help me diagnose this yz250 top end <<PICS>>

[ej207]

So I just picked up this 92 yz250 that needed a new top end... and this being my first bike i need you guys to help me figure out what is going on with this piston and cylinder.

it seems to be chipped? and there's some seizure marks (i believe) ..could it have been a bad gasket? also theres some marks on the cylinder..looks like the piston melted onto it or something..

help!! 👍

[davidl9999]

Looks like powervalve contact. What is the piston orientation? Chips on the intake or exhaust side?

[ej207]

you know what?

that's a great question.

[ej207]

ummm..im guessing its at the intake side cause its facing the rear of the bike

[davidl9999]

Okay, then it's not from the PV. Guessing detonation/lean mix in that case. Check that the reeds/cage are still intact (prolly will be okay). If it were my bike, I'd just continue with the rebuild and look for metal in the bottom end that needs to be cleaned out, and check the reciprocating parts that they meet the tolerances in the manual and there's no play in the bearings. That cylinder looks very repairable, plenty of links to quality shops here if you search for them, if you don't have one already in mind. 👍

[MrBlahh]

bleh I dont know if that cylinder is repairable that's pretty bad

[pixelmonkey380]

cylinder is shot.

thinking from the debris, it's most likely a rod bearing coming apart.

time to tear down and rebuild the bottom too.

chris<pixelmonkey>👍

[spdfrks]

My guess from the pics is your oring was leaking coolant into the cylinder.

[ForsheeMS]

From the looks of the piston it's very possible that debris from a bad crank and/or rod bearing could be the culprit however that normally leaves a lot of dings on the top of the piston. How does the exhaust side of the piston look? A lean seizure is normally worse on the exhaust side of the piston.

The cylinder is fixable. Might need a little welding done but Millennium Technologies could fix it.

[jsantapau]

itsa good question never seen that type of damage before. the piston looks like it was snagging a port or something sticking out of a port but the cylinder damage isn't near a port.

by the looks of the cylinder deck you have some wierd discoloration on the front and clean on the back, also by the marks left in the cylinder that the rings left kinda gives the thought the bike sat for a long time with a slight coolant leak corroding the piston to the cylinder and then started up?

[arnego2]

Hi there

I think that the piston is showing the side of the exhaust site.

My piston have thir number stamp but as your is 92 so it should be that they stamped the number into the piston with a number. The piston I have here has the number on the exhaust side and what else could do that kind of damage?

On my RM125 a powervalve brough and did the same damage. How do the powervalves look like?

regards

Arnego2

[Greg Y]

wash looks pretty close, I suspect either a piece of debris or something came apart below.

was the head/plug mangled?

[BRM]

O.K.

The damage is on the intake side not the exhaust side.

The nikosil has been damaged or flaked off at some stage and the piston has grabbed the bare aluminium.

You will need a replate on the cylinder and a new piston and rings. Check your bottom end also as debris would be in there. To be safe a new bottom end would be in order. Only $170 for a wiseco kit that would get you all the gasket and bearings also.

Good luck

[ej207]

thanks every1 for the replies🤣

im gonna post some more pics up but b4 i do that i found a cylinder jug on ebay for a 91 thats about to end soon

http://cgi.ebay.com/ebaymotors/91-Yamaha-yz250-yz-250-cylinder-jug_W0QQcmdZViewItemQQhashZitem1e5b127f6bQQitemZ130376957803QQptZMotorsQ5fATVQ5fPartsQ5fAccessories

should i just go with this? its a 91 tho..👍

[ej207]

well it looks like im gonna have to replate the cylinder as what BRM is sayin..sounds simple enough lol

but still ..here are some pics for your viewing pleasure🤣

took me 3 hours to pry out that damn wrist pin!👍

o and heres the top end..looks pretty clean..i guess

so it turns out ..the seizure marks are on both the exhaust and intake sides of the piston.

also is that spot on the crank a heat spot? and how does my bottom end look?

[MrBlahh]

you cant tell if a bottom end is good by looking at it, you have to check for slack in the bearings

you also need to check for debris with a flashlight, although it looks like the piston parts stayed on the top and went out the exhaust, I would still check it out