Can somebody PLEASE explain piston markings to me? (These are from a passle of new-to-me 90/6 topend parts)

Stamped onto right-side piston crown is "90_965"

means the piston skirt diameter is 90.965 mm, or second(maximum) oversize

and "R"

Regular, as in Standard, vs low-octane. I know that's confusing. The piston is the regular crown configuration.

and "Sp. 0.035"

That's the recommended piston-to-cylinder clearance in millimeters.


casting batch identification, I think. don't recall ever seeing this linked to any meaningful information.

plus, of course, "Vorn-- "

Which means "Forward". That's the direction you should face when you
ride the bike.

cast into the piston's underside (inside) is "090206"

That's the date on the MOLD that the piston was cast from.

and "+ " and "3" (or is it "+3"?

The left-hand slug has exactly all the same markings with one exception: the
castings in the underside read "+ 4" rather than "+ 3"

It's "+3" and it means the piston weight deviates from spec by three grams heavier. Since it's impossible to make all the pistons weigh the same, they sort them by weight, and used matched sets for factory engine assembly. When you buy pistons over the parts counter, you get what ya get. You could match the two, but one gram will NOT make any appreciable difference in anything.

FWIW, the smoothest R100 I ever rode had piston assemblies that were mismatched by 16 grams! Coincidence, methinks.

-Tom Cutter

(originally posted to the Airheads list)