yamaha04 Posted August 26, 2017 Report Share Posted August 26, 2017 tore motor down due to crank seal. Though I would have my cylinders checked Both ran at 150psi but iv had a very slight knock at idle since I put this motor together years ago. Two different machine shops. And a relative that is a machinist All checked them with the same result. Bore measures. 2.549in. 64.745mm. And pistons measure 2.541in 64.541mm. The pistons are Wiesco prolites and are stamped 513M06475. This explains my slight knock due to piston slap. But how can this happen. Both cylinders I can see the hone marks on. They look brand new to be honest. They measure straight and true. The pistons look new also. I can see the machining marks on them still. This entire rebuild came from passions race engines back in the day when they sold a top end kit for 300$ included Boring. Anyway the issue is. Cylinders seem to be bored to 64.75mm. The the pistons are stamped the same. But they measure 64.541mm. Quote Link to comment Share on other sites More sharing options...
registered user Posted August 26, 2017 Report Share Posted August 26, 2017 the number stamper on the top is only the nominal diam. actual diam is usually a few thou smaller. hard to say why u have a discrepency. get some measuring tools and check for yourself. i dont trust nobody to measure my shit Quote Link to comment Share on other sites More sharing options...
JUSTWANNARACE Posted August 27, 2017 Report Share Posted August 27, 2017 Sounds like the cyls where bored .005 to big or the bike got hot and the piston collapsed. If it's been knocking sence you pit it together I'd say who ever did the bore job was a little off. The should be measured right above the intake ports as this is the widest point on the piston. and should be measured exhaust to intake side as pistons are not actually round. Typical piston to wall clearance is .003 yours is .008 so loose for sure. The stamped number on top of the piston (say 64.75) is irrelevant when it comes to boring and doing the machine work. And each piston will actually very in size a couple though. That's why anyone that knows what they are doing will require to have the pistons in hand when boring. I will even label piston to the cyl (left and right) cause most of the time they will very slightly. Quote Link to comment Share on other sites More sharing options...
yamaha04 Posted August 27, 2017 Author Report Share Posted August 27, 2017 Thanks guys. This brings me to my next question. my bore size indicates the piston I need is 64.75. And that's the number on the pistons I have so do i order the same and hope it's bigger or get the 65.00 and bore to the pistons with 2-3 thou. Tolerance. That seems stupid to me. The pistons should be closer in size to what they listed as. Btw All 3 different guy's measured the pistons at wrist pin level. From intake to exhaust. Quote Link to comment Share on other sites More sharing options...
yamaha04 Posted August 27, 2017 Author Report Share Posted August 27, 2017 Maybe this is why passions racing isn't in the game anymore. If what I am seeing is right they bored the cylinders to 64.75 before they measured the pistons ?????? Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted August 27, 2017 Report Share Posted August 27, 2017 1 hour ago, yamaha04 said: Maybe this is why passions racing isn't in the game anymore. If what I am seeing is right they bored the cylinders to 64.75 before they measured the pistons ?????? Odd. Jim usually set stuff up pretty tight. Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted August 27, 2017 Report Share Posted August 27, 2017 OP, you'll have to order the next size up on the piston and have them bored to fit. Make sure to use a machinist that knows to size the bore to a piston, not some guy who just bores to what some nominal package size says. 1 Quote Link to comment Share on other sites More sharing options...
yamaha04 Posted August 27, 2017 Author Report Share Posted August 27, 2017 Jim was a hell of a guy. 300$. Piston./bore and clean up exhaust port. Wish that deal was still around. Anyway thanks a lot for the help guys. I will get the 65mm pistons and have them fit nice and tight. 2-3thou. Hard to believe these ones I have now still hold 150psi. Same as when I built it years ago. Maybe like 5-6-7 years ago. With 8 thousands of clearance. probably should have tore it back down and checked things out when I herd the slap after start up but I figured it was fresh and should be fine. .. Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted August 27, 2017 Report Share Posted August 27, 2017 I'll go with the collapsed skirt Quote Link to comment Share on other sites More sharing options...
JUSTWANNARACE Posted August 27, 2017 Report Share Posted August 27, 2017 DO NOT set up less than .003 .003-.004 imo anything less you're asking for trouble. Even at .003 with stock cyl alow plunty of warm up time before "getting on it" to prevent "cold seizing" jmo Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted August 27, 2017 Report Share Posted August 27, 2017 I set all my bores up at .0035-.004 . I then tell people to monitor static compression. If it drops more then 7 rering and if it differs side to side 5-7 rering but I don't know many customers that do . After a hone and rings you could be sitting at .005-.0055 and they don't make noise Quote Link to comment Share on other sites More sharing options...
yamaha04 Posted August 27, 2017 Author Report Share Posted August 27, 2017 So if you guys measure a bore at 64.745mm then you would probably go with a 64.75mm piston eh. I don't really want to bore to 65 if I don't have too. Just hone for clearance. But I also don't want to order pistons and have them measure in smaller so in that regard the 65mm would be the best bet. Has anyone measured a set of new pistons that is smaller and out of tolerance. The way I see it is the pistons should be atleast the number on the box. I get it doesn't mean anything but still. Quote Link to comment Share on other sites More sharing options...
registered user Posted August 27, 2017 Report Share Posted August 27, 2017 normally u measure the piston at the bottom of the skirt perpendicular to the pin. sounds like u measured at the pin. your numbers are likely a few thou smaller because the piston is tapered like a cone or egg. I would measure again just to be sure u really do need bigger pistons. but if so then just go to 65mm and bore/hone accordingly AFTER u have pistons in hand. ya everyones uncle is a master machinist but just double check for your own piece of mind Quote Link to comment Share on other sites More sharing options...
blowit Posted August 27, 2017 Report Share Posted August 27, 2017 Agree with above. The very first question to be raised is where on the piston were they measured? Also, I would pull and ring and check the ring end gap for a comparison. When you mic a piston, you are looking for the largest value anywhere on that piston. That happens to be about 10mm below and normal to the wristpin centerline. Not looking to immediately go at shoddy measuring but as being a machinist and engine builder myself, I can firmly say not all machinists understand some specifics of engines, especially 2 strokes. The noises you hear could be coming from the clutch as well. heard it many times. Brandon Mull Engineering Quote Link to comment Share on other sites More sharing options...
yamaha04 Posted August 27, 2017 Author Report Share Posted August 27, 2017 I can move both pistons around fairly freely in the bore. I also questioned the basket. I can grab it and turn it slightly before the gear moves but this takes abit of force Quote Link to comment Share on other sites More sharing options...
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