The letters usually designate the piston Class, which is the size of each piston. You need to know it in case you need new rings, for example.
The bore is a nominal diameter (e.g. 80mm) and the piston is designed to fit into it, with a gap for the rings.
Manufacturing tolerances can vary though. Some bores will be precisely 80mm and others might be 80.1 or 80.2. Pistons equally vary in size by +/- 0.1~-0.2mm.
So the pistons and bores are classified. If you have a slightly bigger bore, you want to match it with a slightly bigger piston and rings.
Class A is the most precise size. Class B I think is +0.2mm bigger.
You can also get Class C which is +0.4 but these are usually reserved for over-size engines (e.g. if you have a lot of bore wear you can skim the bore out to standard + 0.4mm and fit the class C pistons and rings).
If you change pistons for another reason (e.g. cracked) you should only fit a piston the same Class. Obviously if you stick one of your "B" pistons into the "A" bore, it's a bit tighter than it wants (although your bore is probably looser now than when it left the factory) and it could seize.
If you can't find "B" pistons, you have to bore out the cylinders to the 0.4 oversize and fit Class C. You shouldn't run just one oversize (Class C) piston.. but A or B can be mixed in same engine. Your B-B-A-B is probably what came out of the factory.
Ralf S.
I love a useless fact!
