Looking at this crankcase. it is a M20, correct? Part number on it shows up in 1939 list for an M20 best I can see it is 66-1637

email (option): unpob@yahoo.com

Henri. Like M20/21 conrods. Which have a part number cast in to them which from memory is one number away from the listed part number. I guess the raw item has a part number. But a fresh number is listed in the parts book once the item is fettled for use. Ron

email (option): ronpier@talk21.com

Looks like a B31 or B33 to me with the blanked off take off for the oil tell tale and the extra metal around the top of the timing chest where it joins the base gasket face and of course the threads for the push rod tube and cylinder studs

email (option): davmax@ntlworld.com

B model. And you are going to have to repair the magneto platform
Cheers
Pete

email (option): petercomley@web.de

Yes, B Series...They should be dated just below the barrel base joint. It's hard to tell from the photo but they look like 'ZB' cases...Cam followers, guides, cam spindles etc. are all identical to the M Series parts if you wanted to use them elsewhere...

The crankcase isn't really worth repairing unless you have a specific use for it... They have little value unless you have the other half and they are late 'wide flange' cases...

Saying that though, with BSA M and B series crankcases 'odd' halves can safely be used to make up a pair if the crankcase register between the two halves is a good fit and you have the facility to machine the barrel and mag platform faces if required.....Ian

Doh! I never even noticed the B series stuff. Just that we had been talking about strangely numbered BSA items on another forum. Ron

email (option): ronpier@talk21.com

Thanks Guys. I was told this was a OHV but just did not have another already opened to compare. I suppose that number is really a 65 not a 66

There's a date on it. 28 9 55

All done here!

That number was in fact: 66-1617