three piston kits in one month......

[reeseyz125]

ok, today i removed my top end and discovered yet another piton had went to hell. this will be the third kit that i went through this month, and each of them had the same signs. all three pistons were cracked and missing a piece in back on the bottom. does anyone know why this keeps happening? i keep hearing air leak but it seems like the piston is hitting the rod. they all were cracked or cheaped in the same place. i know it cant be slapping because i had the cylinder redone each time. first time was with l.a. sleeve then rehoned the next two times, all three were pro x pistons. im thinking that the wrong crank was installed b4 i bought the bike, anyway i could find out without pulling the motor?. theres a ( B ) on the rod. and can it be installed backwards?

well thanks for the info and reading my novel,lol.

[bitshifter]

I can't imagine that the rod is hitting but if it is there should be a mark on the rod. If you pull the head or just the plug the piston should go up and down smoothly. If it is hitting the rod it will be very hard to turn as it contacts the rod. It sure sounds like an air leak most likely on the stator side. This will lean the mixture out and thus the lube for the skirt. If the rod is to short the piston might be hitting the crank but again I really doubt it. If you are rehoning each time cause of cylinder damage I would look for an air leak.

[TRexRacing]

I mean no offense but you may want to take it to a reputable shop and have them work on it.You're killing yourself and not saving any money let alone the fact you're not getting in any quality riding time.It's far easier to learn how to work on a bike when doing maintenance instead of constant repairs where the problem isn't diagnosed just broken parts replaced.What year is your bike anyway?

[FFRacing11x]

I probably missed your earlier posts so can you post any pics? Standard bore? Model? Tdub

[reeseyz125]

yea i was thinking that as well, its a 2000 yamaha 125 but the guy i bought it from had put a 2002 motor in. i didnt know this until the shop called asking if i was sure and ran the vin# on the motor. but anyway, im not sure whats the bore size. theres other damage on top in front of piston like it was hitting the power valves. ill post pics later once i find the camera. so no offense taken, im just needing help until i take it into a shop.

[FFRacing11x]

You are in luck...I know someone who knows Yz125s really well. let me know. tdub

[bitshifter]

If the top of the piston is melted where the exhaust port is then it is likely an air leak. The piston could be hitting the power valve if it is an oversized piston.

[TRexRacing]

yeah it sounds like a leak and a clearance issue from here.Man I don't envy you.You will know a YZ engine well when you're done though.

[reeseyz125]

see... thats what i asked them when i got the cylinder back. i was seriously thinking that the valves were hitting the piston crushing it. ok so... i can get an air leak through the exhaust port and through the seal with the flywheel right?. what about the clutch side? anyone know how to upload pictures to post?. thanks.



i posted photos in my photo gallery cuz its not letting me do it in the post.

[bitshifter]

Wow, I did not expect that. I looked at the pics in your Member Gallery. I do wish they were bigger. There is a blurry picture of the exhaust side that clearly shows wear. What baffles me is that the exhaust bridge shows no wear at the bottom but wear at the top. If the power valve is contacting the piston it still shouldn't wear at the exhaust bridge. It may just be a bad picture since I can't see it in the other picture to the left that has the piston connected to the rod. The damage doesn't look like a seal leak since I don't see any real dry scoring. It looks more like the piston is smaller than the bore or the bore is really tapered. I'm with TRexRacing on this one, now that the cylinder is off I would take it to get checked. You got a real strange one. If you keep doing the same thing with the same result you must be missing something.
Let us know.

[reeseyz125]

yea thats what i was thinking with the cylinder, im thinking that i should just buy another one cause nothing is making since with this cylinder. the bike is a 2000 but the motor is a 02. that top end could be another year but the shop swears it an 02. theres hardly any damage to the walls but im taking it to another shop to make sure. the piston went in tight but it keep breaking in the back and smashing in the ring at the top,exhaust side. plus that wear on the exhaust port bothers me.

[bitshifter]

I hope the whole engine is the same year. I am pretty sure that the crank for the 2000 is different than the 2002. This would most likely make the cylinder different also.

[reeseyz125]

is there any way that i could tell what yr. is the cylinder? or the crank? is there something i could measure.

[reeseyz125]

so what causes the skirt to break in the back?

[reeseyz125]

ok so i just came back from a shop and i had talked to a tech there. i was told that i might have a ball bearing or some type of metal floating around in the case. he saw the marks on top of the piston that was made. so now im off to the next level of things to do. thanks for all the help. ill post later and let everyone know whats going and if im doing anything wrong.