Bridgestone 50 SPORT User manual

Classic Cycles Technical Resources
BRIDGESTONE
OWNER’S HANDBOOK
& SERVICE MANUAL
50 Sport, 60 Sport, 90, 90 Trail,
90 Mountain, 90 Sport, 175 Dual Twin

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TECHNICAL HANDBOOK
Index
SPECIFICATIONS ...........................................1
BS 50 Sport ................................................1
BS 60 Sport ................................................1
90 cc ......................................................3
175 Dual Twin .............................................5
I. ENGINE DISASSEMBLY & REASSEMBLY ..................7
50 Sport, 60 Sport and 90cc models ...........................7
II. REASSEMBLY ........................................12
175 DT ...................................................15
II. ENGINE DISASSEMBLY ...............................16
III. ENGINE REASSEMBLY ..............................19
IV. INSTALLING ENGINE INTO FRAME ..................24
CYLINDERS AND PISTONS .................................26
A. Construction and Operation: .............................26
CRANKSHAFT AND ROTARY DISC VALVES ................27
A. Construction and Operation .............................27
B. Rotary Valve Modification ...............................28
50-60 SPORT, 90 & 90 SPORT ...........................28
175 DUAL TWIN .......................................29
KICK STARTER MECHANISM ..............................29
Construction and Operation ................................29
CLUTCH ...................................................31
A. Construction ...........................................31
B. Operation ..............................................32
C. Adjustment ............................................32
TRANSMISSION ............................................34
A. Construction and Operation .............................34
B. Gear Shift Mechanism ...................................35
C. Gear Shift Operation ....................................37
D. Gear Ratios ............................................41
CARBURETOR .............................................42
A. Construction and Operation .............................42
B. Functions of Various Parts ...............................44
C. Adjustment ............................................45
TO ADJUST FLOAT LEVEL: ............................46
OIL INJECTION SYSTEM – 90cc .............................47
A. Construction and Operation .............................47
B. Adjustment ............................................48
C. Special Attention to the Oil Injection System ..............49
OIL INJECTION SYSTEM – 175DT ..........................50
A. Construction and Operation .............................50
B. Adjustment ............................................51
C. Special Attention — Pump Modification ...................52
ELECTRICAL EQUIPMENT – 50, 60 Sport, 90cc ...........53
A. Ignition System ........................................53
Contact Breaker ........................................53
Ignition Timing .........................................53
TO SET TIMING .....................................53
Condenser .............................................54
Spark Plug .............................................54
B. Testing the Ignition Coil ................................55
C. Technical Standards ....................................55
D. Lighting System ........................................55
E. Testing the Lighting Coil ................................56
F. Selenium Rectifier ......................................56
G. Battery Charging (50-60 Spt) ............................56
ELECTRICAL EQUIPMENT – 175 DT .......................57
Contact Breakers .......................................57
Ignition timing .........................................57
TO CHECK TIMING - RIGHT CYLINDER ...............57
RIGHT CYLINDER TIMING ADJUSTMENT .............58
TO CHECK TIMING-LEFT CYLINDER ..................58
LEFT CYLINDER TIMING ADJUSTMENT ..............58
B. Ignition Coils ...........................................58
C. Technical Standards ....................................59
D. Generator ..............................................59
E. Selenium Rectifier ......................................60
F. Battery ................................................60
G. Headlight ..............................................60
H. Spark plugs ............................................60
Dry Charged Storage Batteries ...............................61
FRONT FORK .............................................63
Operation ................................................63
Maintenance ..............................................63
Assembling Front Fork ....................................63
REAR SUSPENSION .......................................64
FRONT WHEEL ...........................................65
Removing Front Wheel ....................................65
Adjusting Front Brake .....................................65
REAR WHEEL ..............................................66
Removing Rear Wheel .....................................66
Adjusting Rear Brake .....................................67
PRE-DELIVERY SERVICE .................................68
B/S 90 w/Oil Injection ......................................68
175 DT ...................................................69
TROUBLESHOOTING: ENGINE ...........................71
I. PRELIMINARY CHECK ................................71
II. SPARK PLUG .........................................71
III. ENGINE OVERHEATING .............................74
TROUBLESHOOTING: TRANSMISSION ...................75
I. MISSED GEARS ........................................75
TROUBLESHOOTING: ELECTRICAL ......................76
50, 60 Spt; 90cc ............................................76
I. KEY SWITCH ........................................76
II. CHARGING SYSTEM ................................76
III. BRAKE LIGHT SWITCH ............................76
IV. HEAD LIGHT .......................................77
V. HORN CIRCUIT .....................................77
VI. SELENIUM RECTIFIER ............................77
175 DT ...................................................77
ALL MODELS ............................................78
INSPECTION AND MAINTENANCE .......................79
SERVICE STANDARDS ....................................80
APPENDIX .................................................81
USEFUL FORMULAS AND TABLES .....................81
CONVERSIONS ........................................82
EXAMPLES OF CONVERSIONS ........................83
TEMPERATURE .......................................83
PISTON DISPLACEMENT ..............................83
COMPRESSION RATIO ................................84
HORSEPOWER AND TORQUE .........................85
PISTON SPEED ........................................85
GEAR RATIO ..........................................86
BOLT TORQUE ........................................87

Classic Cycles Technical Resources
1
SPECIFICATIONS
BS 50 Sport BS 60 Sport
'Engine:
(1) Type: 2-stroke, Single Cyl. 2-stroke, Single Cyl.
(2) Piston Displacement: 50cc.(3.15cu.in.) 58cc.(3.54cu.in.)
(3) Bore & Stroke 39 mm x 42 mm (1.53 x 1.65 in.) 42 mm x 42 mm (1.65 x 1.65)
(4) Compression Ratio: 6.60:1 6.70:1
(5) Max. Brake H.P. 5.2 HP/8500 rpm. 5.6 HP/8500 rpm.
(6) Max. Torque: 0.45 kg-m/7500 rpm. 0.48 kg-m/7500 rpm.
(7) Air Intake System: Rotary Disc Valve Rotary Disc Valve
(8) Starting System: Kick Starter Kick Starter
(9) Charging System: A.C. Magneto A.C. Magneto
(10) Ignition System: Flywheel Magneto Flywheel Magneto
(11) Ignition Timing: 19° before T.D.C. 19° before T.D.C.
(12) Spark Plug: N.G.K. B-7 N.G.K.. B-8
(13) Carburetor: AMAL Type, VW15SC. AMAL Type, VW15SC.
(14) Fuel Mixture: 20 to 1 (gasoline) 20 to 1 (gasoline)
(motor oil SAE No. 30) (motor oil SAE No. 30)
(15) Transmission Oil: 0.132 gal. in trans. 0.132 gal. in trans.
case; SAE #20 (winter) case; SAE #20 (winter)
or SAE 10W30 (all season) or SAE 10W30 (all season)
* Performance:
(1) Max. Speed: 85 km/h (53 mph) 88 km/h (55 mph)
(2) Climbing Ability: 1 in 3.5 1 in 3.5
(3) Fuel Consumption: 200mpg/20mph at 30km/h 176mpg/20mph at 30km/h
paved level test road paved level test road
(4) Min. Turning Radius: 1.8m (70.8 inch) 1.8m (70.8 inch)
(5) Acceleration: 14.5 sec. (Stan. st. 0-200 m) 14.0 sec. (Stan. st. 0-200 m)
(6) Braking Distance: Less than 8 m at 35 km/h Less than 8 m at 35 km/h
(26 feet, at 22 mph) (26 feet, at 22 mph)
'Frame &Suspension:
(1) Frame Type: Pressed Steel/Backbone Pressed Steel/Backbone
(2) Front Suspension: Telescopic Fork with Telescopic Fork with
Hydraulic Damper Hydraulic Damper,

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2
'Transmission
(1) Clutch: Manual, Multi. disc in oil Manual, Multi. disc in oil
(2) Transmission: 4 speed constant mesh 4 speed constant mesh
gear & foot control gear & foot control
(3) Gear Ratio: Primary (Hel. gr.) 1 : 4.05 1 : 4.05
Gear Box: 1st: 1 : 2.92 1 : 2.92
2nd: 1 : 1.83 1 : 1.83
3rd: 1:1.32 1 : 1.32
4th: 1 : 1.00 1 : 1.00
Secondary (Chain): 1 : 2.77 1 : 2.54
Total Gear Ratio:
1st: 1 : 32.8 1 : 30.0
2nd: 1 : 20.5 1 : 18.8
3rd: 1 : 14.8 1 : 13.6
4th: 1 : 11.2 1 : 10.3
•Dimensions, Weight:
(1) Overall Length: 1,810 mm (71.3 in.) 1,810 mm (71.3 in.)
(2) Overall Width: 660 mm (26.0 in.) 660 mm (26.0 in.)
(3) Overall Height: 970 mm (38.1 in.) 970 mm (38.1 in.)
(4) Saddle Height: 750 mm (29.5 in.) 750 mm (29.5 in.)
(5) Wheelbase: 1,160 mm (45.7 in.) 1,160 mm (45.7 in.)
(6) Road Clearance: 130 mm (5.10 in.) 130 mm (5.10 in.)
(7) Tire Size (Front): 2.25-17, 4 ply 2.25-17, 4 ply
(Rear): 2.25-17, 4 ply 2.25-17, 4 ply
(8) Tire Pressure (Front): 22.8 lbs/in222.8 lbs/in2
(Rear): 28.4 lbs/in228.4 lbs/in2
(9) Caster: 63 63
(10) Trail: 78 mm (3.07 inch) 78 mm (3.07 inch)
(11) Banking Angle: 45 degrees 45 degrees
(12) Net Weight: 69 kg (152 lbs) 69 kg (152 lbs)
(13) Fuel Tank Capacity: 1.72 gal. 1.72 gal.
including 0.264 gal. reserve including 0.264 gal. reserve
(14) Front Brake: Right Hand Right Hand
(15) Rear Brake: Right Foot Right Foot
'Electrical Equipment:
(1) Head light: 6V-15/15W 6V-15/15W
(2) Tail light: 6V-2W 6V-2W
6V-5W (exclusively C.H.P.) 6V-5W
6V-18W (exclusively C.H.P.) 6V-18W
(3) Stop light: 6V-8W 6V-8W
(4) Battery: 6V-2AH 6V-2AH

Classic Cycles Technical Resources
3
SPECIFICATIONS
90 cc
*Engine:
(1) Type: 2-stroke. Single Cylinder
(2) Piston Displacement: 88 c.c. (5.39 cu-inch)
(3) Bore & Stroke: 50 mm x 45 mm (1.97 x 1.77 inch)
(4) Compression Ratio: 6.8 : 1
(5) Max. Brake H.P. 7.8 HP/7000 rpm. (8.8 HP/8000 - 90 Spt)
(6) Max. Torque: 0.85 kg-m/5000 rpm
(7) Air Intake System: Rotary disc valve
(8) Starting System: Kick Starter
(9) Charging System: A.C. Magneto
(10) Ignition System: Flywheel Magneto
(11) Ignition Timing: 21 degrees before T.D.C.
(12) Spark Plug: N.G.K. B-7H
(13) Carburetor: AMAL Type, VM 15SC (VM 17SC-90 Spt)
(14) Fuel Mixture: Gasoline
(15) Transmission Oil: 0.6 litre (0.158 US gal.) in transmission
SAE #30 (summer); SAE #20 (winter)
SAE #10W30 in all seasons
*Performance:
(1) Max. Speed: 95km/h(60mph) (105 kh/h—65 mph—90 Spt)
(2) Climbing Gradient: 1 in 3
(3) Fuel Consumption: 75 km/L (177 mpg/20 mph) at 30 km/h on paved
flat test road
(4) Min. Turning Radius: 1.8 m (70.8 inch)
(5) Acceleration: 13.0 seconds (0-200 m) (12.0Sec—90 Spt)
(6) Braking Distance: 6 m at 35 km/h (20 feet, at 22 mph)
* Frame Suspension:
(1) Frame Type: Pressed Steel, Backbone Type
(2) Front Suspension: Telescopic Fork with Hydraulic Damper
(3) Rear Suspension: Swinging Arm with Hydraulic Damper

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4
•Transmission
(1) Clutch: Manual, Multiple discs in oil bath
(2) Transmission: 4 speed constant-mesh gear & foot control
(3) Gear Ratio: Primary (Helical Gear): 1 : 3.95
1st: 1: 2.77
2nd: 1: 1.72
3rd: 1: 1.23
4th: 1: 0.924
Secondary (Chain): 1 : 2.43
Total Gear Ratio: 1st: 1 : 26.58
2nd: 1: 16.51
3rd: 1: 11.81
4th: 1: 8.86
'Dimensions & Weight:
(1) Overall Length: 1,830 mm (72.0 inch)
(2) Overall Width: with Standard Handle Bar 660 mm (26.0 inch)
with Flat Bar 580 mm (22.8 inch)
(3) Overall Height: 970 mm (38.1 inch)
(4) Saddle Height: 750 mm (29.5 inch) (30.1 inch—90 Spt)
(5)Wheel base: 1,160 mm (45.7 inch)
(6) Road Clearance: 150 mm (5.9 inch)
(7) Tire Size (Front): 2.50-17, 4 ply
(Rear): 2.50-17, 4 ply
(8) Tire Pressure:
(Front): 1.6kg/cm(22.8 lbs/in) (Same—90 Spt)
(Rear): 2.4 kg/cm (34.2 lbs/in) (28.4 Ibs/in—90 Spt)
(9) Caster: 63 degrees
(10) Trail: 80 mm (3.15 inch)
(11) Banking Angle: 45 degrees (47 degrees – 90 Spt)
(12) Net Weight: 79kg(1741bs)
(13) Fuel Tank Capacity: 7.0 L (1.85 US gal) (2.25 US gal—90 Spt)
including 0.264 gal. reserve (0.396 gal—90 Spt)
(14) Front Brake: Right Hand
(15) Rear Brake: Right Foot
*Electrical Equipment:
(1) Head light: 6V-15/15W
(2) Tail light: 6V-2W
(3) Stop light: 6V-8W
(4) Turn signal light: 6V-8W
(5) Battery: 6V-4AH

Classic Cycles Technical Resources
5
SPECIFICATIONS 175 Dual Twin
*Engine
(1) Type : 2-stroke, Dual Cylinders
(2) Piston Displacement: 177 cc. (10.8 cu-inch)
(3) Bore & Stroke: 50 mm × 45 mm (1.97 × 1.77 inch)
(4) Compression Ratio: 9.5: 1
(5) Max. Brake Horsepower: 20HP/8,000 rpm
(6) Max. Torque : 1.9 kg-m/7, 500 rpm
(7) Air Intake System : Rotary disc valve
(8) Starting System : Kick Starter
(9) Charging System : A. C. Generator
(10) Ignition System : Battery
(11) Ignition Timing: (21 +1,-2) degrees before T.D.C.
(12) Spark Plugs : N G K B-8 H
(13) Carburetors : AMAL Type. VM 17 SC
(14) Engine Lubrication : 2 cycle engine motor oil
(15) Fuel: Gasoline
(16) Transmission oil : 0.8 litre (0.21 US gal.) in transmission case
SAE No. 10W/30 in all seasons or
SAE No. 30 in summer and SAE No. 20 in winter
*Performance
(1) Max. Speed: Over 130km/h(80 mph)
(2) Climbing Gradient: 1 in 3
(3) Fuel Consumption: 55 km/l (129 mpg) at 40 km/h (25 mph) on paved flat test road.
(4) Min. Turning Radius : 1.95 m (76. 8 inch)
(5) Acceleration:
(Standing Start 1/4 mile) Under 18 sec (0-400 m)
(6) Braking Distance : Less than 6 m at 35 km/h (20 feet, at 22 mph).
*Frame &Suspension
(1) Frame Type : Pipe Frame, Cradle Type
(2) Front Suspension : Telescopic Fork with Hydraulic Damper
(3) Rear Suspension : Swinging Arm with Hydraulic Damper

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*Transmission
( 1 ) Clutch : Manual, Multiple discs in oil bath.
( 2 ) Transmission : Dual Transmission, selective 4-speed constant mesh-rotary or
5-speed constant mesh-return by shifting "sportshift" lever
( 3 ) Gear Ratios: Primary (Helical gear) : 1:3.41
Gear Box: 1st 1:2.61
2nd 1:1.67
3rd 1:1.24
4th 1:1.00
5th 1: 0.85
Secondary (Chain): 1: 2.37
Total Gear Ratio : 1st 1:21.19
2nd 1:13.50
3rd 1:10.03
4th 1: 8.10
5th 1:6.86
*Dimensions and Weight
(1 ) Overall Length: 1,885 mm (74.2 inch)
( 2 ) Overall Width : 750 mm (29.5 inch)
with Standard Western type Handlebar
( 3 ) Overall Height: 1,020 mm (40.2 inch)
( 4 ) Saddle Height : 780 mm (30.7 inch)
( 5 ) Wheelbase: 1,235mm (48. 6 inch)
( 6 ) Road Clearance : 150 mm ( 5.9 inch)
( 7 ) Tire Size (Front): 2.50-18 4 ply
(Rear) : 2.75-18 4 ply
(8) Tire Pressure (Front): 1.6kg/cm2(221bs/in2)
(Rear) : 2.0 kg/cm2(23 lbs/in2)
( 9 ) Caster : 64°
(10) Trail : 83.5 mm (3.29 inch)
(11) Banking Angle : 45°
(12) Net Weight : 123 kg (271 lbs)
(13) Fuel Tank Capacity: 10 L (2.64 US gal.)
Including 1.2 litre (0.317 US gal.) reserve
(14) Oil Tank Capacity : 1.8 L (3.8 pint)
(15) Front Brake : Right Hand Operated
(16) Rear Brake : Right Foot Operated
*Electrical Equipment
( 1 ) Head light : 12 V - 35/25 W
( 2 ) Tail light: 12V-8W
( 3 ) Stop light: 12V-25W
( 4 ) Speedometer lamp: 12V-3W
( 5 ) Third gear indicator lamp: 12V-2W
( 6 ) Neutral indicator lamp: 12V-2W
( 7 ) Battery: 12V-6AH

Classic Cycles Technical Resources
7
I. ENGINE DISASSEMBLY & REASSEMBLY
50 Sport, 60 Sport and 90cc models
Most service can be performed on the engine
without removing the engine from the frame. In
fact, all operations which occur before the actual
splitting of the crankcase sections can be
accomplished with the engine mounted in the
cycle frame. For complete engine disassembly, it
is necessary to remove the engine from the frame.
This sequence describes the dismounting of the
engine from the frame, the complete disassembly
of the engine, and engine reassembly.
1. Remove the drain plug from the under side of
the transmission case and completely drain
the crankcase. Replace the drain plug.
2. Turn the fuel cock selector lever to the "O"
position to shut off the fuel between the tank
and the carburetor.
3. Raise the rubber carburetor cap.
4. Remove the carburetor cover from the side of
the transmission case by removing the phillips
head attaching screws.
5. Remove the rubber plug from the access port in
front of the carburetor enclosure.
6. Insert a screw driver through the access
aperture and loosen the carburetor mounting
bolt. (Fig. 2)
7. Disconnect the fuel line from the fuel cock.
8. Remove the carburetor from the carburetor
adapter and allow it to hang suspended from
the throttle cable and starter jet cable.
9. Remove the clutch release arm lock nut and
washer and remove the clutch release arm
from the release screw. (Fig. 3)
10. Disconnect the clutch cable from the clutch
release arm.
11. Unscrew the clutch cable adjusting screw
completely out of the threaded boss on top of
the carburetor enclosure.
12. Pull the clutch cable free of the carburetor
enclosure.
13. Using the special exhaust pipe clamp nut
wrench, (Fig. 4) loosen the exhaust pipe clamp
nut and rotate the exhaust pipe toward you
down and out of the way.
14. Remove the two bolts which connect the
engine hanger to the front of the engine.

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8
Loosen the bolt at the top of the engine hanger
and swing the engine hanger forward.
15. Moving to the left hand side of the machine,
remove the gear shift lever.
16. Remove the magneto cover retaining screws.
17. Remove the two hex head bolts from the lower
half of the chain case and remove the lower
chain case section.
18. Remove the left hand battery cover.
Disconnect the wires running from the
magneto to the battery. Remove the battery
band and remove the battery. Locate the chain
connecting link and chain connector.
Dismount the drive chain from the small
sprocket. Temporarily reconnect the chain to
avoid losing the chain connector. Pull the
magneto wire loom out of the cycle frame.
19. Remove the nuts from the engine mounting
bolts. The nuts are located on the right hand
side of the machine. Withdraw the engine
mounting bolts from the left hand side of the
machine.
20. Lower the engine out of the frame.
21. Place the engine on the bench and remove the
kick starter lever.
22. Remove the cylinder head mounting bolts
using a 10mm socket wrench.
23. Using a 10mm open end wrench, remove the
four cylinder mounting nuts. Lift the cylinder
off the cylinder mounting studs.
24. As soon as the cylinder is removed, insert the
piston seat (special tool) under the piston. This
special tool protects the piston during
disassembly and it also holds the crankshaft
stationary for further disassembly operation.
(Fig. 5)
25. Remove the piston pin snap rings and remove
the piston pin and piston from the connecting
rod. A suitable piston pin puller must be used
in this operation. This is provided in the
Bridgestone special tool kit.
26. Position the engine on the bench, magneto
side up. Hold the magneto stationary with the
stopper wrench (special tool) and, using a
socket wrench, remove the magneto retaining
nut and lock washer. (Fig. 6)
27. Mount the magneto puller (special tool) and
holding the magneto stationary with the
stopper wrench, remove the fly wheel
magneto. CAUTION: When mounting the
magneto puller on the 90cc fly wheel magneto,
insert the three attaching screws just far
enough to engage two or three threads. If
these screws are turned in too far, it is possible
to damage the magneto coils.
28. Remove the wire from the neutral gear
switch. (Fig. 7)

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29. Carefully mark the position of the magneto
armature plate relative to the crankcase with
a prick punch or with a felt marking pen.
These reference marks will make it possible to
reinstall the magneto armature plate in
exactly the same position to avoid the
necessity of major timing adjustments when
the engine is reassembled. (Fig. 8)
30. Remove the three armature plate attaching
screws and remove the magneto armature
plate.
31. Remove the three neutral gear switch cover
mountings screws and remove the switch
cover. Remove the brass contact located under
the cover. This contact is fitted to the end of
the shift drum shaft.
32. Turn the engine over so that the carburetor
side faces up and remove the transmission
case cover screws.
33. With the screws removed, carefully remove
the transmission case cover. CAUTION: Some
oil will remain in the transmission case so use
caution in removing the transmission case
cover to avoid spilling this remaining oil.
34. Remove the 6 clutch thrust plate bolts from
the top of the clutch assembly. Remove the
thrust plate. (Fig. 9)
35. Remove the 6 clutch springs.
36. Flatten the up turned sides of the bend
washer and remove the clutch retaining nut
on the 90cc engine after shifting the
transmission into first gear. Use the shift lever
to shift the gears rather than a pliers to avoid
damaging the splined end of the gear shift
shaft. The sprocket stopper (special tool) must
be slipped over the gear shift shaft on the
opposite side of the engine and the steel pegs
on this special tool are positioned between the
teeth of the small sprocket. This holds the
sprocket stationary, enabling you to remove
the clutch retaining nut. (See figure 10). On
the 50cc and 60cc rotary valve engines, remove
the clutch retaining snap ring to remove the
clutch assembly.
37. Remove the clutch assembly as a unit. NOTE:
On more recent models of the 90cc series, a
thrust spring fits over the drive gear of the
clutch. In these models, remove the clutch
bracket, the splined washer, lift out the gear
drive assembly, and then remove the thrust
spring.
38. With the clutch wrench (special wrench),
remove the pinion gear nut from the end of the
crankshaft. (On the newer models this
retaining nut can be removed with a socket
wrench.) CAUTION: This is a left hand
thread. The crankshaft can be kept from
rotating during the removal of the pinion gear
retaining nut if the piston seat is put in place.
39. Remove the pinion gear from the end of the
crankshaft.
40. Remove the 6 rotary valve cover screws and
remove the rotary valve cover.

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41. To remove the rotary valve disc, insert your
finger up through the bottom of the fuel inlet
port and gently dislodge the rotary valve disc
from the crankshaft collar. If a screw driver is
used to assist in removing the disc, use
extreme caution to avoid damaging any part of
the rotary valve cavity. (Fig. 11)
42. Remove the crankshaft collar and locating
pin. The locating pin may be removed with a
needle-nosed pliers.
43. Remove the washer and spring guide from the
kick starter shaft.
44. Unhook the upper spring hook on the kick
starter return spring by striking the upper
end of the spring on the side opposite the
spring hook. This will pop the spring hook
from the hole in the kick starter shaft.
45. Unhook the lower end of the kick starter
return spring from the crankcase and lift off
the spring.
46. Using the circlip expander (special tool)
remove the circlip from the kick starter shaft.
47. Using an open end wrench, remove the shift
drum stopper pivot screw (Fig. 12). Unhook
the shift drum stopper spring and remove the
shift drum stopper. Inspect it for bends of
abnormal wear. The disc on the end of the
shift drum stopper should rotate freely.
48. Lift the shift arm pawl free of the shift
drum and holding it in this position, carefully
withdraw the shift arm and shaft assembly.
Inspect the shift arm assembly for bends,
damage or abnormal wear. Remove the thrust
receiver.
49. Position the engine on the bench with the
magneto side facing upwards and remove the
crankcase retaining screws. It may be
necessary to use an impact driver to remove
these screws as they are inserted very tightly.
The impact driver will enable you to remove
the screws without damaging the screw heads.
See tool list, figure 1. (The drive sprocket
should be removed as soon as the engine is
turned magneto side up.)
50. With all crankcase retaining screws removed,
carefully tap the crankcase with a soft faced
hammer to break the gasket seal and separate
the crankcase halves.
51. Carefully lift out the two transmission shafts
and gears together with the shift drum-shift
fork assembly.
52. Remove the shift drum/shift fork assembly
from the transmission gears and inspect the
shift forks for bends or abnormal wear. (Fig.
13)

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53. Inspect the two transmission shafts and gears
to be certain that they are all in good
condition. Further disassembly of the
transmission is not necessary unless one or
more of the gears must be replaced. If this is
necessary, carefully note the position of the
gears on the shaft before removing them for
replacement. (Fig. 14)
54. Remove the kick starter intermediate gear
which remains in the crankcase half after the
transmission shaft assembly has been
withdrawn. Note that there is a spacer above
and below this gear. Keep these spacers with
this gear to facilitate reinstallation. (Fig. 15)
55. Withdraw the kick starter shaft and gear
assembly. Notice that there is a spacer on the
end of this shaft. Keep the spacer with this
shaft to facilitate reassembly.
56. Withdraw the crankshaft assembly. Notice
that spacing shims are used on either side of
the crankshaft balance assembly. (Fig. 16) If
the same crankshaft is to be replaced, keep
these spacers on the crankshaft so that the
crankshaft end play will not be affected. The
number and thickness of spacers on this
crankshaft varies from engine to engine.
Occasionally the spacing shims will stick to
the crankshaft bearings. Inspect the bearings
carefully to be certain that these shims have
been removed from both the crankcase
sections if the oil present has caused them to
adhere to the bearings.
This completes the disassembly operation. Wash
all parts (except magneto armature plate) in a
cleaning solvent and inspect all bearings and oil
seals. If it is necessary to replace bearings, the
crankcase must be heated with a small torch in
the vicinity of the bearings to expand it
sufficiently to allow the bearing to be removed
and replaced. When using a torch, keep the torch
constantly in motion to avoid melting the
crankcase castings.
If oil seals are to be replaced, coat them liberally
with oil and tap them into place with a small
hammer. Before beginning the reassembly
operation, oil all bearings and oil seals
thoroughly.

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II. REASSEMBLY
All parts installed inside the two crankcase
halves are installed into the magneto clutch
section.
1. Install the kick starter shaft and gear
assembly. Position the pin on the ratchet arm
into the boss provided for it in the crankcase
half.
2. Oil the kick idler gear assembly thoroughly.
This gear need not be removed from the
transmission case during disassembly as it is
pressed into the counter shaft (clutch shaft)
bearing. It is very unlikely that this gear will
ever be replaced during the life of the engine.
3. Reinstall the kick starter intermediate gear
positioning the spacers above and below the
gear. These spacers are identical and can be
identified since the hole in the spacer is the
same size as the hole in the gear. Place it in
mesh with the kick idler gear and the kick
starter shaft gear and oil the gears. (Fig. 15)
4. We are now ready to install the transmission
shaft assemblies and the shift drum/shift fork
assembly. Before doing so, familiarize yourself
with the positions into which the various
shafts and the shift drum will be inserted by
studying figure 15.
•The splined end of the countershaft (the shaft
with the smaller gears) will be inserted
through the kick idler gear, the gear that is
pressed into the countershaft bearing.
•The end of the drive shaft will be inserted into
the kick starter intermediate gear, the one
which was installed with the spacers above
and below it. The splined end of the shaft will
point up and the small sprocket will be
attached to it when the crankcase sections are
joined. The large diameter end of the shift
drum will be inserted into the large hole in the
casting located directly above the kick idler
gear and the kick starter intermediate gear.
5. Align the countershaft and drive shaft gears so
that all gears are in mesh. Plan your
installation of the shafts so that the longer end
of the countershaft extends through the
bottom of the crankcase section. Now insert
the shift forks into the slots of the sliding
gears of the transmission.
6. Holding both shafts and shift drum together as
a unit, install the transmission assembly into
the crankcase half. When the shafts have been
partially inserted, rotate them slightly to mesh
all gears and properly position the shafts.
7. Install the two remaining spacers, one on the
end of the countershaft and the other on the
end of the kick starter shaft.
8. Be certain that the crankshaft end play shims
are positioned on both sides of the crankshaft
balance and install the crankshaft. See figure
16. If a new crankshaft has been installed, it
will be necessary to put the crankcase gasket
into position, install the other crankcase
section and insert several of the crankcase
retaining screws tightly. The crankshaft
should now be checked for end play. If there is
more than .010" to .015" of end play, additional
shims must be added to the crankshaft.
9. Install a new crankcase gasket locating it on
the steel positioning pins. Install the second
crankcase section. Again check transmission
shaft rotation to be certain that the shafts turn
freely. If found to be ok., reinstall the
crankcase retaining screws and tighten them
securely.
10. Trim the excess portion of the crankcase
gasket flush with the cylinder seat.
11. Install the spacer on the shift arm shaft and
position the shift arm shaft assembly into the
crankcase. (Fig. 17)
12. Insert the shift arm pawl into the shift drum
assembly. Install the thrust receiver and bend
the tabs on the tab washer against the flats of
the bolts when the mounting bolts have been
secured. This tab washer prevents the bolts
from loosening during engine operation. (Fig.
12.)
13. Install the shift drum stopper, collar and
retaining screw. Connect the drum stopper

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13
return spring before inserting the retaining
screw. The collar goes beneath the shift drum
stopper. Tighten the shift drum stopper pivot
screw securely.
14. Install the snap ring on the kick starter shaft.
15. Using a needle-nosed pliers, install the
crankshaft collar locating pin into the
crankshaft.
16. Install the crankshaft collar over the locating
pin.
17. Install the rotary disc valve after coating it
and the valve cavity liberally with oil.
18. Carefully check the large "O" ring around the
outer perimeter of the rotary valve cover and
check the center oil seal to be certain that they
are in good condition. (Fig. 18) Install the
rotary valve cover and the 6 retaining screws.
19. Install the drive sprocket, sprocket washer
and retaining nut onto the transmission drive
shaft. Tighten the nut finger tight. Put the
sprocket stopper (special tool) in position and
tighten the retaining nut securely. Bend the
tabs of the washer against the flats of the
retaining nut to prevent its loosening while
the machine is being operated.
20. Install the clutch collar on the transmission
countershaft after oiling the collar liberally
and checking it for freedom of movement. (Fig.
19) NOTE: On newer models, the clutch collar
is built right into the shaft thereby eliminating
it as a separate component.
21. Install the clutch assembly onto the
countershaft and countershaft collar. Rotate
the clutch slowly while holding the sprocket
stationary in order to engage the drive dogs.
To positively engage the drive dogs, rotate the
kick starter shaft. When the clutch is properly
installed, approximately ½ inch of the
transmission countershaft will protrude
through the clutch assembly. (Fig. 20) NOTE:
On newer models, place the thrust spring over
the clutch drive gear, install the clutch
assembly, except for the clutch bracket, onto
the countershaft, replace the splined washer,
and install the clutch bracket.
22. With the sprocket stopper in place, install the
clutch retaining washer and nut and tighten
the retaining nut securely on the 90cc engine.
Bend the tabs of the washer against the flats
of the nut to prevent rotation of this nut
during engine operation. On the 50 and 60cc
engines, install the clutch retaining spring
clip.
23. Install the kick starter return spring, spring
guide and spacer.
24. Install the pinion gear onto the crankshaft
with the undercut side of the gear facing up.
With the piston seat in place, tighten the
pinion gear retaining nut securely. Remember
that this is a left hand thread.

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25. Install the 6 clutch springs and reinstall the
clutch thrust plate using the 6 thrust plate
bolts. Tighten them securely.
26. If the clutch release push screw has been
removed from the transmission cover, reinstall
it from the inside so that the flats of the
screws (as seen from the outside of the
transmission cover) are in a relatively
horizontal position with the back of the release
push screw flush with the inside of the
transmission case cover. (Fig. 3) Be certain
that the thrust bearing has been liberally
coated with grease and inserted into the push
screw. (Fig. 21)
27. Position the transmission cover gasket on the
locating pins. Install the rubber "O" ring on
the carburetor inlet adapter, which is a part of
the rotary valve cover. (Fig. 22)
28. Install the transmission case cover by
securing all the transmission case cover
screws.
29. Turn the engine assembly over and install the
neutral light switch gasket and contact. With
the transmission in neutral gear, check to be
certain that the contact in the switch cover
touches the contact which is installed onto the
end of the shift drum shaft. Secure the cover.
30. Position the magneto armature plate making
use of the reference marks which you made
prior to removing this assembly. (Fig. 8)
Secure the plate with the 3 armature plate
screws.
31. Reattach the neutral gear switch wire to the
switch cover. Tighten the wire retaining
screw. (Fig. 7)
32. Install the flywheel onto the crankshaft and,
holding the flywheel stationary with the
stopper wrench, tighten the flywheel retaining
nut securely.
33. Install the piston pin onto the connecting rod
to check for fit. It should be snug but rotate
easily. Replace the pin and bushing if
necessary. Oil the bushing thoroughly before
installing the piston pin and piston.
34. Install the piston pin and piston. When
installing the piston, the letters "EX" are to
face the exhaust port (front) on the cylinder. It
is important to position the piston this way
since the piston pin hole is slightly off center
for improved performance. If the piston is
installed backwards, a loud knocking sound
will be heard at low speeds. (Fig. 23)
35. Install the piston pin retaining clips.
36. Check to see that the piston rings are properly
installed. The chrome plated ring is the upper
ring and the cast iron ring is the lower ring.
The rings are to be positioned on the piston so

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that the locating pin on the piston is located in
the ring gap. (Fig. 24) The notch in the ring
gap should face up. If the notch faces down,
the rings cannot be compressed enough to
permit installation of the cylinder.
37. Install the cylinder onto the mounting studs
and install the lockwashers and nuts securely.
38. Install the cylinder head gasket and cylinder
head. Four flat washers are used under the
cylinder head nuts. Tighten the head nuts
progressively and evenly to 125 inch lbs. of
torque.
The engine is now completely reassembled and
ready for remounting in the cycle frame. The
engine mounting procedure is to be done in the
reverse order of the dismounting procedure.
BRIDGESTONE 175 DT ENGINE
DISASSEMBLY AND REASSEMBLY
I. Removing Engine From Frame
1. Drain transmission oil into a suitably large
container.
2. Raise the right hand rubber carburetor cap.
3. Remove the air cleaner duct from the
carburetor cap.
4. Remove the carburetor cover from the right
hand side of the engine.
5. Remove the rubber plug from the front of the
carburetor enclosure and loosen the carburetor
retaining bolt.
6. Remove the right hand carburetor. It can be
allowed to hang suspended from the throttle
cable.
7. Loosen the clutch adjusting screw lock nut and
remove the clutch adjusting screw.
8. Remove the clutch release arm and disconnect
it from the clutch cable.
9. Unscrew the clutch cable adjusting screw all
the way out of the engine.
10. Remove the kick starter lever.
11. Remove the cover plate on which the
transmission selector lever is mounted.
12. Remove the gear shift pedal.
13. Raise the carburetor rubber cap from the top
of the left carburetor enclosure.
14. Remove the air cleaner duct from the rubber
carburetor cap.
15. Remove the left carburetor cover.
16. Remove the left carburetor.

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17. Remove the control cable from the oil injection
pump control arm by prying the cable
attaching bracket open with a screw driver.
18. Remove the throttle cable adjusting screw out
from the transmission cover.
19. Remove the main oil pump supply line from
the fitting on the oil pump and plug this oil
line with a screw to prevent the oil from
draining out of the supply tank.
20. Remove the saddle.
21. Remove the side cover from the left hand side
of the cycle.
22. Disconnect the wiring running between the
AC generator and the battery. These wires are
snap connected.
23. Disconnect the wires from the terminals on
the selenium rectifier. The wiring harness can
then be pulled free of the cycle.
24. Disconnect the master link on the drive chain
and remove the chain from both sprockets.
25. Disconnect the exhaust pipe clamps and
swing the exhaust pipes down, clear of the
engine.
26. Remove the air cleaner.
27. Remove the four engine mounting bolts from
the mounting plates at the front of frame
assembly.
28. Remove the lower rear engine mounting bolt.
29. Remove the upper rear engine mounting bolt.
This bolt should be withdrawn with a pliers
since all of the engine weight is now bearing
against this last bolt.
30. Remove the engine from the cycle frame.
II. ENGINE DISASSEMBLY
1. With the engine placed on a suitable bench,
remove the cylinder heads and cylinders.
2. Remove the piston pin retaining clips and,
using the piston pin pusher from the
Bridgestone Special Tool Kit, remove the
piston pins and pistons.
3. Remove the connecting rod needle roller
bearings.
4. Remove the oil lines from the oil pump.
CAUTION: If the union bolts are removed
from the banjo connectors use care to avoid
losing the ball check valves from the union
bolts. NOTE: The earliest BS175 models were
not equipped with ball check valves in the
union bolts.
5. Remove the two screws which mount the oil
pump to the oil pump gear box.
6. Remove the four oil pump gear box mounting
screws. It will be necessary to rotate the pump
to remove the last two screws.
7. Remove the oil pump and gear box assembly
from the rotary valve cover.
8. Remove the left hand crank case cover.
9. Remove the rotary valve cover. CAUTION: If
it is necessary to pry this cover to remove it
from the crankcase, use extreme caution. Pry
only in the area immediately adjacent to
rotary valve cover locating pin. Prying may
cause damage to the crankcase which will
result in leaks.

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10. Remove the rotary valve from the
crankshaft. Removal can be facilitated by
inserting your index finger through the fuel
intake port from inside the crankcase and
pushing the valve off the crankshaft spline. It
is not necessary to remove the crankshaft
spline.
11. Turn the engine around so that you can work
on its right hand side. Remove the
transmission cover mounting screws and the
transmission cover.
12. Remove the clutch thrust plate by removing
the six mounting bolts.
13. Remove the clutch springs.
14. Place the transmission in any gear. With a
small chisel and hammer, flatten the bend
washer beneath the clutch retaining nut.
15. Place the clutch stopper, a special tool, in
place to hold the clutch stationary while the
clutch retaining nut is removed.
16. Loosen, but do not remove, the generator gear
mounting bolt.
17. Remove the clutch as a unit, by placing the
fingers of both hands around the clutch gear
while holding the center of the clutch in
position with your thumbs. (On more recent
models of the 173 DT, a thrust spring fits over
the drive gear of the clutch. On these models,
remove the retaining nut, the clutch hub, the
splined washer, the remaining clutch
assembly, and then the thrust spring, in that
order.)
18. Insert the aluminum piston seat under the
upper end of one connecting rod to hold the
crankshaft stationary and loosen the pinion
gear retaining nut. THIS IS A LEFT HAND
THREAD. Remove the retaining nut and the
gear from the shaft.
19. Remove the right rotary valve cover.

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20. Remove the right rotary valve and collar. It is
not necessary to remove the spline from the
crankshaft.
21. Mount the flywheel puller in the threaded
holes provided in the generator gear and with
the center bolt of the special tool bearing
against the generator gear bolt, remove the
generator gear from the tapered generator
shaft.
•B/S 173 models after serial No. l6G 11761are
equipped with a timing idle gear also. This
gear is removed at this time.
22. Remove the drum stopper retaining bolt and
drum stopper. Do not lose the spacer washer
beneath the drum stopper.
23. Pull the shift pawl free of the shift drum and
remove the entire gear shift shaft assembly.
Be certain that: the spacer is removed with the
shaft. It sometimes adheres to the crankcase
due to oil surface tension.
24. Remove the kick starter spring seat and the
kick starter shaft return spring.
25. Remove the three screws from the bearing
retainer and remove the bearing retainer.
26. Remove the AC generator from the top of
engine assembly. NOTE: It is not necessary to
remove the generator in order to split the
crankcases.
27. Invert the engine and remove all of the bolts
which join the crankcase sections.
28. Separate the crankcase sections.
29. Remove the crankshaft assembly.
30. Remove the transmission clutch shaft.
31. Remove the transmission drive shaft. If it is
necessary to disassemble the gears from the
transmission shafts, carefully note their
positions before any gears are removed so that
you will be able to reassemble them in the
same order.
32. Remove the neutral gear switch cover from
the shift drum and remove the contact from
the shift drum.