Hi all.
It is an engine with wide cylinder and cylinder head. On the engine block is stamped with the date 14-9-53, under the chain tightener.
Disassembled it turns out that it is an A7 cylinder +40. Cylinder head A10, camshaft 67-334, pistons 1:6.7.
My first motorcycle in 1976 was a 1958 A7, over the years converted to the A7 with 67-1101, 2xconcentric 626 and 67-357, but that's a different story.
Then a B31 1953 the most comfortable of my BSA motor bikes, - after some years it was all sold, then to end up with a GS750.
My second acquaintance with BSA an A10 plunger from 1955, solid and never failed. It was then swapped out with a red LM850 and a red SS900.
Greeting Svend