Dunno if it was just light touch with the punch by the working girls at Hepolite back in the day, but identifying pistons and their oversizes from random sources can be a pain, requiring good illumination and a magnifying glass for the more obscure offerings. Measure to confirm what you think you have. Batch, inspector, date and process numbers can also be present.
Drags have a handy piston cross reference chart for most BSA Pistons, along with Hepolite references, the parent casting number is usually (but not always) to be found somewhere inside under the dirt. Oversize somewhere stamped on the crown, sometimes in small figures at the extreme periphery, but more often towards the centre.
Original BSA Pistons and their numbers do confuse, the catalogue part numbers quoted are for a complete piston, rings, pin'n'all. The number on the piston itself can't be found in the literature. There is more often just a single digit difference. Head scratching? Simple when you know the answer, it identifies the bare piston, but does not feature in published data.
Thanks to the men in white coats, the hard work has been done with regard to running clearances. All the man in the street has to do is bore the cylinder to a finished specified oversize and select the appropriate oversize piston. So working backwards, as Musky says, measure an unknown piston at right angles to the pin, (my preference is just below the pin) and this reading plus a ball park 3-4 thou running clearance will give the piston nominal size, standard, or an oversize of a dimension available at the time, noted in published data. The bore always refers to the barrel dimension, not the actual size of the piston, which is finished bore diameter less running clearance.
Reboring a barrel to suit used pistons was done by measuring the piston, adding a running clearance and machining the barrel to produce the finished honed bore with the correct running clearance for the piston. This is where the machinists' skills made the difference. Sadly a dying art.
Swarfy.
