Facts

BSA MODEL M20 496 Cc SV SINGLE-CYLINDER - 1939-45 (K-M20 AND W-M20)

Viewed as a near failure in the eyes of the War Office in 1936, this model was ultimately to evolve into perhaps the most illustrious and longest serving model in the history of British military motorcycling, not to mention becoming the most numerous type produced for the War Office itself

The military model M20 was produced by BSA in several variations until 1942, when the type was largely standardized, undergoing only minor modifications thereafter until the end of the Second World War. The very earliest examples supplied, the K-M20 models from the 1939 production year, were generally a type constructed from a combination of standard and de-luxe model components with the addition of certain military specified fittings. (However, it is interesting to note that, principally from contract number 294/C.3655 onwards, the factory ledgers detail the type as "De-Luxe").

The military specified additional fittings included the large 8 in Lucas DUl42 headlight, complete with the switch panel and ammeter, a timing-gear cover incorporating a screw-in plug permitting access to the magneto drive-pinion nut, and 'winged' filler caps for both petrol and oil tanks. Other points of interest, concerning certain early W~M20 examples as well as the War Department K~M20 models, include the presence of a semi-rod operated front brake, a screw-in speedometer drive-box on the front wheel brake-plate face, a rather bulbous 3½ gallon fuel tank, and an alloy tappet cover carrying the BSA 'piled-arms' emblem, and the lack of a cylinder-head engine-steady bracket, pillion seat and footrests. Both front and rear number-plates were fitted as standard, and some models also had an oil-pressure button-indicator incorporated in the timing cover. The early military M20 models were fitted with a long field-stand on the rear nearside of the machine, attached to, and pivoted from, a lug brazed on to the upper nearside rear-frame tube (deleted on later models). When not in use this stand was secured horizontally along the rear nearside of the model by means of a spring-clip attached to a stud affixed to the central rear mudguard stay-cum­-lifting handle (also deleted on later versions).

According to the factory ledgers, a number of the later K-M20 models within contract 294/C.3655 were originally destined for customers in Sweden, South Africa and India, despite the war having started, and that even after some six months of hostilities, BSA along with several other manufacturers were still selling their products to overseas governments and commissions including Holland, Ireland, India and South Africa as well as civilian dealers and distributors.

From October 1939 detail changes were made to the W-M20, as the former K-M20 was now known. These included a new less bulbous 3 gallon fuel tank and girder-forks minus the hand-adjusted damper-knob, which was replaced by a simple locknut impossible to adjust while riding. The speedometer driver was relocated to the nearside of the front wheel, the valanced rear mudguard was removed, and finally Jaegar speedometers were fitted to some models.

During late 1940 certain numbers of civilian specification M20 models were purchased by the War Office direct from the BSA factory, mainly to de-luxe specification as applicable to the civilian market of the time, although a few standard examples were also supplied, probably militarized only to the extent of the colour scheme.

The 1941 model differed only slightly from the 1940 model, both front and rear number-plates being removed, and the alloy tappet cover replaced by a plain steel version.

Between 1941 and 1942 further changes to the M20 were made, including the reinstallation of the offside hand-adjusted damper-knob. Service experience of the model, particularly in North Africa where proper roads were few, had shown the need for readily-adjustable fork damping in order to offset wear and possible failure of the fork. The first damper-knobs were made of bakelite, and later of pressed steel. Other changes included the use of a 6 in Lucas DU42 headlight, complete with the hooded, slotted black-out shield, instead of the 8 in DU142 type, and the fitting of the universal War Department pattern L-WD-MCT1A tail­light. By early 1942 a new full-size rear carrier had been fitted to accommodate the newly introduced universal War Department pattern steel pannier-frames and bags, together with a pair of lower support-stays for the frames. Pillion equipment was also now standardized and a pillion seat and footrests were fitted to all production machines. To accommodate the new equipment it was necessary to alter the design and mounting position of the long nearside field-stand, which was now much longer than before. The securing clip was repositioned to sit just above and forward of the nearside rear wheel spindle nut, the stand sitting at an angle of approximately 45º as opposed to near horizontally as before.

Late 1942 saw the deletion of rubber fittings on all new machines, the replacement of the handlebar grips with universal War Department canvas items and the relocation of the horn to the nearside front engine-plate on many machines. Further modifications include the removal of the rib-centered rear mudguard in favor of a simplified plain item, although the front mudguard continued to retain the ribbed center until the end of the war.

During the latter half of 1943 the girder-fork steering-damper was removed from all new machines, the crankcase sump-shield was redesigned to become a pressed steel item incorporating additional protection at the sides for both the engine and frame-rails, and the fuel tank was altered by having the rear offside corner removed to accommodate the substantial hose section leading from the tank-top mounted Universal War Department pattern Vokes air filter to the carburetor air intake. This last modification was carried out on a large proportion of machines destined for service in hot, dusty climates. By early 1945 the Vokes filter was fitted as standard to the fuel tank of all new models irrespective of where they were destined to serve, the filter being secured to the tank by mounting-strips attached to the previously redundant knee-grip locating

holes on either side of the fuel tank. The last notable change to effect the model during the war occurred in early 1945, when the lighting system was altered by removing the ammeter and installing a simple push-button 'change-over' switch for the headlight, the main lighting switch now being of a different pattern, located on a simple bracket beneath the offside of the saddle.

The vast majority of BSA M20 models delivered were employed by the War Department (Army), although smaller quantities were also used by both the Admiralty (Navy) and the Air Ministry (RAF). It is interesting to note that the Air Ministry used quantities of the model fitted with a Swallow-manufactured sidecar, and that a great number of the smaller M20 model contracts throughout the Second World War specified sidecar-lugs for the frame, generally removed from all the larger contracts, which were specifically solo only.

Although intended as a general-purpose motorcycle for convoy escort and long-distance communications duties, due to the sheer number supplied the model was employed ultimately in every theatre of war and for virtually every purpose imaginable, whether suitable or not. It is perhaps for this reason that the model is the one best remembered by and familiar to most ex-servicemen and other individuals when military motorcycles are mentioned. The post-war service of the BSA M20 model further increases its renown, the type being retained in service as the standard War Department motorcycle throughout the national service period of the 1950s and onwards in limited numbers until the end of the next decade, this despite the fact that it was never totally suitable, and was most probably retained only because of the huge quantities of machines and parts available following the end of the war.

Admittedly, by the end of the Second World War the model's reliability was reasonably good, having served through six years of harsh conflict without any major failing. The M20's clutch had always been a problem, especially if contaminated by oil, and would drag or slip when hot, not to mention not fully releasing on occasions (even when new!) due to the limitations of the single spring multi-plate design. The post-war service authorities were so concerned about the problem that a modification directive was eventually issued during 1958, permitting the installation of four small screws to the central spring-nut in an attempt to effect some adjustment to the non-adjustable design! Another M20 idiosyncrasy, never fully resolved, was a tendency for the machine to refuse to start when hot, largely due to the heat from the cylinder evaporating the fuel in the carburetor before it could enter the engine. The model was also prone to backfire through the carburetor causing a fire, and post-war regulations invariably specified the carriage of a fire extinguisher somewhere on the machine.

With so many examples produced, the BSA M20 is a common machine today, although few retain their correct original factory specification, especially the early wartime or pre-war examples. During the course of the Second World War and throughout the post-war period, virtually all M2Os were rebuilt by the military at least once, if not several times. Engines were changed around under service-exchange schemes and rebuilt models were constructed from stocks of parts assembled from all years and contracts. In certain instances, the military authorities would fit a brand-new frame or engine-unit to a rebuilt machine and not stamp the item fitted with a number, further complicating dating such a machine today. 

Source: British Forces Motorcycles