help me understand the piston A and piston B stuff

[Honda5]

Crankshaft / Piston 1986 1987 1988 1989
Piston A for Cylinder A (STD)
13101-HB9-000 13101-HA2-770 13101-HA2-771 13101-HA2-772
Piston B for Cylinder B (STD)
13102-HB9-000 13102-HA2-770 13102-HA2-771 13102-HA2-772


I have a new cyl marked on the sleeve HA2 + 87 with a b stamped on the back

I also have a cyl marked HB9 + 88 and no stamp


so what is the difference in the A and B pistons and how would I know what stock piston to get for the 88?

[Honda 250r 001]

not sure man, maybe its the differnt years, i would get an aftermarket piston anyway.

[zedicus00]

as the tooling for making and boring these at the factory wares it creates slightly different sizes. at the threshold they end the first series and create a new piston to fit the slightly different bore size.

on street bikes where hundreds of thousands are produced ive seen them lettered out to D.

if theres no letter stamped that normally means A.


on a rebuild none of this matters cuz they bore to the piston you provide.

ive also heard a shop say they just always used the A pistons. this is not a shop i would ever deal with.

[GPracer2500]

Here's what my OEM service manual says about the difference between A and B piston sizes.

Standard range and service limit in mm (and inches). The manual's piston to cylinder clearance is there for reference.

[Honda5]

thats the answer I was looking for thanks GPracer2500. i would use a aftermarket piston but this is a little different kind of build. i have spent 2 years and enough $ to buy a new car on a all oem, almost all new 1989 250r and i am down to the last few pieces.