I just finished up a bore job on an old early 70’s R5 piston port 350 Yamaha. The pistons came from HVC Cycle in Nebraska. Give them a call if you ever need some old Vintage parts.
This set is one of the few 2-stroke Japanese cylinders which appears to have removable liners. I think I’ll have a set to repair and reline soon.
My thoughts came up about how to accurately measure piston sizes when doing a bore job. With all of the different suppliers and brands it’s tricky to figure out where to measure the largest diameter.
Some pistons have a mild taper, some have a belly in the center and some a fairly straight.
What works for me: While finish honing the cylinder, I stop at around .0015” clearance and rinse things off. The fine hone filings are enough to jamb up the new piston.
While stroking the piston up & down in the bore, a certain resistance will be felt. It probably would be good to paint the pistons with Dykem layout dye, but I didn’t on this project. After a short time you will see the largest diameter point from the wear marks. Each piston needs to be measured since production runs may vary in diameter tolerance. Sometimes you’ll find the largest diameter isn't where you expected it to be. When the dial gauge is set you’ll have the correct dimensions to fit the parts accurately. That final skirt clearance fit is one of the most critical measurements in your engine.
Just got his photo from a happy customer in North Carolina.
Wash your Hands