de Nederlandse BSA ownersclub - the twin buyers guide

BSA twins - the twin buyers guide

serial numbers

All BSA twins have the engine number stamped on the drive side crankcase directly below the cylinder barrel joint. Frame numbers usually appear on the left side of the steering head. Ignore any raised numbers, which usually identify castings. From 1965, frame numbers moved to the front engine mounting lug - although on post 1971 frames the headstock was again used.

Engine and frame number sequences are:

Model	Year	Engine	Frame
Rigid A7	1947	XA7-101	XA7-101
Golden Flash	1950	ZA10-101	ZA7S-6001
Star Twin (sprung frame)	1951	AA7S-101	ZA7S-14001
A7 (swinging arm frame)	1954	CA7-101	CA7-101
Shooting Star	1954	CA7SS-101	CA7-101
A10 (swinging arm frame)	1954	CA10-101	CA7101
Road Rocket	1955	CA10R-601	CA7-7001
Super Rocket	1958	CA10R-6006	FA7-101
A10	1959	DA10-4616	FA7-8522
A7	1960	CA7-7101	GA7-101
Rocket Gold Star	1962	DA10R-5958	GA10-101
Super Rocket	1963	DA10R-8197	GA7-23643
A50 Star Twin	1962	A50-101	A50-101
A65 Star Twin	1962	A65-101	A50-101
Rocket	1964	A65B-101	A50-5501
Royal Star	1966	A50R-101	A50C-101
Thunderbolt	1966	A65T-101	A50C-101
Lightning	1966	A65L-101	A50C-101
Spitfire	1966	A65S-101	A50C-101
Firebird	1968	A65FB-	101

Matching engine and frame numbers from 1969 simplified identification. Better still, a two letter code after the model designation pinpointed the month of build and the model year.
This is the code:

Letter	Month	Year	Letter	Month	Year
A	January		H	July	1973
B	February		J	August
C	March	1969	K	September
D	April	1970	N	October
E	May	1971	P	November
G	June	1972	X	December

The model year usually began in August when production restarted after the summer holidays. This allowed time for stocks of complete machines to be built up before the new season's models were launched at the autumn Motorcycle Show.
Example: Matching engine and frame numbers A65T-BD04887, identify a Thunderbolt (A65T) built in February (B) of the 1970 (D) model year.