I have a BSA with frame number WM20 100649 and engine number 48233. There is no WM in front of the numbers, the only thing on the case is the 48233 number. What do I have here?
Thank, Ben in Georgia

Ben, most of the resident experts are probably off to Normandy this week. It was certainly BSA practice to stamp the prefixes so there must be another explanation for yours. Perhaps replacement parts. WD contracts were generally supplied with 10% spare engines and gearboxes and 5% spare frames and the evidence suggests that these were unnumbered.

I'd suggest posting a picture of the complete machine and close-ups of the numbers.

Thanks, for the reply. I forgot to mention in the original message that the 48233 is not a stamp, it is a casting number. Guess it has to be some kind a a replacement case.

I run my 16H with unstamped replacement cases. It seems more accurate than faking a number and most authorities rely on the frame number.

Yours would be from contract S2603 and would originally have carried the WD serial no. C5211349.

What year would my bike be?

The factory records for these later bikes haven't survived but the Ordnance records for S2603 show that delivery was scheduled for 2400 per month commencing December 1943 at WM20 96818.

There are no specific dates given but a reasonable guess would be January / February 1944.

The 'cast' number isn't an engine number but a casting or part number. Engine and frame numbers were always stamped..
I have an NOS set of replacement engine cases with no number on them...and it's not the first set I've had..
Numbering was done at replacement by the workshop carrying out the work and stamp sizes/prefixes etc. show variations, unlike the original 'factory' fitted components which follow a standard pattern...Ian

email (option): ian@wright52.plus.com