Hello,

My RT3 has been hard to start and was intermittently cutting out when riding. Its time to get it straight for the spring. The magneto didn't test right and is getting sent out for a rebuild and new condenser. The base gasket or exhaust seals were leaking. The left side seals are leaking. The decompression valve looks to be leaking. Etc.... I have a question about the piston.

The piston and cylinder bore looked pretty good. The bike had great compression, but the wrist pin was pretty hard to get out. I plan on changing the piston, wrist pin, clips, bearing, and getting the bore honed to be safe. I don't know if the piston is standard size or not. The cylinder was 79.87mm and the piston was 79.84mm give or take.

Is this the standard size piston or the 1st over?

Bill

I should have checked the manual before posting. The manual says the bore is 80mm. It must be on the stock bore. Good news. There is a piston and ring kit with wrist pin and clips on ebay for $50 shipped.

That's a steal for an RT3 so hopefully it's correct!

Original Yamaha pistons will have any oversize stamped on the top, so:-

no stamp - standard
0.25 - 1st oversize
0.5 - 2nd oversize
0.75 - 3rd oversize
1 - 4th oversize

I think that companies such as Wiseco supply pistons, but in different oversizes.

The piston that came out the bike has a 77 and an arrow pointing forward on the front of the piston and 975 stamped on the rear of the piston. The underside of the piston says Izumi and Yamaha.

The part number listed on the ad is 308-11631-00-96. It seems to jive with other part numbers that I found for standard pistons.

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