this is a reply back from him with other parts for sale....

" parts for sale, engine in bits # KB 26 771, engine # KB 24 1191,
engine # KB 461, engine # KB 21 696 only bottom half, ZB 31 23823 with ally
ZB 350 head and barrel, ZB 31 frame. some to think about."

curt

It'll fit, but you will need a different tank with cutaway underneath.

this the underside of my tank, it has the cutaway i think, or is it more than this..curt

http://s305.photobucket.com/user/sevenup_photos/media/BSA%20M21%201939/bsam21parts2004.jpg.html?sort=3&o=37#/user/sevenup_photos/media/BSA%20M21%201939/bsam21parts2004.jpg.html?sort=3&o=37&_suid=1373751306250006500050452778572

Your tank might be OK for clearance i have a feeling that BSA made all the post war "M" tanks with clearance for OHV engines so may well be OK can't say definitely but try it first, the highest points are 2 bolts & the decompressor cable which are where the tunnel is anyway.
pics look fine nothing horrible .. typical rocker cover damage it must be where people don't slacken tappets off before removal etc because guess what.. mine have both also been welded there.
Piston needs identification (from number inside) as it looks low comp but prob makes it a good choice.
crankcases, cams, head, barrel all look correct for a KM24 (39)
Price is not greedy at all.

Exhaust downpipe shape is different to 38 also if he has one there, its flat at the front.
People wrongly point at the fins either side of the pushrod tunnel on mine assuming they are broken or modified.
All the "M" engines & frames widened 1939 onward so your engine will fit your M21 frame but you would actually have more trouble putting it in a 38 M24 frame.

cheers Cycle Fred, all info is a help.all being well should have by next week..curt

Well i really like riding my 1938 engined bike .... its on a 45" straight through pipe so you can hear all the bangs
: D
Still amused that they produced an engine that you have to guess the tappet clearances on.
But even considering they had to add those two bolts either side of the pushrod tunnel as an afterthought i really do think the head & particularly the barrel on these engines are nothing short of a work of art.

these engines along with some m20 frames will be for sale at stanford hall this coming sunday. stall F14 ask for eric... curt

Fred...At least you don't have to guess the valve guide clearances like the 16H... ...Ian

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