Become a Member: Get Ad-Free Access to 3,000+ Reviews, Guides, & More

1937 BSA WM20

1937 BSA WM20 Background

The 1937 BSA WM20 was part of BSA’s massive wartime contribution to Britain’s war effort. 1937 was its first year of production that would last until the late 1950s. The “W” in the model designation signifies that it was specifically-deisnged to be a military bike (just like Harley’s wartime WLA). Designed to be simple, rugged and reliable, it was a side-valve (flathead) 500cc air-cooled single with a cast-iron top end. It had a rigid frame and BSA’s pre-war girder front end. Everything on the bike was extra-heavy duty for military use. The M20 was designed by Val Page as a heavy-framed sidecar rig, so the engine had tons of low-end torque. When sent to the British Army for testing (without the sidecar), they didn’t like it saying it was heavy, slow and had poor ground clearance. But it was saved by its toughness, reliability and ease of maintenance and repair. Of course, BSA was the world’s largest motorcycle maker at the time, and also a huge defense contractor, and that probably helped. In the end, the Army, the Royal Navy, and the RAF placed huge orders for the M20, along with Sweden, South Africa and India.

1937 BSA WM20 War Record

Our feature bike was one of the first of 126,000 M20s that served during the War, making it the most-produced motorcycle of World War II. The M20 and WM20 were also one of the longest-serving military motorcycles in British motorcycle history. BSA’s Small Heath factory was bombed by the Germans in 1940, but mega-corporation BSA had 67 other factories scattered around England, so production quickly resumed. Despite being heavy, rather slow, and having minimal ground clearance, the M20 was favored by British troops for its ruggedness, reliability and easy of repair in the field.


Engine type:


Bore & Stroke:

Compression ratio:

Fuel system:

Ignition system:

Horsepower output:

Primary drive:



Final drive:

Suspension, front:

Suspension, rear:

Brake, front:

Brake, rear:

Air-cooled, side-valve single

496cc / 30.27 ci

82mm X 94mm


Amal carburetor

Lucas 6V magnetodyno

13 hp @ 4200 rpm


Multi-plate, wet

4-speed, right-foot shifting


Girder forks


7-inch SLS

7-inch SLS

Check out these BSA BOOKS

BSA Motorcycles: The Final Evolution

The BSA Gold Star: Motorcycle History

Illustrated Bsa Buyer’s Guide (Motorbooks International Illustrated Buyer’s Guide)

Bsa Twin Restoration

BSA Unit Singles: The Complete Story including the Triumph Derivatives

Bsa Motor Cycles: Since 1950 (British Motor cycles since 1950)

BSA Unit Twins

BSA 500 & 650 Twins: The Essential Buyer’s Guide

Triumph and Bsa Triples

How to Restore Triumph Trident T150/T160 & BSA Rocket III: YOUR step-by-step colour illustrated guide to complete restoration (Enthusiast’s Restoration Manual)

For more like these, visit our BSA BOOK STORE


12 Chapters & over 100 original photos. My best work yet.
ONLY $4.99!
Thank you,
Andy Tallone, your humble author

Click here to buy now:
The Random History of Classic British Motorcycles, Volume 1

Comprehensive Vintage Motorcycle Price Guide 2020-2021