BSA M20 The BSA M20 was a British motorcycle made by Birmingham Small Arms Company ( BSA ) at their factory in Small Heath , Birmingham . Although initially viewed as a near failure by the War Office in 1936 , the M20 evolved into one of the longest serving motorcycles in the history of British military motorcycling , as well as becoming the most numerous type produced for World War II with 126 , 000 in active service . Many are still in use around the world today . Development . At the outbreak of World War II BSA were Britain ' s largest motorcycle manufacturer with a long history of armaments supply to the armed forces . Designed by Val Page the BSA M20 started development in 1937 as a heavy - framed sidecar model with a simple 500 cc single cylinder side valve engine . It had low compression and plenty of low - end torque through a standard BSA gearbox . Early K - M20 models from 1939 were made from standard civilian parts with the addition of military fittings , such as a large 8 - inch Lucas DUl42 headlight ( fitted with a black out mask ) , a timing - gear cover with a screw - in plug for access to the magneto drive - pinion nut and special filler caps for the petrol and oil tanks . These early military M20 models were also fitted with a long spiked prop stand on the rear nearside pivoted from a lug brazed on to the rear frame tube . Factory ledgers show that BSA exported K - M20 models to Sweden , South Africa and India , as well as civilian dealers and distributors . M20 factory bombing . BSA workers employed making the M20 were killed in an air raid on the BSA factory in Armoury Road , Small Heath , Birmingham on the night of Tuesday the 19th of November , 1940 . The factory was one of the main targets for the Luftwaffe and at 9 . 25pm a low flying aircraft dropped two bombs which destroyed the southern end of the BSA building in Armoury Road . Rescuers included BSA ' s own fire brigade who pumped the Birmingham and Warwick canal dry putting out the fire . As well as 53 workers killed , 89 were injured and it was six weeks before the last of the bodies could be recovered . Much of the factory and equipment was destroyed or damaged but BSA had 67 factories so work was transferred elsewhere and production of the BSA M20 continued . Modifications . From October 1939 the K - M20 was designated the W - M20 and modified to include girder - forks and removal of the valanced rear mudguard . During late 1940 some civil specification M20s were purchased by the War Office which were civil models with a military paint scheme . In 1941 front and rear number plates were removed and between 1941 and 1942 active service use in North Africa showed the need for easy adjustment of the fork dampers . Special damper knobs were made of bakelite ( later replaced with pressed steel ) . The DU142 headlight was replaced with a smaller 6 in Lucas DU42 , with a hooded , slotted black - out shield , and a universal L - WD - MCT1A tail light was fitted . From 1942 there was a shortage of rubber so handlebar grips and foot pegs were replaced by canvas covered metal items and production was standardised , with only minor modifications until the end of World War II . By early 1942 a new large rear carrier was fitted to hold universal WD steel pannier - frames and bags . This meant repositioning the long prop stand to forward of the nearside rear wheel spindle nut . In 1943 further modifications were made include redesign of the crankcase sump shield and fitting of the Vokes high capacity air cleaner , which was a box shaped canister on top of the fuel tank and was intended to assist operations in dusty environments . In early 1945 a push - button switch was introduced for the headlight and the main lighting switch relocated to a bracket beneath the offside of the saddle . Military service . The M20 failed on its first submission to the War Office in 1936 due to ' unacceptable engine wear ' . The prototype machine required a replacement piston and cylinder after approximately 6 , 000 miles . In 1937 three more M20s were submitted to the War Office for testing . Two of these machines only just passed the 10 , 000 mile suitability tests at Military Vehicles and Engineering Establishment , MVEE , Chertsey , while a third machine was used in service trials . In 1938 when a small batch was commissioned . Criticised for being heavy and slow , with poor ground clearance it was saved by its reliability and ease of maintenance . As the need for transport quickly gained pace orders were placed for larger quantities . Most BSA M20 motorcycles were used by the British Army but the Royal Navy and the Royal Air Force also commissioned M20 ' s from BSA . Designed as a general - purpose motorcycle for convoy escort and dispatch use , the M20 saw action in almost every theatre of war . After the war the BSA M20 model continued in military service throughout the national service of the 1950s and in smaller numbers until the end of the 1960s , partly due to the low cost and easy availability of spare parts but also as a result of six years of harsh conditions with no serious failures . Civilian versions . The BSA M20 was a reliable and affordable form of post war transport , so BSA repainted the khaki WD models black and they became particularly popular as a sidecar motorcycle . The Automobile Association placed a large order and their yellow and black M20 combinations became a familiar sight on British roads . The 1945 wartime cast iron engine continued in production until 1951 , when an alloy cylinder head was introduced but otherwise it remained largely unchanged . Spares began to become harder to find in the 1950s , however , and the simple design was overtaken by modern twin cylinder motorycles , so it was discontinued in 1956 — but the 600cc M21 went on until 1963 when it was also discontinued . 112 MPH BSA M20 . As well as being painstakingly restored by enthusiasts to accurate wartime specification , the M20 has also been developed by specialists for speed trials . A 1938 BSA M20 ridden by Bill Jenkins from Dallas achieved a top speed of 108 mph at Bonneville Salt Flats making it the fastest M20 for this venue in the world . The original side - valve engine was extensively modified and fuelled with nitrous oxide . The intake port fed by the carburetor , originally horizontal , was re - drilled at a steep angle into the cylinder casting so it pointed up at the underside of the intake valve . The motorcycle did however retain the long BSA prop stand that hinged under the seat and clipped to the rear mudguard when not in use — so it looked like a stock M20 . In October 1995 an M20 fitted with a BSA Gold Star crankshaft sprocket and running on a 95 % methanol / 5 % acetone mix ridden by Pat Jeal achieved a terminal speed of 112 mph at the disused airfield of RAF Elvington , Yorkshire .