I recently tore down my top end on my 1991 KX 500 to freshen things up. I installed a wiseco STD piston, and it ran great but rattled ALOT. I tore it back down, and inspected the old and new pistons extensively. The old piston measures 85.94 mm and the wiseco measured 85.84mm, at least using my caliper. I'm aware that the wiseco is slightly smaller for thermal expansion. I'm borrowing an inside micrometer today, so I should be able to get the bore measured up tonight.
The old piston has a "C" stamped on the crown.
Has anyone heard of a "C" piston size?? All I can find online is 0.50 mm over sizes in Wiseco for over-bored engines, and all I can find in OEM is 1 size, part number 13001A (13001A: PISTON-ENGINE,B) but NO C pistons. With the dealer its the smae OEM number, 13001A: PISTON-ENGINE,B
I know my YZ 250 had different bore sizes, but I'm not sure how much difference it really makes, and I'm not sure why they would call it PISTON-ENGINE B if there wasn't a "C"
