yz85 problem and piston questions????

[TCYFudge]

Hey everybody. i need a little help with my yz85. a little bit of a long story but please bear with me.

i searched my problem and couldn't find anything. anyways... my piston clearance came out to 49.5mm (i was told by my mechanic) and stock is 47.5mm. my last piston (same one in now) is a 49.5mm. after breaking it in and everything, it was seizing. that obviously means the piston was too big correct?

well.. i kept that same piston because i didnt have any money and it never seized again after shaving the high spots. well just the other day i was having an unusual a problem at WOT. it was stuttering and didn't want to give it all it had. i had severe bogging problems at WOT mainly from 3rd to 6th gear. the bike would pull the front up like normal but once at a higher gear it would bog down and almost die if kept on the throttle. so i messed with jetting, mainly the main jet because i had problems at WOT. nothing seemed to help.

then all the sudden the bike was dying like it never had before and killed the engine. i pulled the plug and it looked great. a nice tan to it. then i put the plug back in and tried to kick it and there's no compression. i get home and pull off the cylinder and piston and stuff. i put the piston ring in the cylinder and its looks pretty badly warped. my piston doesn't look that bad, other than the spots it was seizing before. but then i put the ring onto the piston and then the piston into the cylinder and one side had a bad gap in it where i can see light.


now onto my questions.
1. do you think i should get a whole new top end or just a new ring? if you say top end, should i get a 48.5mm piston because the last 49.5mm was seizing?
2. also, do you think that would be the cause of my bogging and loss of power and no compression?

p.s. i was having this problem after switching from castor 927 to yamalube 2r. I'm not sure if that has anything to do with it though.

 

[Jasle]

sounds like you went way too long between piston or ring swaps. We usually get about 20-30 hours on a ring and 60-90 on a piston.

if it were me i'd swap the entire piston kit. I'd also have someone measure the clearances to see if the cylinder is out of round. you have a nik-a-sil cylinder but they can still get worn. where was it siezing? could be too tight of a clearance or just not warmed up properly. you an get a 4 corners siezure from running too hard without warmup.


if you try to upsize you need to bore and re-coat the cylinder. it costs $50 more to bore/coat than just recoat. if you send you cylinder in to usnicom.com and ask for a big bore/eri gorr porting you will have a fully bithin setup. lots of bottom end which is what the yz is missing.

 

[TCYFudge]

I probably only have around 25 hours max on the piston.
im thinking of downsizing because it looked like a multi point seizure on the intake side and exhaust side. when it was first seizing its was only the exhaust side but when i just checked, there seemed like there were more scuff marks on the intake side. i cleaned the aluminum off the cylinder already with muriatic acid. I'm leaning towards the 48.5mm piston kit because the 49.5mm piston really looks too big for the cylinder and all the scuff marks around it makes it seem like it is definitely way too big. I make sure i warm up my bike enough though. i think i should have my cylinder checked for bore too though.

 

[ohmthis]

Absolutely have your bore checked by a "Reputable" shop. Don't buy any parts until you get it measured correctly. Don't ever put a smaller size piston in a larger bore. It can and will slap the skirt and break the skirt off the piston. That can cause the bottom end to get crap in it and tear that up as well. Just get it measured, fixed if bad, buy new top end kit, Now you know it's right.

 

[Jasle]

I probably only have around 25 hours max on the piston.
.

a 1mm difference is HUGE. that is .040" of an inch. you should only have about .003" clearance.

You NEED to measure the cylinder and see which piston you need. A,B,C you can get different size pistons to fit stock bore cylinders. your cylinder should be stamped.

and 25hours could be way too long depending on the cleanliness of the filter and if the rider over revs it. if the cylinder has been ported it can eat up pistons if they didn't chamfer the ports correctly.

 

[TCYFudge]

ok
i always have my bike maintained. i clean the filter after every ride and never over rev it. but thanks for telling me about the .003" of clearance. im going to check that right now. that clearance is only the piston to cylinder wall with the ring on correct?

when i put the ring on the piston and put it into my cylinder. i can feel and hear the piston scraping the cylinder and see and feel a gap between the ring and the cylinder. if i am correct, i shouldn't see or feel any of that right?

also, the right size piston shouldn't seize right? maybe it would have a tendency to if it was new?

 

[ohmthis]

Put the piston in the bore without the rings on it. Use a feeler gauge and stick it in between the piston and bore. That will help you out as far as size. Remember if you stick in a .008" feeler gauge; there is a .004" gap completely around the piston. You still need to have someone measure the bore to make sure it's size along with it tolerance on the out of round spec. Good luck!