90S pistons : opinions sought!!!!!

[the quinner]

I recently bought some standard size, good condition, 90S pistons off AUS Ebay and presumed that they were the same size, weight, etc. Wrong!!! The pistons are a 89.970mm (3.5421") 'B' bore code and a 89.980mm (3.5425") 'C" bore code

I'm trying to make some sense out of some spare parts I have on the shelf...

Are your pistons actually stamped 89.970mm and 89.980mm? That what is stamped into the crown of the piston? Reason I ask...I have a set of R90S pistons that are stamped 89.975. The cylinders are stamped "B" and everything from the bike is original and low mileage. I've seen both 89.970mm and 89.975mm given as the spec. for "B" size.

[hools100RS]

Yep quinner, the pistons are stamped 89.970mm and 89.980mm. Seems like a mystery to me!!! An 89.975 mm must be right at the end of the 'B' spec. Cheers.....hools.

[Deleted User 62]

I just checked my pair of R90S slugs and jugs: 89.975 pistons and "B" cylinders

[vanzen]

Do not lose sight of the fact that the letter (A, B, C) designations
represent a range of measured sizes
for both cylinders and pistons
that could be fit together – to achieve an established production build tolerance.

Production build tolerances will be somewhat lax
compared to the specs used to "blueprint" ... i.e. the best possible fit.