BSA M type gearbox
BSA M type Gearbox, 1938 -1947, 1948 -1963, identification data, dimensional notes on identifying and replicating spacers, shims, circlips etc.
 
Tips for living with a BSA gearbox.
 
Updated 8 Jan 2012.
 
Identifying shells and covers:
66-3028 Gearbox only with fixed fittings, 1938-1947 (shell or case with bushes, oil slinger and ballrace fitted) JM19-JM23, KM20-KM24, WM20 to 1947, has casting number JHL 66-3030 on right-underside, (Due to polishing this number has been obliterated on some pre-war gearboxes; due to die wear this number is barely discernable, unreadable on some WD gearboxes) has integral oil filler (1/2” BSPP thread) at right rear; no provision for proper oil seal at countershaft sprocket.
 
66-3140 Gearbox with fixed fittings, 1948-1963 (shell or case with bushes, oil slinger and ballrace fitted) Early versions of 1948 and later M shells have casting number JHL only (same position as earlier box, apparently earlier casting dies were modified and early number obliterated at same time), have provision for oil seal, but no oil filler which was deleted from main shell and moved to outer cover for 48.
 
66-3140 Gearbox with fixed fittings, 1948-1963 Later versions of 1948 and later M shells have casting number 66-3141 and PBM within a hexagon below number, the entire number is located on left-underside.
 

66-3064 Gear box cover outer, foot change, 1938 JM23, KJM22 some carry over into actual KM 1939 models (on outside, hole bored for the screw/slotted nut for kickstart return spring is sunk deeply into cover; cover has deep groove for rubber boot in clutch arm pillar, has casting number 66-3064 D (inverted) on inside; has strengthening ribs around kickstarter shaft; has no external protrusion on outside at front in indented area, has no machined clearance slot for corresponding 37-38 narrow kickstart spring) 37-38 covers were normally polished.
 
66-3102 Gear box cover outer, foot change, ‘39-’40 KM20, KM21, KM22, KM23, KM24, (on outside there is a boss, raised slightly above cover for hex locknut on screw for kickstart return spring; inside of cover has 66-3064 D casting number, number is inverted, but 3064 is crossed out and 3102 added above, inside of cover has machined slot at rear of kickstart boss for additional spring clearance for wide spring, has no strengthening ribs around kickstarter shaft; has an external protrusion on outside facing forward at front in indented area) 39 & 40 civilian covers were originally polished. WD covers were rough cast, “war finish”, not polished.
 
66-3102 Gear box cover outer, foot change, early WW2 production as for 39-40, but as casting dies wore during production, the internal casting numbers became less readable, until they all but disappeared. WD covers were rough cast, “war finish”, not polished.
 
66-3102 Gear box cover outer, foot change, later WW2 production casting dies changed, now has casting number 66-3102 only, in smaller font and now number is in normal orientation; no groove for clutch arm rubber boot; still has an external protrusion on outside at front in indented area). ex WD covers were rough cast, “war finish”, not polished.
 
66-3102 Gear box cover outer, foot change, probably very late WW2 and later production, casting dies again changed, 66-3102 still in small font and normal orientation but now number is much more distinct, no groove for rubber boot; external protrusion on outside at front in indented area has been deleted by altering casting die, die line is visible from where die insert was installed) ex WD and immediate post war covers were rough cast, “war finish”, not polished.
 
1948-1963 M outer covers have horizontal clutch arm and large screw-in filler/adjustment plug.
24-4215 Gearbox cover inner with fixed fittings foot change, JM23, KM20, KM21, JKM22, KM22?, KM23, KM24, G14
 

1948-1963 M inner covers incorporate speedometer drive, have bearing retained by circlip and are wider than 37-47 M types.

 
Gearbox small parts:
 
K139 mainshaft key, C10-M24 & G14 (7/32” x 7/32” square x 5/8” long, rounded ends, goes in milled slot in clutch taper of mainshaft) 
 
2-433 Gearbox key, mainshaft, Identical 1948 and later key for M box. Replacements can be quickly made from industrial key steel 7/32” x 7/32” square, cut to length and file or grind ends. 
 
15-270 clutch rod gland washer, felt, (later cork) 1937-1947 (3/16” ID x 1.00” OD x 3/16” thick, stretches over ¼” Ø pushrod, fitted between ratchet assembly and inside of outer cover, supposed to prevent oil travelling along pushrod to clutch and also a secondary “seal” to prevent water ingress and oil leaks at clutch arm, felt acts as an oil lubrication wick and seal?, cork sort of works for a while, often missing, possibly replace with oil proof closed-cell neoprene foam?)
 
15-4035 gear control quadrant plunger (detent for selector quadrant, large, hollow, pointed one end, 0.308” OD x 0.200” ID x 0.875” long) Same part and number used in 1938-1963 M gearbox.
 
15-4219 Ratchet lever sleeve clip ring 1937-1947 (wire circlip, retains gear indicator disc, 0.800” ID x 0.053”Ø wire, for less painful removal, can be replaced with a Seeger type circlip with ears for circlip pliers)
 
15-4224 operating plate indexing plunger 1937-1963 (detent, for pedal mechanism, fits horizontally in steel insert in outer face of inner cover, small, hardened, solid, pointed one end, 0.213” OD x 0.625” long)
 
15-4225 operating plate indexing plunger spring 1937-1963 (compression spring for detent, fits horizontally in steel insert in outer face of inner cover, 0.188 OD x 0.510” long x 0.020” Ø wire x 8 coils, when rebuilding gearbox ensure there is no rust or other potential fracture points on spring, check that detent spring does not become coil bound in operation)
 
15-4232 gear control operating lever locking washer (tab washer, 3/8” clearance hole, D shaped, 0.030” thick) Same part and number used in 1938-1963 M gearbox.
 
15-4233 gear control quadrant spacing collar, (0.568” ID x 0.812” major OD x 0.685” minor OD x 0.280” long.) Same part and number used in 1938-1963 M gearbox.
 
15-4234 pawl carrier arm washer/shim 0.024” Same part and number used in 1938-1963 M gearbox.
 
15-4235 pawl carrier arm washer/shim 0.022” Same part and number used in 1938-1963 M gearbox.
 
15-4240 Operating shaft clip ring 1937-1963 (wire circlip, prevents gear lever shaft moving inwards, 0.570” ID x 0.053” wire, as shaft seldom needs removal and its corresponding groove for original clip is quite shallow and radiused for the round wire, I do not recommend replacing with a Seeger circlip)
 
15-4246 cover screw locking strip (fits to inner cover, under outer cover, 0.500” wide x 0.030” thick x 2.640” long, 2 x 0.280” holes at 1.900” centres) 
 
No known part number: Mainshaft bush for clutch pushrod, not all shafts are bushed, most bushed shafts found have had bush pressed into clutch end only, however examples of shafts with bushes both ends and internal end only also exist, no data on when used, bush not shown or referred to in any civilian parts books, not shown in military parts books, may be a BSA reclaim of incorrectly machined shafts? May be a field repair?.
 
Check your mainshaft, good centrality of pushrod, combined with minimal clearance is conducive to good clutch operation and reduces incidence of gearbox oil finding its way into clutch.
 
24-68 gear control quadrant plunger spring (compression spring for detent, fits into angled steel insert on inside of inner cover, 0.185” OD x 1.100” long x 0.030” wire x 15 coils; when rebuilding gearbox ensure there is no rust or other potential fracture points on spring, otherwise you will be revisiting your gearbox internals sooner than you may prefer and check that detent spring does not become coil bound in operation) Same part and number used in 1948 and later M gearbox.
 
24-4225 (2) gearbox oil retaining washer, one of these is often found to be missing from gearboxes, one goes in case before bearing, second one goes on top gear sleeve between gear and bearing, 1.383” ID x 2.500” OD x 0.040” thick. Same part and number used in 1948 and later M gearbox.
 
CAUTION! for close ratio gearboxes, reduce OD of 24-4225 installed between bearing and gear, otherwise layshaft gear teeth may chew into outer edge of 24-4225. Or substitute with a 65-3509 1948 and later inner washer in this location. 
24-4232 mainshaft felt washer, M19-M24 & G14, (fits up against outer end of top gear sleeve, gear can be re-machined for a proper oil seal, 22 x 28 x 4, slightly chamfering and polishing the mainshaft’s sharp step inboard of clutch taper facilitates assembly of mainshaft past lip of seal).
 
24-4235 oil flinger washer, 0.755 ID x 1.82” OD x 0.040” thick, goes on mainshaft after first gear, before 24-4237 and inner cover, often missing from gearboxes, same part and number used in 1948 and later M gearbox.
 
24-4236 oil retaining washer small, (mainshaft, goes inside bearing recess of inner cover, before bearing is installed, 0.754” ID x 1.600” OD x 0.040” thick. Dimensions are fairly critical as it rotates in a recess behind bearing.)
 
24-4237 mainshaft distance piece, M19-M24 & G14, (spacing washer, 0.755” ID x 1.225 OD x 0.125” thick, dimensions fairly critical as it spins inside lip of inner cover, goes next to 24-4236 inside bearing recess of inner cover, same part and number used in 1948 and later M type gearbox.
 
24-4238 control shaft collar felt washer M19-M24 & G14 (fits over 15-4233, 5/8”ID x 1” OD x 3/16” thick. Intended as an oiling wick deleted 1941? See tips.)
 
24-4256 kickstart ratchet spring 1938 M19-M24 and all G14 1937-1963 (coil compression spring, 0.825” ID x 0.053” wire x 0.5” long, 3 coils. When rebuilding gearbox ensure there is no rust or other potential fracture points on spring, otherwise you will be revisiting your gearbox internals sooner than you may prefer. If a substitute or unknown spring is employed, check that ratchet spring does not become coil bound in operation.
 
24-4261 kickstart stop rubber M19-M24 & G14 1937-1963 (originally made from  oil resistant rubber, some repros quickly turn to goo, replacements can be cut from nitrile [nitrile is industrial oil proof rubber] I have also cut replacements from polypropylene plastic kitchen cutting boards, suggest you buy the wife a new cutting board before you pinch her old one and sacrifice it for your cause!)
 
24-4263 sprocket locking washer, M19-M24 (6 x internal spline tangs x 2” OD x 0.030” thick, same part and number used in 48 and later M gearbox).
 
24-4278 kickstarter ratchet sleeve nut, locking washer, M19-M24 & G14 (tab washer) 0.565” ID plus tang x 1.250” OD x 0.040” thick. same part and number used in 1948 and later M type gearbox.
 
24-4300 kickstart crank spring 1937-1938 M19-M24 and all G14 (0.140” Ø wire, approx four coils) as with corresponding cover and other parts there may have been some carry over into 1939 at least. When rebuilding gearbox ensure there is no rust, or other potential fracture points on spring, otherwise you will be revisiting your gearbox internals sooner than you may prefer. 
 
66-3058 kickstart crank spring 1939-1960 M models M20-M24 (0.140” Ø wire, approx six coils, i.e. significantly wider than 37-38 spring) when rebuilding gearbox ensure there is no rust or other potential fracture points on spring, otherwise you will be revisiting your gearbox internals sooner than you may prefer. Same part and number used in 1948 and later M type gearbox.
Regardless of the advice of modern salespeople, if correctly made, a 66-3058 will fit the cover for which it was intended, which is 1939 and later. If your bike has the wrong outer cover it is not the problem of the parts salesperson, or the spring.
 
65-3135 kickstart crank spring 1940 B29 & B30, 1948 B31 & B32 (0.125” Ø wire, approx four coils and has longer hook for quadrant, but hook fits heavyweight M quadrant ok)
65-3135 is from 350cc lightweight box, but appears to be a viable substitute for 24-4300.
 
M588 kickstart crank spring anchor pin 1937-1947 (¼” 26tpi x 3/4” cheese head screw)
 
66-3066/1 kickstart crank spring anchor pin nut 1937-1938 (¼” x 26 sleeve nut with screwdriver slot, fits in corresponding deep recess of 66-3064 outer cover)
 

EB102/1 kickstart crank spring anchor pin nut 1939-1947 (thin ¼” x 26 hex nut, locked against cover without washer)

 
2-443 kickstarter crank spring anchor pin nut 1948-1960 (thin ¼” x 26 hex nut, locked against cover without washer, appears the part number was changed)
 
65-3169 gear position indicating peg (riveted into outer cover, goes rusty, can be remade in stainless).
65-3509 gearbox sprocket oil seal washer, 1.383 ID x 2.125” OD x 0.035” thick, from post WW2 M gearbox, note that it is 0.005” thinner than original, but still useful for use with close ratio gearsets as OD is already reduced, will also be OK as inner shield in std box, if sealed bearing used.
 
66-3084 control shaft bush, gearbox JM19 – KM24 and G14, all M gearboxes to 1963, steel, fits in main shell. Bore seldom wears, often found to be worn on face resulting in excessive end float, can be removed, built up with bronze, re-machined and refitted.
 
No known part number: Closing-off disc for 66-3084, use saucer type “welch”, “core”, “freeze” or “expansion” plug, 7/8” size, these are available from automotive suppliers in steel, brass and stainless steel.
 
66-3071 mainshaft packing washer, 0.005”, M19-M24 (shim washer, 0.755” ID x 1.225” OD x 0.005” thick, install on outside of first gear on mainshaft after 24-4235 flinger washer against 24-4236, use as required to eliminate excessive end float of mainshaft), same part and number used in 1948 and later M gearbox. 
 
66-3072 mainshaft packing washer 0.010” M19-M24 (shim washer, 0.755” ID x 1.225” OD x 0.010” thick, install on outside of first gear on mainshaft after 24-4235 flinger washer against 24-4236, use as required¸ same part and number used in 1948 and later M gearbox 
 
66-3086 control shaft bush packing washer (Shell end shim washer, 1 1/8” ID x 1 ¼” OD x 0.010” thick, for adjusting positioning and end-float of control shaft, used as required under flange of large control shaft bush in main gear case) 1948 and later M gearbox part number, handy for setting up early boxes.
 

66-3087 control shaft bush packing washer, small. (Inner cover end shim washer ¾” ID x 7/8” OD x 0.010” thick, for adjusting positioning and end-float of control shaft, used as required under flange of control shaft bush in inner cover) 1948 and later M gearbox part number, handy for setting up early boxes

 
Gear box tips:
 
It would appear that over their long lifetime many WDM20s (and other models) have been repaired by having 1937-1938 outer covers and kickstart springs fitted.
 
I can only surmise that at some time there was a stock of the early spares and a shortage of the correct covers and or springs.
 
The earlier cover being perceived to be less prone to fracture near kickstart may have contributed.
 
The change to the 6 coil 66-3058 spring required the removal from cover of metal supporting the K/S bush.
 
It would appear that someone insisted on a snappier return of kickstart lever, but ultimately this may have resulted in cracked/broken covers.
 
Having said that, it is my experience that cover fracture is usually the result of improper use of the valve lifter and excessive wear, particularly on the OD of the layshaft bush, the blind protrusion of which, serves a secondary function of supporting the kickstart quadrant, but is poorly lubricated. Packing grease into the cavity of kickstart quadrant is recommended.
 
All outer covers should be checked to ensure good condition of the ¼” x 26 TPI thread for kickstart spring screw M588 and helicoiled if thread condition is at all doubtful.
 
This little M588 screw works very hard at its job and ensuring it is well supported by the cover thread and locked into place by external locknut is the key to long term reliability of M588.
 

Many kickstart springs have been bodged by having a new eye or hook formed after the original broke, which alters the force acting on the spring, this, together with poor quality “pattern” springs, may account for the perceived variation in the relationship of the 'start and end' positions of the spring ends relative to each other.

 
Original gaskets are approx 0.010” thick.
 
Do not use thicker and softer gaskets as end-float is affected and joint may also “fret” with resultant subsequent extrusion of gasket, loosening of all fasteners and lubricant loss.
 
Gaskets can be eliminated with modern sealants, but maintaining end-float is the major consideration.
 
Old-stock gaskets which have dried out and shrunk can be salvaged by soaking in water, but be aware as gasket is compressed a small amount of water may be introduced into your newly rebuilt gearbox.
 
If you are experiencing leaks from high gear bushes, but don’t yet want to strip down to fit a sealed ballrace, together with a proper oil seal to high gear sleeve, you may want to consider using a suitable Thixotropic lubricant (thickens when stopped, becomes liquid when stirred up during operation. i.e. thickens when you park!) Apparently Scott owners also use this stuff in their gearboxes to reduce or prevent the “parking puddle”.
 
If you fit sealed ballraces, consider leaving both seals in place on both bearings, although initially counter-intuitive, the bearing will actually last longer because no contaminant metal chips can enter bearing. Many modern gearboxes use fully sealed bearings for this reason. Another example is modern car and truck alternators where pre lubricated sealed bearings give amazing life while working far faster, harder, hotter and for much longer than any M20 gearbox is likely to. 
 
To collect and remove ferrous particulates and thus potentially increase the life of your gearbox, epoxy a small magnet into the usually present hole in the internal threaded end of your drain plug. Of course after epoxy sets you have to refit drain plug and refill with oil. Inspection for particles during routine oil changes gives peace of mind when minimal “fur” detected and early warning of impending “troubles” if a significant increase of magnet “fuzz” is noted, or big chunky bits are suddenly found. 
 
The gear-change and kick-start mechanisms inside the outer cover are not well lubricated in use, even in a regularly used machine these mechanisms are often found to be rusty.
 
Examine the small gear change tension springs very carefully, discard if wear or rust pits are found.
 
When either of these small springs fail you can only change gears in one direction and when you get to the end of that direction, that is the gear in which you will stay, until you strip the outer cover and replace the spring.
 
For long term rust prevention, it is possibly a good idea to have all the small parts of the outer cover’s internal mechanism cadmium plated, but not the springs. 
 
Modern spray-on “sticky” chain lubricants are excellent for lubricating the mechanisms inside the outer cover; coat all mechanism parts liberally before fitting outer cover.
 
Gear-shifting of ‘37-‘47 boxes can be improved by fitting the 1948 and later M shifter-fork assembly, which has smaller diameter pins and corresponding narrower and thus better defined cam tracks. It fits into early M boxes with no other modifications, obviously you must check end-float; check that dogs are fully engaging and also centralized, and all other gearbox adjustments, as per BSA service manual (book 101/BC2A), which is available on Henk’s DVD and elsewhere.
 
Much of the internals of 1948 and later M boxes will fit directly into 1937-1947 M boxes.
 
End float: One method is to adjust to achieve zero end-float without a gasket in “dry” assembled box. (each shaft must rotate with fingers, but no perceptible “clunk” from end-float), then use 0.010” gasket to provide end-float during final assembly. 
 
Check end-float of both gear shafts and selector shaft.
 
Dry assemble all 3 shafts for initial inspection and familiarisation.
 
Note end floats (if any) and centrality of dogs.
 
Due to the fact that the box may have to come apart a number of times during checking and shimming of shaft end float, dog engagement and centrality, my preferred method uses specially ground and honed “slip-fit” main-shaft bearings, until positioning of internals has been completed, then strip and final assemble with new bearings and gasket/sealant.
 
Due to lack of backlash at gear dogs and many small moving parts between gear lever and selector dogs, the BSA M box will never shift as quickly and positively as more modern designs. However with careful and thoughtful “fettling” they can be very sweet, if slightly slow shifters. 
 
If thoughtfully overhauled and properly maintained, the BSA M gearbox is reliable and will remain so indefinitely.
 
“First, do no harm”
 

Neale Gentner.