BSA motorcycle frames can be identified by deciphering the code stamped on the steering tube of the motorcycle. If the bike has a number plate, as is the case on later model years, the plate with the ID number is in the same location. The BSA numbering system varied significantly from its inception in 1933 until the discontinuation of the brand in 1973. It is critical to have a frame number identification guide to identify your BSA motorcycle frame. Fortunately, these guides are available free online.
Copy the serial number you'll find stamped on the steering tube of the BSA motorcycle. This number will have a letter prefix, followed by between four and nine additional alphabetical or numeric characters. If it is unreadable, the number is also sometimes located on the original title. The prefix letters indicate the actual model of the BSA. For example, a "C" prefix indicates one of BSA's "C" model motorcycles such as the C15 Starfire, built between 1963 and 1965. BSA's popular Lightning models, built between 1966 and 1969, have an "A" prefix.
Visit a website such as sunnymeadcycles.com or classicmotorcycles.org.uk where you will find a PDF file or generalised listing for the frame identification numbers of all the BSA motorcycles built between 1933 and 1973.
Cross-reference your frame number with the numbers listed at one of the websites. This will tell you exactly the model and year of production of your motorcycle frame, as well as whether it was a standard, military or police-issue motorcycle.
Your BSA motorcycle may have different numbers for the engine and frame of the motorcycle. This does not mean that it is a motor not originally equipped with the motorcycle. BSA used different numbering systems for their frames and engines. For example, a numbers-matching 1946 BSA B32 has an engine code of XB32-101, while the frame number is XB31-101.