BSA M20 User manual

Book No. 100/BC2C
Air Publication No. 1657B, Vol.1. Parat2
Maintenance Manual
And
Instruction Book
For
MOTOR CYCLE (SOLO)
500 c.c. s.v.
Model M20

INDEX – GENERAL
Pages
Pages
Brakes
28
Front Forks
23
Carburetter
10
Gearbox and Gearchange
25
Clutch
21
Hubs
27
Charging System
36
Ignition System
32
Electrical Wiring System
30-31
Lighting and Accessories
42
Engine Adjustments
8
Lubrication System
3
Engine – Complete dismantling
12
Lubrication Chart
4-5
Engine – Decarbonising
11
Steering Head
28
Engine – re-assembly
16
Transmission
20
Engine – removal from frame
12
Useful Data
2
INDEX – DETAILS
Pages
Pages
BRAKES
Ignition System
- continued
Adjustment – Relining
28
Slipping clutch – sparking plug -
CARBURETTER
Suppressor - immobiliser
33
Mixture – Needle position – Pilot adjustment –
Re-assembling and testing slipping clutch
35
Throttle stop.
10
Lighting and Accessories
ENGINE ADJUSTMENTS
Headlamp – Tail lamp – Cables - Lighting
Oil-pressure valves – Exhaust valve lifter
8
switch
42
Tappets – Ignition timing
8-9
Horn
43
ENGINE DISMANTLING
Wiring diagrams
30-31
Cylinder head – Cylinder barrel – Valves
6-7
Valve grinding – Valve guides, removal
FRONT FORKS
and replacement
11
Adjustment
28
Piston and Rings – Checking ring gap –
Dismantling – fitting new spring – re-assembly
29
removing engine from frame
12
Removing Magdyno pinion
13
GEARBOX
Oil pump, removal and dismantling –
Removal – dismantling
23
“Splitting” crankcase – removing
Dismantling Gearchange – Re-assembling
Bearings
14
Gearbox
24
Removing cam spindles – dismantling
Re-assembling gearchange
25
flywheels – reboring cylinder - fitting
Replacing gearbox
26
new cylinder lining.
15
ENGINE RE-ASSEMBLY
HUBS
Flywheel assembly and alignment
16
Rear – adjustment – dismantling and
Replacing oil pump – tappets – replacing
Re-assembly
27
bearings – re-assembling crankcase -
Front
27-28
Replacing timing gears & magdyno
17
Replacing piston – cylinder barrel - timing
LUBRICATION
Cover – cylinder head – exhaust valve
Engine lubricating system
3
lifter – refitting engine in frame.
18
Lubrication chart
4
ELECTRICAL EQUIPMENT
Hubs – Brake cam spindles - Speedometer
Charging System
Drive – Filters – Rear chain – Dynamo -
Dynamo
Gearbox
5
Testing – Removal and Replacement
36
Oil pump – removal – dismantling and
Dismantling – Brushes - Commutator
37
Re-assembly
14
Field coil, testing and removing -
Armature – Bearings
38
TRANSMISSION
Re-assembly
39
Clutch – adjustment
20
Cut-out and Regulator
39
Clutch – dismantling
21
Cleaning contacts – setting regulator -
Clutch – re-assembly
22
Cut-out
40
Chaincase - removal
12
Ammeter –Removal and Replacement
41
Chaincase - replacement
18
Battery – Care of
41
Chain adjustment
20
Ignition System
Wheel alignment
21
Magneto lubrication – adjustment – testing -
Cleaning contact breaker – H.T. cable -
SHOCK ABSORBER – Adjustment
28
Pick-up
32
Removal – dismantling – removing and
STEERING HEAD
testing armature
34
Adjustment
28
Re-assembly
35
Dismantling – re-assembly
29

1

USEFUL DATA
Engine bore
82 mm
Engine stroke M20
94 mm
Engine stroke M21
112 mm
Engine Capacity
496cc
Petrol tank capacity
3 gallons
Oil tank capacity
5 pints
Gearbox capacity
1 pint
Inlet tappet clearance (cold)
.010”
Exhaust tappet (cold)
.012”
Compression M20
4,9:1
Compression M21
5:1
Tyres
3.25/3.50-19
Tyre Pressure
22psi
Piston ring gap
.008 - .012”
Piston clearance Bottom of skirt
.0035 - .0055”
Piston Ring side clearance
.002" - .004"
Ignition timing
7/16” BTDC
Magneto point gap
.012”
Spark plug gap
.012 - .018”
Carburetter- Jet
170
Carburetter- Needle
2nd notch
Engine Sprocket
19 teeth
Clutch sprocket
43 teeth
Gearbox Sprocket
18 teeth
Rear Wheel Sprocket
42 teeth
Primary Chain
95 links
Gear Ratio- Top
5.3
Gear Ratio- 3rd
7.0
Gear Ratio- 2nd
10.9
Gear Ratio- 1st
15.8

2

THE LUBRICATION SYSTEM
Fig. 1. Lubrication System
The engine lubrication system is of the dry sump type
operated by a double gear pump, situated in the
bottom of the crankcase on the right-hand side.
All oilways are internal except for the supply and
return pipes from the tank. The oil flows from the
tank to the supply pump (the top pair of gears) and
thence past the pressure valve (A) to the two oilways
feeding the cam spindles, and along the hollow
mainshaft to the big end bearing. After lubricating
the big end and circulating through the engine in the
form of a mist, the oil drains down through a filter in
the bottom of the crankcase, from which it is drawn
by the return pump (lower pair of gears) past ball
valve (C), and delivered up the return pipe to the
tank, where it passes through a fine mesh filter into
the tank itself.
Incorrect seating of the ball valve (A), will allow oil to
transfer from the tank to the engine, whilst the
machine is stationary. If the ball valve (C) should get
stuck in its seating, there will be no return of oil to the
tank. To check the oil circulation open the tank filler
cap and remove filter cap whilst the engine is
running. Oil should be seen issuing from the return
pipe from the crankcase. The tank and crankcase
should be drained every 2,000 miles and replenished
with clean oil.
Any restriction in the pressure release pipe in the
tank will cause an increase in pressure inside the oil
tank, and will result in leakage of oil at the filler cap.
This can be put right by inserting a length of flexible
wire into the pipe at its lower end (just in front of the
rear mudguard) and pushing the wire right up the
pipe, thus clearing any obstruction.
To remove the oil tank filter for cleaning, release the
tank filler cap, release the filter tap thus exposed, and
lift filter out. The filter should be placed in a can large
enough to cover it with petrol, and thoroughly
washed. Before replacing make sure that it is quite
dry of petrol.
The pump filter can be withdrawn after removing the
cover plate (B) and should be thoroughly washed
with petrol, dried and replaced.
NOTE. It is not advisable to remove the oil pump
unless the pump is definitely faulty.
3

LUBRICATION
Fig 2. – Lubrication Chart

4LUBRICATION CHART
No
PART
Lubricant
Type of
Lubrication
Daily
250 Miles
(Inclusive)
General
FRAME GROUP
3
Front fork (top)
CG-1 (AL)
1 nipple
Grease Gun
2
Front fork (centre)
CG-1 (AL)
1 nipple
Grease Gun
4
Steering Stem (top)
CG-1 (AL)
1 nipple
Grease Gun
5
Steering Stem (bottom)
CG-1 (AL)
2 nipple
Grease Gun
7
Steering Head (top)
CG-1 (AL)
1 nipple
Grease Gun
6 Steering Head (bottom) CG-1 (AL) 1 nipple Grease Gun
8
Saddle nose pivot
CG-1 (AL)
1 nipple
Grease Gun
11
Front wheel hub
CG-1 (AL)
Re pack
-
Re pack w shops every 5000 miles
12 Rear wheel hub CG-1 (AL) Re pack - Re pack w shops every 5000 miles
BRAKE GROUP
9 Brake pedal CG-1 (AL) 1 nipple Grease Gun
16
Brake cam (front)
CG-1 (AL)
1 nipple
Grease Gun
15
Brake cam (rear)
CG-1 (AL)
1 nipple
Grease Gun
Bowden control wire
OE-30
Oil Can
Few Drops
Foot brake linkage
OE-30
Oil Can
Few Drops
ENGINE GROUP
1
Engine oil tank
OE50
5 pints
Replenish
Replenish
Drain & refill at 1000 miles (AO17)
19
Primary chain case
OE50
Reservoir
Replenish
Replenish
Drain & refill at 1000 miles (AO17)
1/20 Oil Filters OE50 Wash in petrol every 2000 miles
IGNITION GROUP
Advance Retard Cable etc
OE30
Oil Can
Few drops
Contact breaker tappet
OE30
Oil Can
One drop
22
Contact breaker cam
CG-1 (AL)
Hand smear
Slight smear
Generator (drive end)
WB-2
Re pack
Re pack w shops
21 Generator (commutator end) OE30 Oil Can Few drops
FUEL GROUP
Air cleaner
N/A
Carburetter control cables
OE30
Oil Can
Few drops
Throttle handle bar grip
OE30
Oil Can
Few drops
TRANSMISSION GROUP
10
Speedometer drive
CG-1 (AL)
1 nipple
Grease Gun
Speedometer cable
OE30
Oil can
Few drops
14 Gear box GO-90 1 pint Replenish Drain & refill each 6000 miles No 18
12
Clutch push rod
OE-50
Oil can
Few drops
Clutch bowden control wire
OE30
Oil can
Few drops
Clutch handle bar grip
OE30
Oil can
Few drops
Rear chain
OE30
Oil can
Few drops
Wash in paraffin and soak in oil OE-
50 every 2000 miles. Workshops
5

THE ENGINE – EXPLODED VIEW
Fig. 3. Top half of engine (exploded view)
6

THE ENGINE – EXPLODED VIEW
Fig.4. Crankcase half of engine (exploded view)
7

ENGINE ADJUSTMENTS
Which can be carried out without dismantling
OIL PRESSURE VALVES
As described under the heading “How the Lubrication
System Works” on page 3 there are two ball valves
incorporated in the lubrication system to prevent the
transfer of oil from the tank to the crankcase.
The spring loaded valve is located in the delivery
passage between the pump and the big-end, and lies
behind the hexagon plug at the lowest point of the
timing cover (see Fig. 5).
Fig. 5. Pressure valve in timing cover.
Should any foreign matter lodge between the ball and
its seating oil will gradually transfer from the tank
when the machine is left standing, and when the
engine is started up there will be a heavy discharge
of blue smoke from the exhaust.
To rectify, remove the plug, spring and ball. The
simplest way of removing the ball is to hold the hand
close to the orifice and gently turn the engine over,
when the ball will be forced out.
Clean the ball and the seating, and if on replacing
there is still doubt as to whether the ball is seating
properly, insert a small punch against the ball and
deal it a sharp tap with a light hammer. Finally
replace the spring and plug.
The other ball valve is located between the return
pump (Fig. 6), and apparent failure of the return
pump may be due to this ball having stuck in its
seating.
To rectify, remove the pump cover plate, insert a
piece of wire into the valve orifice and lift the ball off
its seating. Should the trouble keep recurring it may
be necessary to fit a new base plate to the pump.
On no account remove the oil pump unless it is
absolutely necessary.
EXHAUST VALVE LIFTER
The peg on the exhaust valve lifter inside the tappet
chest must always be well clear of the collar on the
exhaust tappet (see Fig. 7), otherwise the engine will
be noisy and the tappet clearances seriously
affected. Failure to check that there is clearance at
this point may result in a badly burnt exhaust valve.
Adjustment is carried out by means of the cable
adjuster at the side of the tappet chest.
Fig. 6. Ball valve below return pump.
TAPPET ADJUSTMENT
Before any attempt is made to adjust tappet
clearances, check that the exhaust valve lifter is
correctly adjusted as explained in the previous
paragraph.
To check and adjust tappet clearances, it is most
essential, owing to the special design of the cam
form (see Fig. 8), that the following procedure be
adhered to.
Fig. 7. Tappet and exhaust valve lifter.
Rotate engine forward until the inlet valve has just
closed (until tappet is just free to rotate).
Now adjust the exhaust tappet clearance to .012”.
Turn engine forward again until the exhaust tappet
clearance is just taken up (but before valve actually
starts to lift).
Now adjust inlet tappet clearance to .010”
8

Fig. 8. Instructions for setting tappets
The actual adjustment is carried out by releasing the
locknut (B) (Fig. 8), holding the tappet with a spanner
on the flat (C), and screwing the tappet head (A)
either up or down. When correct clearance is
obtained, the locknut must be tightened against the
tappet head. It is advisable, after locking up, to
check clearance again, to make sure that the
adjustment has not been affected.
Tappet adjustment should always be carried out with
the engine dead cold, and the clearances
recommended above regarded as a minimum,
especially in the case of the exhaust valve.
IGNITION TIMING
It is a rare occurrence for the magneto pinion to
slacken off and disturb the ignition setting, and it is
not advisable to interfere with the setting unless it is
known to be at fault.
It is however advisable to check over the timing after
carrying out any adjustment to the magneto contact
points, as a slight variation of the points tends to
advance or retard the timing. (Opening the points
advances timing, closing them retards timing).
If the timing requires re-setting, first remove the
timing cover, and in so doing take care not to
damage the small nozzle in the timing cover which
feeds oil to the hollow crankshaft.
With the cover removed, take off the nut locking the
magneto pinion on its shaft, and with the aid of a
magneto pinion extractor (Fig. 10) release the pinion
on its taper. (Note that the pinion is held on its shaft
by a plain taper only, and can only be released with
safety by using the proper extractor.)
Fig. 9. Ignition timing,
Check that the fully open gap is correct to gauge (not
exceeding .012”).
To re-set timing, turn engine forward until piston
reaches top dead centre on the compression stroke
(see Fig. 9). Now turn engine until piston has
descended 7/16”. With ignition control at full advance
turn contact breaker in its direction of rotation until
the points are just about to open (not more than .002”
open). Lightly tighten the magneto pinion nut and
carefully check figures and positions. Then tighten
nut properly and re-check.
Fig. 10. Magneto pinion extractor.
9

CARBURETTOR
To maintain the efficiency of the carburettor it should
be cleaned periodically by entirely dismantling it and
washing each part in clean petrol.
Renew any worn parts, particularly in the needle
valve if the head has a distinct ridge at the point of
seating, throttle valve if excessive side play is
present or taper needle and clip, if it is possible to
rotate the needle freely in the clip. When re-
assembling, make sure that the taper needle is
refitted into the correct groove, is securely locked by
the clip, and that it enters the central hole in the top
of the jet block. Also verify that the needle valve
enters the top of the float chamber easily, the mixing
chamber flange joint is airtight, and the needle valve
clip registers correctly in it’s groove. It will, of course,
be necessary to reset pilot adjusting screw.
NEEDLE POSITION . Needle positions are counted
from the top of the needle and the groove nearest the
top is No. 1.
THROTTLE STOP.The position of the throttle valve
is set by means of the throttle stop screw (See Fig 4),
the throttle control being closed during this
adjustment. Alternatively, if the screw is adjusted
clear of the throttle valve, the engine will be shut off
in the normal way by the control.
Fig. 11. Carburettor adjustments.
PILOT ADJUSTMENT.To weaken the slow running
mixture, screw the pilot air adjuster outwards and to
enrich the slow running mixture, screw the adjuster
inwards.
Screw the air adjuster home in a clockwise direction.
Warm up the engine, close the air lever and set the
throttle about 1/8th
SPARKING PLUG
open. Gently close the throttle
when the mixture will prove too rich unless air leaks
are present. Gradually unscrew the pilot air adjuster,
when the engine speed will increase and must be
again reduced by gently closing the throttle, until by a
combination of throttle positions and air adjustment,
the desired idling is secured.
MIXTURE STRENGTHS.Weak mixture is indicated
by difficult starting, a tendency for the engine to spit
back through the carburettor (indicated by blue fumes
from the air intake). The engine knocks, and runs hot
with loss of power. The spark plug electrode shows
indications of intense heat, and the mica insulation
becomes white. If spitting back occurs, raise the
needle in the throttle valve. Test by lowering the air
valve gently. Engine revs will rise when the air valve
is lowered slightly below the throttle valve.
Rich mixture indications are heavy “thumpy” running
with emission of black smoke from the exhaust pipe.
As the throttle is opened heavy blowback of fuel is
observed from the carburettor air intake. If the
engine speed does not increase progressively as the
throttle is raised, lower the needle in the throttle
valve.
The normal needle setting is with the clip in No. 2
groove.
The machine is supplied with a K.L.G. type F70
sparking plug, and is of a three-piece construction.
After dismantling, the lower (taper) portion should be
scraped clean of all carbon deposit.
Note: Earlier models are fitted with type L777 plug –
a three-point plug with mica insulation. Where mica
insulation is used, the mica must on no account be
scraped, but cleaned with petrol and a rag. The
inside of the body should be well scraped, and the
earth point cleaned.
When re-assembling, verify that the internal washer
is in place before inserting the electrode. Having
tightened the gland nut, set the earth point to give a
gap of 0.015” to 0.018”. This may mean bending the
earth point towards the centre electrode, or if the gap
is too narrow, prising it outwards. The centre
electrode must not be levered towards the earth
point. The external washer should be replaced if it is
broken or has been completely flattened.
SYMPTOMS OF MINOR PLUG TROUBLES.
Misfiring especially at high speeds and under heavy
pulling at lower speeds, invariably indicates that the
gap setting of the plus is too wide, whilst erratic slow
running can be accounted for by too narrow a gap
setting. An over rich mixture will result in trouble in
the form of an excess deposit of soot on the internal
insulation of the plus with consequent “shorting”
inside the plug. A faulty high tension cable, or the
magneto contact points being out of adjustment will
also account for the plug misfiring.
10

ENGINE DISMANTLING for DECARBONISING
When decarbonising, it is not necessary or desirable
to dismantle the cylinder barrel, unless it is suspected
that the valves, pistons or its rings are the cause of
some trouble. It is sufficient to remove the cylinder
head and gasket thus exposing the piston and
valves.
REMOVING CYLINDER HEAD
To detach cylinder head, disconnect sparking plug
lead, remove steady strap and the 10 cylinder head
bolts. Head can then be lifted off.
Rotate the engine until the piston is at the top of its
stroke and scrape it with an old penknife, taking great
care not to damage the piston crown. Then clean the
cylinder head and replace, tightening the bolts in the
order shown in Fig. 24.
If the valve seats are suspected of gas leakage, due
to insufficient tappet clearances or other causes
these should be remedied. It is possible to grind in
the valves in position, but it is preferable to remove
the barrel from the crankcase so that the work may
be carried out on the bench, and at the same time
the piston and rings inspected.
REMOVING CYLINDER BARREL
To remove cylinder barrel, first turn off petrol taps
and detach carburetter. This can be tied to frame out
of the way. Next, the exhaust pipe and silencer
should be removed complete. The exhaust valve
lifter should be unscrewed from the tappet chest until
the eccentric peg on the lifter is clear of the tappet
head. Uncouple the exhaust valve lifter by removing
Fig. 12. B.S.A. Valve removing tool.
the pin at the lever end. Now remove the five
cylinder base nuts (four outside and one inside
tappet chest), and cylinder barrel can be lifted off.
When removing the cylinder barrel, the simplest way
is to lift it up and tilt it forwards into the front angle of
the frame. The piston should be steadied as it
emerges from the barrel to prevent possible damage.
Cover the crankcase mouth with rag to prevent dust
and grit falling in.
Fig. 13. Inserting valve guides
REMOVING THE VALVES
To remove the valves an extractor as shown in Fig.
12 may be used. If the proper extractor is not
available, the valves may be removed by laying the
cylinder barrel on a bench (valve heads downwards)
and compressing the valve springs with the aid of a
piece of tube (suitably slotted), while an assistant
removes the cotters. Clean all carbon from the ports
and check valve guides for wear.
FITTING NEW VALVE GUIDES
If new guides are to be fitted, the old ones may be
extracted (from below) by means of a simple punch
(consisting of a bar of steel of not more than 5/8”
diameter – Fig. 13). The new guides can be driven in
from the top with the same punch and it is important
that the dimensions from the top of the guide to the
cylinder head joint (as shown in Fig. 14) should be
carefully observed. After the new guides have been
inserted, the valve seats should be re-cut with a pilot
cutter to ensure concentricity of seats and stems (see
Fig. 14). Note that the exhaust valve guide only has
it’s upper end counterbored.
GRINDING IN VALVES
If the old valves are to be retained, they should only
be ground in if the seating shows slight pitting. If
badly pitted, they should be refaced, otherwise
excessive grinding
11

will wear away the seat in the cylinder barrel and
cause the valve to become pocketed. Take great
Fig. 14. Valve seating cutter and depth of guide.
care to remove all traces of grinding compound
afterwards. A light spring, inserted under the valve
head, considerably simplifies valve grinding, which
should be continued until the valve face shows a
smooth surface all the way round. If the machine has
covered a considerable mileage, the valve springs
may need replacing. Refit the valves, springs and
cotters with the aid of the tool shown in Fig. 12 after
valve stems have been lubricated.
PISTON AND RINGS. The gudgeon pin is located
by means of wire circlips which must be removed by
means of a tang of a file or similar tool. Withdraw the
gudgeon pin, thus freeing the piston and immediately
after its removal mark the inside of the piston so that
it can be re-assembled in its original position.
If inspection of the piston rings shows that they are
stuck, prise them out very carefully, and clean them.
Remove any carbon from the grooves and rings, but
before replacing them, check the gap with a ring in
the cylinder. If the gap is excessive, new rings must
be fitted having gaps of between .008” and .012”
when in position.
At this stage it is advisable to check the big end
bearing for wear. Turn engine until piston is at top of
stroke, and resting both hands on sides of crankcase
mouth, hold connecting rod between fingers and
thumbs and feel for up and down play. It should be
remembered that, even though there may be a little
play present, it will not necessarily mean sudden
failure of the bearing, though it will inevitably become
worse. Where play seems excessive, and apparent
big end noise has been noticed when engine is
running, the engine should be completely dismantled,
and a new big end assembly fitted.
Dismantling for decarbonising and piston inspection as described so far is carried
out without removing the engine from the frame. Assembly from this point is
described on Page 18.
REMOVING ENGINE FROM THE FRAME AND
COMPLETE DISMANTLING
The procedure for the removal of the engine from the
frame and dismantling will be described from the
point reached in the previous section when the
cylinder head and barrel have been removed. The oil
pipes must next be disconnected, but first the oil tank
should be drained. Alternatively the pipes can be
disconnected and suitably plugged.
Detach the leads to the dynamo (both of which are
held by a small plate and one screw), and then the
earth wire adjacent to the contact breaker housing.
Follow these with the sparking plug lead.
The magneto control cable can be readily detached
from the handlebar lever.
REMOVING CHAINCASE
The oil bath chaincase follows next. Take off the
footrest and then undo all the screws round the rim of
the chaincase. The nuts of these screws are welded
to the other half of the case and so cannot be lost.
When the outer chaincase cover is taken off, careful
note should be made of the positioning of the cork
washers and distance pieces, to facilitate
replacement. Before removing the chain loosen
clutch as described in next paragraph, and then
dismantle engine shaft cush drive. Tap the lock
washer clear of the slot in the cush drive retaining nut
and
12

Fig. 16 Engine Shaft pinion extractor
unscrew the latter. Then withdraw the spring and
cam sleeve, leaving the sprocket and chain in
position. Next, take off the clutch.
REMOVING CLUTCH.
This can be accomplished with the aid of an
extractor (shown in Fig. 30) after removal of the
clutch outer cover, the actuating cap and the
central sleeve nut. The extractor screws into
the thread provided inside the clutch centre.
Now uncouple the chain, the spring link being of
the usual “hairpin” type. Take off the clutch as a
unit and then the cush drive. There now
remains the inner half of the chaincase, which is
held to the crankcase by three bolts, wired
together for locking purposes, and by a nut
attaching the rear chainguard to the case. The
nut can be released easily after the chaincase is
pulled off the crankcase register.
The bolts holding the crankcase to the front and
rear engine plates can now be removed and it is
advisable to release the gearbox bolts in the case of
the rear plates, since the latter clamp both gearbox
and crankcase lugs between them. The frame bolt at
the bottom of the front engine plates should be
slackened off so that the plates may be swung
forward, greatly facilitating removal of the engine.
DISMANTLING THE ENGINE
It is advisable before commencing to dismantle the
engine to construct a simple fixture such as that
shown in Fig. 15 on which the engine can be
mounted. Alternatively, a lug on the crankcase may
be clamped in a vice and the crankcase itself
supported on the bench.
Attention may next be given to the crankcase portion
of the engine. Take off the timing cover, and if any
difficulty is experienced in releasing the screws, if will
facilitate matters if a long screwdriver is
used, and the head given a sharp tap with a
mallet. On some models an oil tell-tale is
fitted on the timing cover and this must also
be taken off. It is possible that the jointing
compound on the case between the cover
and crankcase will not allow the cover to be
removed easily and in this event, the lugs on
the end of the cover should be used to tap it
off. Take care not to damage the small
nozzle in the timing cover which feeds oil to
the hollow crankshaft; if it should be refitted
in a bent condition it will foul the mainshaft,
and break off eventually, thus starving the
big end and piston of oil.
REMOVING MAGDYNO PINION
Next, the magdyno pinion should be
removed. Since the pinion fits on to a taper
shaft difficulty may be experienced in
removing it. It is not advisable to attempt to
prise the pinion off with levers, as there is a
grave risk of breaking the timing case, but it
will come off quite simply provided an
extractor to that shewn in Fig. 10 is used.
Note that there is a special oil seal fitted in
the timing case, behind the magneto pinion.
It is only necessary to release the magdyno
strap bolt, when the straps can be swung on
one side, and the magdyno lifted off. The latter is
located by dowels only, and if any shims were fitted
below the magdyno they should be carefully
preserved.
13
Fig. 15. Angle bracket for mounting engine.

Fig. 17 Oil pump spindle locking plunger
The engine shaft nut should be removed and the
plate holding the timing gears in position is detached
by removal of the six fixing bolts, three of which
screw into the crankcase casting and have coarse
threads, while the remaining three screw into the
pinion spindles and have fine threads. All the pinions
can now be withdrawn with the exception of the
engine shaft pinion which may require an extractor.
The latter is shown in Fig. 16, and in order to prevent
damage to the engine mainshaft, a flat headed pin of
suitable dimensions should be inserted in the oil hole,
in the manner illustrated. If the pinions are re-bushed
they should be reamed out to .6255”/.6250” for the
cams and .7505”/.7495” for the idler pinion. The
correct size for the outrigger bearing in the timing
gear plate is .815”/.814”.
Before the oil pump spindle is released it is first
necessary to remove the locking plunger which is
exposed after removal of the timing cover (Fig. 17).
Take care not to lose the loose washer covering the
plunger. If the latter cannot easily be removed with
the fingers, a timing cover screw should be screwed
into the plunger, when it can easily be withdrawn. If it
is necessary to remove the pump take off the sump
cover plate, together with the filter and joint washers,
and remove the two bolts holding the pump in
position, thus releasing the pump. These two bolts
are the ones with spring washers under the heads;
the other two bolts hold the pump parts together and
should not be disturbed unless it is strongly
suspected that the pump is giving trouble.
The crankcase is now ready for “splitting”. Release
all the bolts around the crankcase joint face (the
magneto strap hinge pins also act as bolts and the
nut on these must be removed) and draw each half of
the crankcase off the engine mainshaft. Where
single lipped roller bearings have been used in the
engine, the outer race will remain in the crankcase
and if necessary can be pressed out later. It should
be remembered that the outside bearing on the drive
side has its outer race retained in the crankcase by
means of a spring ring which must be removed
before extracting the race.
Ball bearings will usually be left on the shafts after
removal of the crankcase halves, but should they
remain in the crankcase, they may be pressed out of
the gearside in an arbor press as shown in Fig. 19.
On the drive side the inner bearing must first be
tapped out with a punch, projecting through the outer
bearing and, working all round the
Fig. 19. Ballrace extraction (gearside)
14

Fig. 20 Cam pinion spindle extractor.
Bearing to give even extraction (Fig. 18).
These operations will be considerably
helped if the crankcase is first warmed, the
most suitable method being by dipping in
boiling water.
If it is desired to remove the cam pinion
spindles, they can easily be taken out by
means of an extractor (Fig. 20). Do not
remove these spindles unless
absolutely necessary. If the tappets
require renewal, then the cam spindles and
tappet guides must be withdrawn so that
the tappets can be drawn out downwards
into the timing case. The exhaust tappet
requires special treatment, and should not
be replaced by an inlet tappet. The tappet
guides unscrew upwards out of the
crankcase.
The final item is the flywheel assembly. Remove the
locking plates holding the crankpin nuts and take off
the latter. They will require an unusually large
leverage and it may be necessary to add a piece of
tubing of suitable size to the spanner before sufficient
purchase can be obtained.
The crankpin is a taper fit on the flywheels and can
be released by a sharp blow with a mallet.
It is now only necessary to decide which parts require
renewal, and the following points may be of
assistance in making these decisions.
In the event of big-end wear, we do not advise the
fitting of oversize rollers; the whole big end assembly
(consisting of crankpin, rollers and connecting rod),
should be changed. All these components are
carefully matched by the B.S.A. Co., and supplied in
complete sets ready for fitting.
The bore of a cylinder when new is between 3.2295”
and 3.2280” (82mm) and when the bore (measured
at right angles to the gudgeon pin) shows wear to the
extent of .010” or more, the liner should then be
rebored to ½ mm. oversize (3.2487” – 3.2477”) and a
½ mm oversize piston fitted. Subsequently, the liner
may again be rebored, to 1 mm. oversize (3.2684” –
3.2674”) and a 1 mm. oversize piston fitted.
When wear develops after the second rebore, it is
necessary to fit a new cylinder liner. A suitable screw
or hydraulic press giving a pressure of between 5
and 7 tons is necessary – first to press out the old
liner (which must be pressed out from the base of the
cylinder) and then to insert the new liner, which is
pressed in from the top of the cylinder. Owing to the
possibility of the liner “closing-in” during the fitting
process, it must be ground to a finished diameter of
3.229” – 3.228” when in position.
It is also necessary to grind two scoops at the skirt of
the liner at right angles to the gudgeon pin to provide
clearance for the connecting rod (see illustration
below).
A standard piston and rings must of course be fitted
when a new liner is used. The piston should be
selected so that the clearances between the skirt and
the liner fall within the prescribed limits given in
Technical Data (page 2).
Wear in the mainshaft bearings will be readily
apparent and bearings showing signs of damaged
balls, rollers or tracks should be replaced. Special
internal clearances are specified for mainshaft
bearings used on B.S.A. motor cycles, and these are
“000 clearance” for roller bearings and “00 clearance”
for ball bearings. It is not advisable to fit bearings
with any other clearance.
Two scoops diametrically opposite, ground after liner
is pressed in.
15

RE-ASSEMBLING THE ENGINE
The need for extreme cleanliness cannot be over-emphasised.
Parts should be thoroughly cleaned and all trace of any antirust preparations with which new parts may be
coated must be removed.
All bearing surfaces should be liberally smeared with engine oil when assembling.
FLYWHEELS
If the big end assembly is to be renewed it is as well
to check the weight of the new components against
those which have been removed. A slight variation in
the weights is inevitable, but provided that the
discrepancy does not exceed 1 ½ oz no further
action need be taken. This tolerance should not be
exceeded since in the first instance the flywheels
have been balanced to suit the original parts, and the
balance may be adversely affected if the weight of
the new components varies considerably from that of
the original ones.
The driving side flywheel should now be fitted to the
crankpin (this is the side with the keyway) and the nut
tightened up by hand. Fit the timing side flywheel
and again tighten the crankpin nut by hand.
In order properly to tighten the crankpin nuts, the
whole flywheel assembly must be held rigidly. For
this purpose, it should be mounted in a large vice
(fitted with lead clamps) with the driving side flywheel
uppermost. If a large enough vice is not readily
available an alternative method is to fix rigidly to the
bench in a vertical position, two 1 1/16” diameter
posts, the distance between their centres being 3
7/8”. Midway between the posts a hole of 1” diameter
should be bored in the bench to receive the
mainshaft. The flywheel assembly is mounted on
these posts so that they pass through the holes
bored in the flywheels and the driving side flywheel
should be uppermost. Tighten the crankpin nut very
firmly, using a tubular extension to the spanner as
when dismantling, and fit the locking plate and screw.
Suitable packing under timing side “vee” block to
compensate for smaller diameter bearing.
Fig. 21. Checking flywheel alignment.
Now turn the assembly over, so that the gearside
flywheel is on top and tighten the crankpin nut lightly.
The grub screw in the end of the crankpin must be
To bring flywheels parallel when sides opposite
Crankpin are converging insert wedges as shown and deal sharp
blow with mallet.
Fig. 22. Method of correcting flywheels out of
alignment. Note that above illustrations are
greatly exaggerated.
riveted over or centre-punched to prevent its
unscrewing. If it unscrews serious damage may
result to the engine. Check that the side clearance of
the connecting rod in the flywheels does not exceed
.012” and is not less than .010”.
The flywheels will now be aligned only very
approximately and further steps must be taken to
ensure that the wheels are aligned as true as
possible. Two of the actual (or similar) bearings to
be used in the engine should be fitted to the
mainshafts and the latter mounted on vee-blocks.
The flywheels must be trued up, both on faces and
rims, for which purpose a dial micrometer is
necessary (Fig. 21), and after the wheels are trued to
within at least .005” tighten the timing side crankpin
nut fully. A mallet or lead hammer applied to the
flywheels will provide a sufficiently heavy blow for
final truing, and will not harm the flywheels (Fig. 22).
The shafts must not be struck. The shafts should be
finally trued to within .002” maximum.
16

Fig. 23. Valve timing marks.
CRANKCASE
Withdraw the bearings from the shaft and press them
into their appropriate positions in the crankcase
halves. A new washer will be required behind small
drive bearing and a new retaining ring must be fitted.
In the case of single lipped roller bearings only the
outer race can be so fitted. Do not omit the retaining
ring which holds the driving side bearing in position,
and check the ends of the spacing sleeve between
the bearings are parallel to within .002”. In order that
the inner bearing and the sleeve will stay in position it
is advisable to lay the crankcase half on a bench with
the outer bearing lowest.
Fit the oil flinger washer to the driving side mainshaft
and note that this washer is bent over in one place to
prevent accidental movement when fitting. If a new
washer is being used it should be bent in a similar
manner to the one which has been removed. Insert
the driving shaft carefully into the crankcase, taking
care not to disturb the flinger washer. The shaft
should fit into the bearing without the use of
unnecessary force and although the shaft must be a
fairly tight fit in the bearings, it should be possible to
assemble it by hand. If necessary ease the shaft
with emery cloth, carefully cleaning off any trace of
emery afterwards.
It is advisable to attend to the timing side of the
crankcase before continuing further. Replace the oil
pump driving spindle together with its locating pin
(see Fig. 17) and then fit the oil pump in position.
The fibre washer between the pump and crankcase
should be smeared with jointing compound, but an
excessive amount must not be used, since any
surplus will be squeezed out and may find its way
into the oil passages. The pins securing the oil
pump must not be screwed up too tightly. Check that
the pump spindle can be rotated between finger and
thumb.
Now replace the tappets and guides, the latter being
screwed well home, and insert the cam pinion
spindles. These should be pressed home taking
great care to keep them dead
square, and must be fitted so that
the flat on the spindle shoulder is
parallel to the tappet foot, for which
it provides clearance and
consequently its position is most
important.
Assembly of the crankcase will be
made easier if the flywheel assembly,
together with the driving side portion
of the crankcase fitted on as
previously explained, is mounted in a
vice. Lead clamps must be used and
the splined portion of the shaft held.
The mainshaft bearings may now be
pressed into the gearside half of the
crankcase and the latter replaced on
the mainshaft. Bolt up the crankcase and check that
the flywheels, etc., spin easily. Fit sprocket centre,
tighten up, and verify also that the connecting rod is
centrally disposed in the crankcase mouth. Provided
that the connecting rod is not visibly out of centre,
there is no necessity for any adjustment to be made.
If the connecting rod is out of centre, it will be
invariably be towards the driving side of the
crankcase. In this event a shim will have to be made
and inserted between the driving side flywheel and
the oil flinger washer. It may be also that the
distance sleeve between the driving side bearings
has become a little worn on its end faces, and a new
component (one specially chosen so that its length is
on the maximum limit) will rectify the connecting rod
alignment. The maximum length for the distance
sleeve is 1.005” and the minimum is 1.000”.
When the connecting rod alignment is found correct,
remove the gearside half of the crankcase and clean
the joint of any compound used previously. Fit the
magdyno straps on their hinge pins, smear jointing
compound lightly on the crankcase joint face and
again bolt up the crankcase. Check that top of the
crankcase, where cylinder base flange fits, is dead
flat.
TIMING GEARS
Replace the engine shaft pinion, taking special care
to note that the worm is engaging properly with the oil
pump spindle and that rotation of the flywheels drives
the pump.
The cam pinions are interchangeable and
consequently the timing marks are duplicated on both
pinions. This should not cause any difficulty when
timing the valves if it is remembered that the dash
mark only is used for the inlet cam and the dot for the
exhaust cam. (Fig. 23).
The magdyno can now be fitted to the crankcase and
its straps loosely coupled up. Make sure that the
dowels in the base engage properly in their holes in
platform and that any packing shims are refitted.
Refit the idler pinion between the inlet cam pinion
and the magdyno pinion, but do not replace the
pinion retaining plate at this stage.
An oil sealing washer is fitted behind the magdyno
pinion and this should be temporarily removed.
Replace the magdyno pin on its taper; it need
17