Today in Motorcycle History

BSA M20 Military Motorcycles

BSA was the largest motorcycle manufacturer in Britain at the outbreak of the Second World War, and perhaps naturally the largest supplier of motorcycles to the armed services, its involvement with the services having started during the First World War. During 1929, three BSA S29 De-Luxe models were purchased from the company by the War Office for participation in the forthcoming seven-make, but the testing was cancelled prior to completion because of hasty decisions within the higher echelons of the War Office at the time. During the early 1930s the War Office began to favour multi-cylinder engines, believing them to be quieter than single-cylinder versions, and BSA supplied sizeable quantities of their 498 cc War Office V-twin model to both the War Office and the Air Ministry between 1933 and 1937, with minor changes to each contract.

By 1935, War Office opinion about machine suitability had changed yet again, and it now came out in favour of either 350 cc or 500 cc capacity single-cylinder machines of either SV or OHV configuration - twin-cylinder machines had proved to be no quieter after all. BSA was thus to submit its current W35-6 model for War Office evaluation, alongside products from six other principal motorcycle manufacturers of the time.

Following completion of the 1935 War Office trials, however, the W35-6 model failed to meet with War Office approval. In the following year, BSA submitted another of its products for testing - its first 496 cc SV single-cylinder motorcycle, the first M20. Completion of the testing this time, however, showed the machine's engine to have suffered unacceptable wear and it was thus declared unsuitable for War Office procurement.

Undefeated, BSA was to submit a further three improved versions for War Office evaluation during 1937 and two of these were subjected to the standard military 10,000 mile reliability trial. Following the completion of the trial the MEE considered the reliability of the model as only 'fair', although it did note the type as 'suitable for WD requirements'. Thereon commenced a near thirty-year love hate relationship between the British military and the BSA M20 model, an affair that was to continue in certain isolated instances until 1971.

In 1937 the War Department (Army) received a batch of BSA B20 machines for employment as military training purposes. A second order followed that was delivered during early 1938. Also in 1938 the first sizeable War Office order was placed with BSA for the M20 model, along with a small order for some of the company's 'Gold Star' competition machines as well. In 1939 further quantities of M20 models were ordered, as well as quantities of military specification dO lightweight models, principally intended for military training. Following the outbreak of war in September of that year orders for the M20 model continued, although following the collapse of France in mid-1940 BSA, along with most other manufacturers, was inundated with War Office demands for machines of all types and that year supplied quantities of M20s, M2 is, Cl is, Cl2s, B29s, M22s, M23s and M24s, many simply militarized civilian destined models. (1)

By 1939 military thinking had concluded that any purpose-built military motorcycle had to be of a lightweight design, with a specified upper-weight limit, a reasonable performance and other stated qualities necessary for service use. So, following several prototypes in 1940 and one production batch in 1941, the W-B30 350 cc lightweight service model was suddenly cancelled by the War Office during late 1941, just as mass production was about to commence. As a result the service-rider was to be sentenced to the M20 model for the remainder of the war and beyond.

During 1944 a War Office standardization committee was established to assess the possibility of a new universal motorcycle for the services. Yet again military opinion changed, now back in favour of twin-cylinder machines. BSA offered a twin-cylinder SV machine of 500 cc capacity for War Office consideration. The proposed new requirement and the establishment of a standardization committee were the result of the Ministry of Supply becoming increasingly concerned about the numerous types of machines then employed throughout the services and the subsequently heavy demands for spares and repair facilities - a complicated situation to administer if nothing else. Development of the twin-cylinder machine, however, was slow and lacked the urgency of earlier years, particularly as the end of the war was now in sight. It was not until well after the end of the Second World War, during the late 1940s, that the results of the 'twins' trials were available. The motorcycle eventually chosen was Triumph's 500 cc SV-twin TRW model. In many ways BSA was to have the last laugh, as their M20 model was to continue in service as the standard military model right up to the end of the 1950s, and then remain serving in limited numbers, notably overseas and with Territorial units, until the end of the following decade.

In British military service, the M20 model actually outlived the TRW Triumph, designed as replacement for the ancient BSA. The M20 further served alongside the 350 cc OHV unit-construction BSA W-B40 service model, which was introduced during the early 1960s and which served in limited numbers up to the mid-1980s. The last M20 model was not retired from military service until the beginning of the 1970s.



Following the failure of the company's W35-6 model during the 1935 trials, BSA submitted a new model for War Office evaluation during 1936 in a further bid to win another government contract. The M20 test model was initially evaluated by the MEE at Famborough, Hampshire, where it showed itself to be at least comparable to Norton's similar 16H model. In light of this, a full reliability trial of the type was initiated. Completion of the trial was not as successful as had been hoped, although the model did improve on the performance of the previously tested W35-6 model by at the least completing the trial programme. The old BSA curse of cylinder-bore and piston wear was again highlighted during the course of the test, a new cylinder; piston and other engine components having to be fitted at the 6,538 mile stage. The result was that the MEE considered the reliability of the model as 'fair' and thus not entirely suitable for military service.



Having noted the comments of the MEE about the W35-6 model of 1935 and the original M20 model of the following year, BSA supplied a further three M20 models to the War Office under contract 294/C.991 during 1937. These three had been fitted with improved components to offset the previously noted reliability and wear problems. Two of the three examples purchased were delivered to the MEE for use in the standard War Office 10,000 mile reliability trial, while the third example was delivered to a normal service unit for general use and assessment. The two examples tested showed improved reliability figures on completion of the trials, although they were still only considered 'fair' as opposed to 'good'. This said, however, the military authorities did note the model as suitable for their requirements. Thereafter, the military model M20, as it was known to literally thousands of British servicemen and others, was born.

It should be noted that these pre-production M20 models differed to a considerable degree from the later, more commonly encountered wartime production version. Differences were to include mechanical as well as cycle components, although the model was not generally altered in design to the version familiar to many until midway through the 1939 production year.

The M21 model was not generally a faster machine than the M20 model. Although it had a larger capacity engine, this was designed primarily to provide a greater degree of flexibility for pulling a sidecar, in the nature of low-speed, high-gear engine torque, rather than additional speed. It is possible that several examples of the military supplied M21 model were ultimately to be fitted with sidecars for various purposes.


Tested in August 1940 against a standard Norton 'Big 4' and also an experimental 596 cc OHV Norton combination, this machine was found to perform better than the 'Big 4', but only comparable to the 596 cc combination. However, its top speed (46 m.p.h.) was considered too low for service use and its petrol consumption unacceptable. On the plus side, it was considered easier to handle than the two Nortons (largely because of its narrower track) and was less affected by the sidecar wheel's drag. Needless to say it was not recommended for service use and was considered to need redesigning. Specification wise, it could carry three men and 1 cwt of equipment, weighed 5 3/4 cwt, and had 3.50-19 tyres; how the sidecar wheel drive worked is not known.


Few details are known about this model, but it is probable that it was a lightweight M20 and it used an experimental frame (EX315). Other manufacturers at this time (such as Norton) were also looking at lightening their current War Department models as an alternative to the special lightweights currently under development.

1939~5 (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, and became the most numerous type produced for the British Armed Forces.

The military model M20 was produced by BSA in several variations until 1942, when the type was largely standardised, 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 DU142 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 omitted from 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 available to the civilian market, although a few standard examples were also supplied, probably militarised 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-MCT1 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 he was also now standardised and a pillion seat and footrests were fitted to all production machines. To accommodate the new equipment it was necessary to re 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 45 degrees 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-centred rear the mudguard in favour of a simplified plain item, although the front mudguard ( continued to retain the ribbed centre 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 of carburettor 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 o all during the war occurred in early 1945, when the lighting system was altered by .y to 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, just 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 rear the Second World War specified sidecar-lugs for the frame, generally removed aard 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 the 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 the other individuals when military motorcycles are mentioned. The post-war service the of the BSA M20 model further increases its renown, the type being retained in service as the standard War Department motorcycle throughout the national 1945 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 the 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 carburettor before it could enter the engine. The model was also prone to backfire through the carburettor 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 M20s 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.

1. Model designations need verification.
2. Rob (wd16h at writes, "The picture with the caption "BSA-1941c-M20-Patrol-PNG-Vogel.jpg" does not only show BSA M20's but the troop is led by a Norton WD16H motorcycle." (see British Resources)

(Edited Dec. 2012)