the pistons are stamped BSA 68-251 now according to the books they should be 68-250 standard 68-252 first rebore 68-254 second rebore. I cannot find anywhere a record of 68-251 can anyone tell me what these pistons are.
68-0250/252/254 are the part numbers for the
piston assemblies, (piston, gudgeon pin, rings) as the pistons were not generally available separately, so the '251' marking could be the number for the bare piston. Or it could be that the 251 pistons are a different compression ratio to the early 7.25:1 pistons?
the M has something to do with boring out of the cylinders.
It was standard procedure to accurately measure the bore sizes after (boring and honing) and then grade them (as L, M or H) according to whether they were found to be very slightly slightly under, slightly over, or dead-on the nominal bore size.
L M or H pistons would then be selected to give the ideal running clearance with a particular set of barrels.
So your barrels should also have an 'M' stamp to match the pistons?