1954 BSA Single Engin ID

[Mick M]

I have just joined the BSA owners club and thought I would look up the details of my bikes I have two BSA's.  An A65 and a B31.
The BSA single I thought was a B31 350cc. from 1954
Having noted the Engine and Frame numbers I am now confused.
Engine number ZB33 13759
Frame number CB31.6523

I can find no information on a B33 engine number beginning ZB.  Is it a B33 not a B31?  Is it from 1954?

Can anyone shed any light on this?

Would be very grateful, thank you

[Ian C]

Hello Mick.  For the period we are concerned with, BSA frames were normally stamped with the designation for the lowest engine capacity model with that same frame, so for example your CB31 prefix on the frame number could either be a B31 or B33. From the records I have access to, your frame number corresponds to a 1955 model year bike with swinging arm (a plunger frame was also available prefixed with BB31S). The engine number beginning with ZB33 is from a 1952 model year bike and would possibly have been fitted in either a rigid or plunger frame. HTH

[Mick M]

Thank You for your help. 
So the bike is probably a 1955 B33?  Registration info is vague and needs updating as it says its 1954.  It is a swinging arm frame.  The magneto was playing up that is why it was taken off the road.

Regards Mick 

[Ian C]

It could have been built in late 1954 as the Factory began producing the next years model range after its shutdown in July, so any bike built from around August 1954 to July 1955 would be a 1955 model year machine. As to whether it should be a B31 or B33, would only be determined by a dating certificate or do you have the original buff logbook? The engine definitely didn't start life with your frame.