws
30 2 Governor Gasket - (Kits 3 & 4)
31 1 Governor Adapter - (Kits 3 & 4)
32 2 Bolt with Washer - (Kits 3 & 4)
30
Governor
Gasket (2) 31
Governor
Adapter
32 Bolt
with
Washer
(2)
Crankcase
Alignment
Pins
Alignment
Bushings
See Figure 10
for Port Details.

10
FIGURE 16 – BA-921®CLOSED ROOM COMPRESSOR EXPLODED VIEW.
See Figure 11 for
Port Details.
28 Crankcase
Cover
21 Crankcase
13 Cylinder Head
16 Valve Plate
Assembly
15 Cooling Plate
4 Unloader Cover
2 Unloader Cover Cap Screws (2)
10 Unloader Piston
6 Unloader Balance Piston
14 Head Gasket
(2)
9 O-Ring
5 Unloader Cover Gasket
27 Bottom Cover
Gasket
12 ST-4™Safety Valve
8 Spring
1 Head Cap Screws (6)
(include washers)
7 O-Ring
17 Inlet Reed Valve/
Gasket
20 Bearing
29 Cap
Screws (4)
11 O-Ring
30
Governor
Gasket (2) 31
Governor
Adapter
32 Bolt
with
Washer
(2)
Crankcase
Alignment
Pins
Alignment
Bushings
Kit Notes:
Kit 1: Cylinder Head Gasket
Kit (K022563)
Kit 2: Unloader Kit (5008557)
Kit 3: Governor Adapter Kit
(5008561)
Item Qty. Description
23 1 Rear Bearing (see Fig. 15)
24 1 O-Ring (see Fig. 15)
25 1 Rear End Cover (see Fig. 15)
26 4 Cap Screws (see Fig. 15)
27 1 Bottom Cover Gasket
28 1 Crankcase Cover
29 4 Cap Screws
30 2 Governor Gasket - (Kit 3)
31 1 Governor Adapter - (Kit 3)
32 2 Bolt with Washer - (Kit 3)
Item Qty. Description
1 4 Head Cap Screws
2 2 Unloader Cover Cap Screws
3 1 Unloader Cover Splash Shield
4 1 Unloader Cover
5 1 Unloader Cap Gasket - (Kit 2)
6 1 Unloader Balance Piston - (Kit 2)
7 1 O-Ring - (Kit 2)
8 1 Spring - (Kit 2)
9 1 O-Ring - (Kit 2)
10 1 Unloader Piston - (Kit 2)
11 1 O-Ring - (Kit 2)
12 1 ST-4™Safety Valve
13 1 Cylinder Head
14 2 Head Gaskets - (Kit 1)
15 1 Cooling Plate
16 1 Valve Plate Assembly
17 1 Inlet Reed Valve/Gasket - (Kit 1)
18 1 Piston
19 1 Connecting Rod
20 1 Bearing
21 1 Crankcase
22 1 Crankshaft (see Fig. 15)

11
IN SERVICE OPERATING TESTS
Compressor Performance: Build-up Test
This test is performed with the vehicle parked and the
engine operating at maximum recommended governed
speed. Fully charge the air system to governor cut out (air
dryer purges). Pump the service brake pedal to lower the
system air pressure below 80 psi using the dash gauges.
As the air pressure builds back up, measure the time
from when the dash gauge passes 85 psi to the time it
passes 100 psi. The time should not exceed 40 seconds.
If the vehicle exceeds 40 seconds, test for (and fix) any
air leaks, and then re- test the compressor performance.
If the vehicle does not pass the test the second time,
use the Advanced Troubleshooting Guide for Air Brake
Compressors, starting on page A-1 of this document to
assist your investigation of the cause(s).
Note: All new vehicles are certified using the FMVSS
121 test (paragraph S5.1.1) by the vehicle manufacturer,
however the above test is a useful guide for in-service
vehicles.
Optional Comparative Performance Check
It may be useful to also conduct the above test with the
enginerunning at highidle (instead ofmaximum governed
speed), and record the time taken to raise the system
pressure a selected range (for example, from 90 to 120
psi, or from 100 to 120 psi, etc.) and record it in the
vehicle’s maintenance files. Subsequent build-up times
throughoutthe vehicle’sservice lifecan thenbe compared
to the first one recorded. (Note: the 40 second guide in
the test above does not apply to this build-up time.) If the
performance degrades significantly over time, you may
use the Advanced Troubleshooting Guide for Air Brake
Compressors, starting on page A-1 of this document, to
assist investigation of the cause(s).
Note: When comparing build-up times, be sure to make
anallowance forany air systemmodifications whichwould
cause longer times, such as adding air components or
reservoirs. Always check for air system leakage.
LEAKAGE TESTS
SeethestandardAirBrakeSystemandAccessoryLeakage
test on Page A-14 (Test 2).
Note: Leakage in the air supply system (components
beforethesupply reservoir-such asthegovernor,airdryer,
reservoir drain cocks, safety valve and check valves) will
not be registered on the vehicle dash gauges and must
be tested separately. Refer to the various maintenance
manuals for individual component leakage tests and the
Bendix “Test and Checklist” published in the Air Brake
System Handbook (BW5057) and on the back of the Dual
Circuit Brake System Troubleshooting Card (BW1396).
CYLINDER HEAD
Check for cylinder head gasket air leakage.
1. With the engine running, lower air system pressure to
60 psi and apply a soap solution around the cylinder
head. Checkthegasket betweenthe cylinder headand
valve plate assembly and the inlet reed valve/gasket
between the valve plate assembly and crankcase for
air leakage.
2. Noleakage ispermitted. If leakage is detected replace
the compressor or repair the cylinder head using a
genuine Bendix maintenance kit available from an
authorized Bendix parts outlet.
INLET, DISCHARGE & UNLOADER
In order to test the inlet and discharge valves and the
unloader piston, it is necessary to have shop air pressure
and an assortment of fittings. A soap solution is also
required.
1. With the engine shut off, drain ALL air pressure from
the vehicle.
2. Disconnect the inlet and discharge lines and remove
the governor or its line or adapter fitting.
3. Apply 120-130 psi shop air pressure to the unloader
port and soap the inlet port. Leakage at the inlet port
should not exceed 50 sccm.
4. Apply 120-130 psi shop air pressure to the discharge
portand thenapply andrelease airpressure tothe inlet
port. Soap the inlet port and note that leakage at the
inlet port does not exceed 20 sccm.
If excessive leakage is noted in Tests 3 or 4, replace or
repairthe compressorusing genuineBendix replacements
or maintenance kits available from any authorized Bendix
parts outlet.
Whileit ispossible totest forinlet, discharge,and unloader
piston leakage, it may not be practical to do so. Inlet and
discharge valve leakage can generally be detected by
longercompressor build-upand recoverytimes. Compare
current compressor build-up times with the last several
recordedtimes. Makecertain totestfor airsystemleakage,
as described under In Service Operating Tests, before
making a determination that performance has been lost.
Unloader leakage is generally exhibited by excessive
compressor cycling between the loaded and unloaded
condition.
1. With service and supply system leakage below the
maximum allowable limits and the vehicle parked,
bring system pressure to governor cut-out and allow
the engine to idle.

12
2. Thecompressor shouldremainunloadedfora minimum
of 5-10 minutes. If compressor cycling occurs more
frequently and service and supply system leakage is
within tolerance replace the compressor or repair the
compressor unloader system using a genuine Bendix
maintenancekit availablefrom authorizedBendix parts
outlets.
COMPRESSOR REMOVAL & DISASSEMBLY
GENERAL
The following disassembly and assembly procedure is
presented for reference purposes and presupposes that
a rebuild or repair of the compressor is being undertaken.
Severalmaintenance kitsareavailable andthe instructions
provided with these parts and kits should be followed in
lieu of the instructions presented here.
MAINTENANCE KITS & SERVICE PARTS
Since the compressors have a different head and
crankcase design, be sure to only use replacement
parts specifically applicable.
Note: In particular, the heads and head gaskets are
not interchangeable between the two compressors
covered in this document.
Section 1: Standard Compressor
Cylinder Head Gasket Kit.......................................................5008558
Unloader Kit ...........................................................................5008557
Governor Adapter Kit .............................................................5008561
Compressor Seal Kit (crankcase)...........................................5008559
Components and kits for Caterpillar Engines
CAT C7/C9 Cylinder Head Assembly Kit ...............................K023601
CAT C11/C13/C15/C18 Cylinder Head Assembly Kit ............K023600
CAT C7/C9 Inlet Check Valve .................................................801580
CAT C11/C13/C15/C18 Inlet Check Valve ...............................801592
CAT C7/C9 Engine Seal Kit....................................................5012367
CAT C11/C13/C15/C18 Engine Seal Kit.................................5012369
CAT ST-4™Discharge Safety Valve (7/8"-14 thrd.) ...................801116
Components and kits for DDC Engines
DDC Inlet Check Valve ............................................................802192
DDC Series 60 Engine Seal Kit..............................................5012371
DDC ST-4™Discharge Safety Valve (M16-1.5 thrd.).................800534
Section 2: Closed Room Compressor
Cylinder Head Gasket Kit.......................................................K022563
Unloader Kit ...........................................................................5008557
Governor Adapter Kit .............................................................5008561
DDC Series 60 Engine Seal Kit..............................................5012371
DDC ST-4™Discharge Safety Valve (M16-1.5 thrd.).................800534
All components shown in Figures 15 and 16 with a key
number are available in kits and/or as individual service
parts.
IMPORTANT! PLEASE READ AND FOLLOW
THESE INSTRUCTIONS TO AVOID PERSONAL
INJURY OR DEATH:
Whenworking onoraround avehicle,the followinggeneral
precautions should be observed at all times:
1. Park thevehicle on alevel surface, applythe parking
brakes, and always block the wheels. Always wear
safety glasses. Where specifically directed, the
parking brakes may have to be released, and/or
spring brakes caged, and this will require that the
vehicle be prevented from moving by other means
for the duration of these tests/procedures.
2. Stop the engine and remove ignition key when
working under or around the vehicle. When
working in the engine compartment, the engine
should be shut off and the ignition key should be
removed. Where circumstances require that the
engine be in operation, EXTREME CAUTION should
be used to prevent personal injury resulting from
contact with moving, rotating, leaking, heated or
electrically charged components.
3. Do not attempt to install, remove, disassemble
or assemble a component until you have read
and thoroughly understand the recommended
procedures. Use only the proper tools and observe
all precautions pertaining to use of those tools.
4. If the work is being performed on the vehicle’s
air brake system, or any auxiliary pressurized air
systems, makecertain to drain theair pressure from
all reservoirs before beginning ANY work on the
vehicle. If the vehicle is equipped with an AD-IS®
air dryer system or a dryer reservoir module, be
sure to drain the purge reservoir.
5. Followingthevehiclemanufacturer’srecommended
procedures, deactivate the electrical system in a
manner that safely removes all electrical power
from the vehicle.
6. Never exceed manufacturer’s recommended
pressures.
7. Never connect or disconnect a hose or line
containing pressure; it may whip. Never remove
a component or plug unless you are certain all
system pressure has been depleted.
8. Use only genuine Bendix®replacement parts,
components and kits. Replacement hardware,
tubing, hose, fittings, etc. must be of equivalent
size, type and strength as original equipment and
be designed specifically for such applications and
systems.
9. Components with stripped threads or damaged
parts should be replaced rather than repaired. Do
not attempt repairs requiring machining or welding

13
unless specifically stated and approved by the
vehicle and component manufacturer.
10.Prior to returning the vehicle to service, make
certain all components and systems are restored
to their proper operating condition.
11. For vehicles with Antilock Traction Control (ATC),
the ATC function must be disabled (ATC indicator
lamp should be ON) prior to performing any vehicle
maintenance where one or more wheels on a drive
axle are lifted off the ground and moving.
REMOVAL
In many instances it may not be necessary to remove the
compressor from the vehicle when installing the various
maintenance kits and service parts. The maintenance
technician must assess the installation and determine the
correct course of action.
These instructions are general and are intended to be
a guide. In some cases additional preparations and
precautions are necessary. In all cases follow the
instructions contained in the vehicle maintenance manual
in lieu of the instructions, precautions and procedures
presented in this manual.
1. Block the wheels of the vehicle and drain the air
pressure from all the reservoirs in the system.
2. Drain the engine cooling system and the cylinder head
ofthe compressor. Identify anddisconnectall air, water
and oil lines leading to the compressor.
3. Removeas muchroad dirtandgrease fromthe exterior
of the compressor as possible.
4. Remove the discharge and inlet fittings, if applicable,
and note their position on the compressor to aid in
reassembly.
Note: If a cylinder head maintenance kit is being
installed, stop here and proceed to PREPARATION FOR
DISASSEMBLY. If replacing the compressor continue.
5. Remove any supporting bracketing attached to the
compressorand notetheir positions onthe compressor
to aid in reassembly.
6. Remove the front flange mounting bolts and remove
the compressor from the vehicle.
7. Inspectdrive gearand associateddrive partsfor visible
wear or damage. If the compressor drive gear is worn
or damaged, the compressor must be replaced. Refer
the Engine Manufacturers service manual to address
the associated engine drive parts.
8. If the compressor is being replaced stop here and
proceed to “Installing the Compressor” at the end
of the assembly procedure. (Note: Replacement
compressors come with the drive gear pre-assembled
on the compressor.)
PREPARATION FOR DISASSEMBLY
Remove the balance of road dirt and grease from the
exteriorof thecompressor witha cleaningsolvent. If a rear
endcover or endcover adapteris used on the compressor
being worked on, mark it in relation to the crankcase. It is
recommended, but not specifically necessary, to mark the
relationships of the cylinder head (13), cooling plate (15),
valve plate assembly (16), and crankcase (21).
A convenient method to indicate the above relationships
is to use a metal scribe to mark the parts with numbers or
lines. Do not use marking methods such as chalk that can
be wiped off or obliterated during rebuilding.
Prior to disassembly make certain that the appropriate
kits and or replacement parts are available. Refer to
Figure 15 for the standard compressor and Figure 16 for
theClosed-roomcompressor duringtheentire disassembly
and assembly procedure.
What follows is a description of a complete disassembly,
actual maintenance may only need to include portions of
these instructions.
CYLINDER HEAD
Section 1: Standard Compressor (refer to Figure 15)
1. Remove the discharge safety valve (12) from the
cylinder head (13).
2. To restrain the spring force exerted by balance piston
spring (8) of the unloader assembly, hold the unloader
cover (4) in place while removing the two unloader
cover cap screws (2) and spray shield (3). Carefully
release the hold on the unloader cover until the spring
force is relaxed, then remove the unloader cover.
3. Remove the unloader cover gasket (5).
4. Remove the balance piston (6), its spring (8) and the
unloader piston (10) along with its o-rings (7, 9 & 11)
from the cylinder head (13).
5. Remove the four hex head bolts (1) from the cylinder
head.
6. Gently tap the cylinder head, cooling plate (15) and
valve plate assembly (16) with a soft mallet to break
the gasket seal between the valve plate assembly and
the crankcase (21). Lift the cylinder head with cooling
plate and valve plate assembly off the crankcase.
7. Remove the metal inlet reed valve/gasket (17).
8. Gently tap the cylinder head, cooling plate and valve
plate assembly with a soft mallet to break the gasket
seals. Thenseparatethe cylinderheadfromthe cooling
plate (15) and valve plate assembly and remove the
two gaskets (14) between them.

14
Section 2: Closed Room Compressor (refer to Figure 16)
1. Remove the discharge safety valve (12) from the
cylinder head (13).
2. To restrain the spring force exerted by balance piston
spring (8) of the unloader assembly, hold the unloader
cover (4) in place while removing the two unloader
covercap screws(2). Carefully releasethe holdon the
unloader cover until the spring force is relaxed, then
remove the unloader cover.
3. Remove the unloader cover gasket (5).
4. Remove the balance piston (6), its spring (8) and the
unloader piston (10) along with its o-rings (7, 9 & 11)
from the cylinder head (13).
5. Remove the six hex head bolts from the cylinder head.
Note: The five hex bolts located towards the perimeter
of the cylinder head retain the cylinder head directly to
the crankcase. The single hex bolt in the center of the
cylinder head holds the cylinder head, cooling plate
and valve plate assembly together; independent of the
crankcase.
6. Gently tap the cylinder head, cooling plate (15) and
valve plate assembly (16) with a soft mallet to break
the gasket seal between the valve plate assembly and
the crankcase (21). Lift the cylinder head with cooling
plate and valve plate assembly off the crankcase.
7. Remove the metal inlet reed valve/gasket (17).
8. Gently tap the cylinder head, cooling plate and valve
plate assembly with a soft mallet to break the gasket
seals. Thenseparatethe cylinderheadfromthe cooling
plate (15) and valve plate assembly and remove the
two gaskets (14) between them.
CRANKCASE COVER
1. Remove the four crankcase cover cap screws (29)
securing the crankcase cover (28) to the crankcase
(21). Usingasoft mallet,gentlytapthe crankcasecover
tobreak thegasket seal. Removethe crankcasecover
gasket (27).
2. In the case of the Caterpillar C11 and C13 engine
application, the BA-921®standard compressor utilizes
an “oil jet” that sprays oil under the piston for purposes
of cooling. This oil jet is part of a special crankcase
coverthatisused strictlyontheBA-921®compressorfor
theC11 and C13engine installation (Figure13). Refer
to section OPERATION – Lubrication for description
of the system. To disassemble, perform the following
steps. (Refer to Figure 17.)
a. Remove the oil supply line from the engine at the
inlet to the special crankcase cover.
b. Removethe metaloilsupply tubeatthe compressor
oil supply port and at the outlet fitting of the special
crankcase cover.
NOTE: Mark position of the special crankcase cover. It
must be re-installed with the same orientation to assure
proper operation of the oil jet.
c. Remove the four crankcase cover cap screws
securing the special crankcase cover to the
crankcase. Using a soft mallet, gently tap the
crankcasecover tobreak thegasket seal. Remove
the crankcase cover gasket (27).
REAR END COVER (If Present)
1. Removethe fourend covercapscrews (26)thatsecure
the rear end cover to the crankcase.
2. Remove the rear end cover from the crankcase.
Remove the o-ring seal (24) from the end cover.
CLEANING OF PARTS
GENERAL
All parts should be cleaned in a good commercial grade of
solvent and dried prior to inspection.
CYLINDER HEAD ASSEMBLY
1. Carefully remove all gasket material adhering to the
cylinder head (13), cooling plate (15), valve plate
assembly (16) and cast iron crankcase (21). Make
certain not to scratch or mar the gasket surfaces.
Pay particular attention to the gasket surfaces of the
head.
2. Remove carbon deposits from the discharge and inlet
cavities of the cylinder head, cooling plate and valve
FIGURE 17 – VIEWS OF SPECIAL CRANKCASE COVER
WITH OIL JET ASSEMBLY FOR CAT C11/C13 ENGINE
APPLICATIONS
Crankcase Cover with
Oil Jet Assembly
Compressor
Oil Supply
Port
Lubricating Oil
From the Engine
Enters Here
Lubricating
Oil outlet
Side View
Pre-formed
Metal
Oil Supply
Tube
Engine
Oil Supply
Line

15
plate assembly. They must be open and clear in both
assemblies. Make certain not to damage the head.
3. Remove rust and scale from the cooling cavities and
passages in the cylinder head, cooling plate and valve
plate assembly and use shop air to clear debris from
the passages.
4. Check the threads in all cylinder head ports for
galling (e.g. abrasion, chafing). Minor thread chasing
(damage) is permitted.
5. Make certain the unloader vent passage under the
unloader cover (4) in the head is open and free of
debris. NOTE: This only applies to the unloader cover
on the Standard Compressor.
INSPECTION OF PARTS
CYLINDER HEAD, COOLING PLATE, VALVE
PLATE ASSEMBLY AND UNLOADER
MECHANISM
1. Carefully inspect the head gasket surfaces on the
cylinder head (13) for deep gouges and nicks. Also,
inspect the cylinder head for any cracks or port thread
damage. Ifdetected, thecompressormustbe replaced.
If large amounts of carbon build-up are present in
the discharge cavity such that it restricts the air flow
through the cylinder head, the compressor should be
replaced.
2. Carefullyinspect bothsidesofthe headgasketsurfaces
on the cooling plate (15) for deep gouges and nicks.
Also, inspect the cooling plate for any cracks or other
damage. If found, the compressor must be replaced.
3. Carefully inspect the valve plate assembly (16) gasket
surfaces (both sides) for deep gouges and nicks. Pay
particular attention to the gasket surface. An inlet
reed valve/gasket (17) is used between the valve
plate assembly and crankcase. This gasket surface
must be smooth and free of all but the most minor
scratches. Ifexcessive marringor gougingis detected,
the compressor must be replaced. If large amounts of
carbonbuild-uparepresent onthetwomain surfaces, in
thetwodischarge valveholes or betweenthe discharge
valve and the discharge seat, the compressor should
be replaced.
4. If the unloader assembly has been removed from the
cylinderhead, theunloader assembly mustbe serviced
using an unloader kit. (See Maintenance Kits, page
12.)
5. If large amounts of carbon build-up are present on the
unloaderpiston (10)seator orificeor ifthereturn spring
exhibits compression set, the unloader components
must be replaced with an unloader kit.
REAR END COVER (If Present)
Check for cracks and external damage. Check the
crankshaft rear bearing diameter in the rear end cover
(25) for excessive wear, flat spots or galling. Check the
rear support bracket threaded holes and hydraulic pump
attachment threaded holes (if present) for thread damage.
Minor thread chasing is permitted but do not re-cut the
threads. If any of these conditions are found, replace the
compressor.
CRANKCASE
Check the cylinder head gasket surface on the deck (top)
of the crankcase (21) for nicks, gouges, and marring.
A metal gasket is used to seal the cylinder head to the
crankcase. Thissurface mustbe smoothand freeof allbut
themost minorscratching. Ifexcessive marringor gouging
is detected, the compressor must be replaced.
ASSEMBLY
General Note: All torques specified in this manual are
assembly torques and typically can be expected to fall off
after assembly is accomplished. Do not re-torque after
initial assembly torques fall unless instructed otherwise.
Acompiled listing of torque specifications is presented on
page 18.
INCH POUNDS TO FOOT POUNDS
To convert inch pounds to foot pounds of torque, divide
inch pounds by 12.
Example: 12 Inch Pounds = 1 Foot Pound
12
FOOT POUNDS TO INCH POUNDS
To convert foot pounds to inch pounds of torque, multiply
foot pounds by 12.
Example: 1 Foot Pound x 12 = 12 Inch Pounds
CRANKCASE COVER
1. Position the crankcase cover gasket (27) on either the
crankcaseorcrankcase coverandinstall thecrankcase
cover on the crankcase using the four cap screws.
“Snug” all four cap screws then torque to 62-71 inch
pounds (7-8 N•m) using a crossing pattern.
2. In the case of the Caterpillar C11 and C13 engine
application, the compressor utilizes an “oil jet” that
sprays oil under the piston for purposes of cooling.
This oil jet is part of a special crankcase cover that is
used strictly on the compressor for the C11 and C13
engine installation (See Figure 14). Refer to section
OPERATION–Lubricationfordescriptionofthe system.
To re-assemble, follow the instructions below.

16
a. Position the gasket (27) on either the crankcase or
the special crankcase cover and install the special
crankcase cover on the crankcase using the four
cap screws. Note: Make sure that the cover is
orientated in its original position. “Snug” all four
cap screws then torque to 62-71 inch pounds (7-8
N•m) using a crossing pattern.
b. Assumingthat thefittings atthe outletof thespecial
crankcasecoverand compressor oilsupplyporthad
not been removed (fittings still torqued in place),
position both ends of the metal tube over the two
fittings and run the fittings down to finger tight then
torque to 130 - 150 inch pounds (16 – 17.5 N•m). If
atorque wrenchisnot available,run fittingsdownto
finger tight. Then tighten the fittings ¼ to ½ turns.
REAR END COVER (If Present)
1. Install the o-ring (24) on the rear end cover.
2. Orient the rear end cover (25) to the crankcase
using the reference marks made during disassembly.
Carefully install the rear end cover in the crankcase
making certain not to damage the crankshaft bearing
surface.
3. Install the four end cover cap screws (26) or studs.
“Snug” the screws then tighten to 195 to 213 inch
pounds (22-24 N•m) using a crossing pattern.
CYLINDER HEAD ASSEMBLY
PART ONE: HEAD INSTALLATION
Section 1: Standard Compressors
1. Notethe positionof the protrudingcrankcase alignment
pinson the deck(top) ofthe crankcase(21). Install the
metal inlet reed valve/gasket (17) over the alignment
pins on the crankcase.
2. Positionthe valveplateassembly (16)onthecrankcase
so that the alignment pins in the crankcase fit into the
corresponding holes in the valve plate assembly.
3. Position one of the embossed metal head gaskets
(14) over the alignment bushings protruding from the
cooling plate. Position the second embossed metal
head gasket over the alignment bushings on the
opposite side of the cooling plate (15). When properly
positioned, the outline of the two embossed gaskets
match the outline of the cooling plate.
4. Installthe coolingplatewiththe headgasketsontovalve
plate assembly by lining up the alignment bushings
on the cooling plate over oversized countersunk holes
of the valve plate assembly. Again, when properly
installed, the outline of the cooling plate matches the
outline of the valve plate assembly.
5. Position and install the cylinder head (13) over the
alignment bushings protruding from the cooling plate.
Whenproperly installed, theoutline ofthecylinder head
assemblywill matchthe outlineof thecooling plateand
valve plate assembly.
Note: To assist with correct installation, the alignment
bushings only fit into two of the four cylinder head bolt
holes.
6. Install the four hex head cylinder head bolts (1) and
snugthem,then tightenevenlytoa torqueof 265to292
inch pounds (30-33 N•m) using a crossing pattern.
CYLINDER HEAD ASSEMBLY
PART ONE: HEAD INSTALLATION
Section 2: Closed Room Compressors
1. Note the position of the protruding alignment pins on
the deck (top) of the crankcase (21). Install the metal
inlet reed valve/gasket (17) over the alignment pins on
the crankcase.
2. Positionthe valveplateassembly (16)onthecrankcase
so that the alignment pins in the crankcase fit into the
corresponding holes in the valve plate assembly.
3. Positionand installone ofthe embossedmetal gaskets
(14) over the alignment bushings protruding from the
coolingplate.Position andinstall the secondembossed
metal gasket (14) over the alignment bushings on
the opposite side of the cooling plate. When properly
installed, the outline of the two embossed gaskets
match the outline of the cooing plate.
5, 11
1, 7 6, 12
4, 10
2, 8
3, 9
Sequence Torque
(N•m)
1 . . . . . . . . . . . 20
2 . . . . . . . . . . . 20
3 . . . . . . . . . . . 20
4 . . . . . . . . . . . 20
5 . . . . . . . . . . . 20
6 . . . . . . . . . . . 20
Sequence Torque
(N•m)
7 . . . . . . . . . . 31-34
8 . . . . . . . . . . 31-34
9 . . . . . . . . . . 31-34
10 . . . . . . . . . 31-34
11 . . . . . . . . . 31-34
12 . . . . . . . . . 31-34
FIGURE 18 - CLOSED ROOM COMPRESSOR HEAD BOLT
TORQUE SEQUENCE.

17
4. Install the cooling plate onto valve plate assembly by
lining up the alignment bushings on the cooling plate
overthe oversizedcountersunk holesof thevalve plate
assembly. Again, when properly installed, the outline
of the cooling plate matches the outline of the valve
plate.
5. Positionand installthecylinder headoverthe alignment
bushings protruding from the cooling plate. When
properly installed, the outline of the cylinder head
assemblywill matchthe outlineof thecooling plateand
valve plate assembly.
Note: The alignment bushings will only fit into two of the
cylinder head bolt holes.
6. Install the 6 hex head cylinder head bolts and washers
and snug them (finger tight), then torque the bolts in
the sequence specified in Figure 18.
PART TWO: UNLOADER INSTALLATION
These instructions apply to both the Standard and
Closed Room Compressors.
7. Applya coatingof the lubricantprovided intheunloader
kit into the unloader bore (both diameters). Apply
additional lubricant to the three o-ring grooves on the
unloader piston (10). Note that the o-ring (7) installs
inside the top of the unloader piston. Install the three
o-rings(7, 9, 11)into the appropriateo-ring grooves on
the unloader piston (10). Apply another coating of the
lubricant onto the exposed o-ring surfaces and onto
the large diameter of the balance piston (6). Install
the unloader piston with the pre-installed o-rings into
the cylinder head unloader bore making certain not to
damage them in the process.
8. Install the balance piston spring (8) in the unloader
piston.
9. Apply a coating of lubricant to the largest diameter of
the balance piston. Install the small diameter end of
the balance piston through the center of the spring.
10.Install the unloader cover gasket (5) on the cylinder
head making certain both screw holes align.
11. Position the unloader cover (4) on top of the balance
piston making certain the stamped logo is visible.
12.Press and hold the unloader cover in place on the
cylinder head, place the spray shield (3) (Standard
Compressors only) over the outboard hole (in order to
cover the vent slot in the unloader cap). Install both
unloader cover cap screws (2). Torque the cover cap
screws (2) to 62 to 71 inch pounds (7-8 N•m).
INSTALLING THE COMPRESSOR
1. Install the appropriate gasket or o-ring on the front
flange of the compressor. Make certain oil supply or
return holes in the gasket are properly aligned with the
compressor and engine. On the Detroit Diesel S60
engine installations, an additional o-ring is required to
seal the oil feed passageway between the compressor
and engine gear case cover. Gasket sealants are
not recommended. Secure the compressor on the
engine and tighten the mounting bolts per Engine
Manufacturers recommended torque requirements.
2. Installany supportingbracketsonthe compressorinthe
same position(s) noted and marked during removal. If
a rear support bracket was on the original installation,
handtighten thebolts on bothends beforetorquing the
bolts. Note:It isimportant thatthe rearsupport bracket
is flush to both surfaces before the bolts are torqued.
3. Inspect all air, oil, and coolant lines and fittings before
reconnecting them to the compressor. Make certain
o-ring seals are in good or new condition, the threads
areclean andthe fittingsarefree ofcorrosion. Replace
as necessary.
4. Install the discharge, inlet, coolant and governor
adapter fittings, if applicable, in the same position on
thecompressor notedand markedduring disassembly.
See the Torque Specifications for various fitting sizes
and types of thread at the rear of this manual. Tighten
all hose clamps.
5. Before returning the vehicle to service, perform
the Operation and Leakage Tests specified in this
manual. Pay particular attention to all lines and hoses
disconnected during the maintenance and check for
air, oil, and coolant leaks at compressor connections.
Also check for noisy operation.
TESTING THE REBUILT COMPRESSOR
In order to properly test a compressor under operating
conditions, a test rack for correct mounting, cooling,
lubricating,and drivingthe compressor isnecessary. Such
testsarenot compulsoryiftheunit hasbeencarefullyrebuilt
by an experienced person. A compressor efficiency or
build-uptest can be run whichis nottoo difficult.Anengine
lubricated compressor must be connected to an oil supply
line of at least 15 psi pressure during the test and an oil
returnline mustbeinstalled tokeep thecrankcase drained.
Connectto thecompressor dischargeport, areservoir with
avolume of 1500cubic inches,including the volume of the
connecting line. With the compressor operating at 2100
RPM, the time required to raise the reservoir(s) pressure
from85 psito 100 psishould notexceed 5seconds. During
this test, the compressor should be checked for gasket
leakageand noisyoperation, aswellas unloaderoperation
and leakage. If the compressor functions as indicated,
reinstall on the vehicle connecting all lines as marked in
the disassembly procedure.

18
BA-921®COMPRESSOR SPECIFICATIONS (ALL)
Typical weight:
Standard Compressor (DDC Model) ............................... 66 lbs.
Closed Room Compressor (DDC Model) ........................ 55 lbs.
Number of cylinders ......................................................................... 1
Bore Diameter ....................................................... 3.622 in. (92 mm)
Stroke ................................................................... 2.126 in. (54 mm)
Calculated displacement at 1250 RPM .............................. 15.8 CFM
Flow Capacity @ 1800 RPM & 120 PSI ............................... 11.6 CFM
Flow Capacity @ 3000 RPM & 120 PSI ............................... 16.5 CFM
Approximate horsepower required:
Loaded 1800 RPM at 120 PSIG............................................... 4.6 HP
Unloaded 1800 RPM................................................................ 0.8 HP
Minimum coolant flow at maximum RPM ..................... 2.5 Gals./Min.
Maximum inlet air temperature ................................................. 250°F
Maximum discharge air temperature ........................................ 400°F
Minimum oil pressure required................................................. 15 PSI
Minimum oil-supply line size ................................................ 3/16" I.D.
Minimum unloader-line size ................................................. 3/16" I.D.
Minimum Governor Cut-out Pressure...................................... 120 PSI
TORQUE SPECIFICATIONS: Standard
Compressor
Assembly Torques in inch pounds (in. Ibs.)
M8x1.25-6g Cylinder Head.............265-292 In. Lbs. (30-33 N•m)
M5x0.75-6g Unloader Cap .................... 62-71 In. Lbs. (7-8 N•m)
M8x1.25-6g Governor Adapter.......195-213 In. Lbs. (22-24 N•m)
M8x1.25-6g Rear End Cover..........195-213 In. Lbs. (22-24 N•m)
M6x1.00-6g Crankcase Cover ............. 62-71 In. Lbs. (7-8 N•m)
Inlet Port Fittings
1 3/16"-12 UN-2B (Aluminum Cylinder Head)
straight fitting............................841-925 In. Lbs. (95-104 N•m)
adjustable (w/ jam nut)...............597-655 In. Lbs. (67-74 N•m)
M27x2-6g (Cast Iron Cylinder Head).....885-980 In. Lbs. (100-111
N•m)
Discharge Port Fittings
7/8"-14 UNF-2B (Aluminum Cylinder Head)
straight fitting..............................509-553 In. Lbs. (57-62 N•m)
adjustable (w/ jam nut)...............354-389 In. Lbs. (40-44 N•m)
M22 x 1.5-6g (Cast Iron Cylinder Head)..531-575 In. Lbs. (60-65
N•m)
Water Port Fittings
3/4"-16 UNF-2B (Aluminum Cylinder Head)
straight fitting..............................265-292 In. Lbs. (30-33 N•m)
adjustable (w/ jam nut)...............248-274 In. Lbs. (28-31 N•m)
M18 x 1.5-6g (Cast Iron Cyliner Head)....354-395 In. Lbs. (40-45
N•m)
Unloader Port Fittings
1/8"-27 NPT ................................................................ 2 - 3 TFFT1
M10 x 1.5-6g..................................120-145 In. Lbs. (14-16 N•m)
Safety Valve Port
7/8"-14 UNF-2B .............................230-257 In. Lbs. (26-29 N•m)
M16x1.5-6H ...................................230-257 In. Lbs. (26-29 N•m)
Oil Port
7/16"-16 UNF.................................150-170 In. Lbs. (17-19 N•m)
Special Attachments – Cat C11/C13 Engine Crankcase Cover
w/ Oil Jet and Associated Hardware (Figure 17)
1) Crankcase Cover (In/Out) Oil Fittings
7/16"-16 UNF .............................150-170 In. Lbs. (17-19 N•m)
2) Compressor Oil Supply Fitting
7/16"-16 UNF .............................150-170 In. Lbs. (17-19 N•m)
3) Metal Tube (Tube Ends)
7/16"-16 UNF .........................130-150 In. Lbs. (16-17.5 N•m)
Option: Run fittings down finger tight. Tighten ¼ to ½
turns.
1Note: TFFT = Turns From Finger Tight
TORQUE SPECIFICATIONS: Closed Room
Compressor
Assembly Torques in inch pounds (in. Ibs.)
M8x1.25-6g Cylinder Head......270-305 In. Lbs. (30.5-34.5 N•m)
M5x0.75-6g Unloader Cap ....................62-71 In. Lbs. (7-8 N•m)
M8x1.25-6g Governor Adapter......195-213 In. Lbs. (22-24 N•m)
M8x1.25-6g Rear End Cover.........195-213 In. Lbs. (22-24 N•m)
M6x1.00-6g Crankcase Cover ............. 62-71 In. Lbs. (7-8 N•m)
Inlet Port Fittings
M27x2-6g. Inlet Port Fittings ...991-1089 In. Lbs. (112-123 N•m)..
Discharge Port Fittings
M22x1.5-6H ................................814-912 In. Lbs. (92-103 N•m)..
Water Port Fittings
M18x1.5-6H ..................................593-637 In. Lbs. (67-72 N•m)
Unloader Port Fittings
1/8"-27 NPT ............................................................... 2 - 3 TFFT1
Safety Valve Port
M16x1.5-6H ..................................230-257 In. Lbs. (26-29 N•m)
1Note: TFFT = Turns From Finger Tight

A-1
Appendix A
Advanced Troubleshooting Guide for Air Brake Compressors
Air brake charging system:
Slow build (9.0).....................................A-9-10
Doesn’t build air (10.0)............................. A-11
Air dryer:
Doesn’t purge (14.0)................................A-12
Safety valve releases air (12.0)...............A-12
Compressor:
Constantly cycles (15.0) ..........................A-12
Leaks air (16.0)........................................A-13
Safety valve releases air (11.0) ............... A-11
Noisy (18.0) ............................................A-13
Reservoir:
Safety valve releases air (13.0)...............A-12
INDEX
Air Coolant
Engine
Oil
Compressor leaks coolant (17.0)....................A-13
Oil consumption (6.0) ......................................A-9
Oil Test Card results (1.0).................................A-4
Oil is present:
On the outside of the compressor (2.0)......A-5
At the air dryer purge/exhaust
or surrounding area (3.0)........................A-5
In the supply reservoir (4.0).................... A-6-8
At the valves (5.0).......................................A-8
At air dryer cartridge (7.0)...........................A-9
In the ping tank or compressor
discharge aftercooler (8.0)......................A-9
Symptom Page Number
(1) Oil Leakage at Head Gasket .....A-14
(2) System Leakage .......................A-14
(3) Compressor Discharge and
Air Dryer Inlet Temperature...........A-15
(4) Governor Malfunction ................A-14
(5) Governor Control Line ...............A-15
(6) Compressor Unloader ...............A-15
BASIC™Test Information........ A-16-18
Test Procedures
Maintenance Schedule and
Usage Guidelines (Table A)..... A-3
Symptom Page Number
Maintenance & Usage Guidelines
Theguide consistsof anintroduction toair brakecharging systemcomponents, atable
showingrecommendedvehiclemaintenanceschedules,andatroubleshootingsymptom
and remedy section with tests to diagnose most charging system problems.

A-2
Introduction to the Air Brake Charging System
Powered by the vehicle engine, the air compressor
builds the air pressure for the air brake system. The air
compressoristypicallycooled bytheenginecoolant system
and lubricated by the engine oil supply.
The compressor's unloader mechanism and governor
(along with a synchro valve for the Bendix®DuraFlo™596
air compressor) control the brake system air pressure
between a preset maximum and minimum pressure level
by monitoring the pressure in the service (or “supply”)
reservoir. When theairpressurebecomes greaterthanthat
of the preset “cut-out”, the governor controls the unloader
mechanismof thecompressorto stopthe compressorfrom
building air and also causes the air dryer to purge. As the
service reservoir air pressure drops to the “cut-in” setting
ofthe governor, the governorreturns thecompressor back
to building air and the air dryer to air drying mode.
As the atmospheric air is compressed, all the water vapor
originally in the air is carried along into the air system, as
well as a small amount of the lubricating oil as vapor.
The duty cycle is the ratio of time the compressor spends
buildingairtothetotalenginerunningtime. Aircompressors
are designed to build air (run “loaded”) up to 25% of the
time. Higher duty cycles cause conditions that affect air
brake charging system performance which may require
additionalmaintenance. Factors that addto theduty cycle
are: air suspension, additional air accessories, use of an
undersizedcompressor,frequentstops, excessiveleakage
from fittings, connections, lines, chambers or valves, etc.
The discharge line allows the air, water-vapor and
oil-vapor mixture to cool between the compressor and air
dryer. The typical size of a vehicle's discharge line, (see
column 2 of Table Aon page A-3) assumes a compressor
with a normal (less than 25%) duty cycle, operating in
a temperate climate. See Bendix and/or other air dryer
manufacturer guidelines as needed.
When the temperature of the compressed air that enters
the air dryer is within the normal range, the air dryer can
removemost ofthe chargingsystem oil. Ifthe temperature
of the compressed air is above the normal range, oil as
oil-vapor is able to pass through the air dryer and into the
air system. Larger diameter discharge lines and/or longer
discharge line lengths can help reduce the temperature.
The discharge line must maintain a constant slope down
fromthe compressorto theair dryerinlet fitting toavoid low
points where ice may form and block the flow. If, instead,
ice blockages occur at the air dryer inlet, insulation may
be added here, or if the inlet fitting is a typical 90 degree
fitting, it may be changed to a straight or 45 degree fitting.
For more information on how to help prevent discharge
line freeze-ups, see Bendix Bulletins TCH-08-21 and
TCH-08-22 (see pages A-19-21). Shorter discharge line
lengths or insulation may be required in cold climates.
Theair dryer contains a filterthat collectsoil droplets, and
a desiccant bed that removes almost all of the remaining
water vapor. The compressed air is then passed to the air
brake service (supply) reservoir. The oil droplets and the
watercollectedareautomatically purgedwhenthegovernor
reaches its “cut-out” setting.
For vehicles with accessories that are sensitive to small
amounts of oil, we recommended installation of a Bendix®
PuraGuard®systemfilter,designedto minimize theamount
of oil present.
Air Dryer
Reservoir Drain
Service Reservoir
(Supply Reservoir)
Compressor
Governor
(Governor plus Synchro valve
for the Bendix®DuraFlo™596
Compressor)
Discharge
Line Optional “Ping” Tank
Optional Bendix®PuraGuard®
System Filter or PuraGuardQC™
Oil Coalescing Filter
The Air Brake Charging System supplies the
compressedair forthebraking systemas wellas otherair
accessories for the vehicle. The system usually consists
of an air compressor, governor, discharge line, air dryer,
and service reservoir.