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How's this piston look?

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today was my first time pulling apart an engine! Took the cylinder off and it looks good. Was ported by a previous owner apparently.

Anyways, is this a terrible looking piston? I've never looked at one before.

Pic:

ImageUploadedByThumper Talk1392515877.052316.jpg

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I'm no expert,I've only done my own engine for the 1st time couple weeks ago,but I just saw this pic....so I thought I'd post it for you.

Your piston looks pretty close to the center pic that is perfect-as far as running rich or lean anyways. Hows the cylinder look? Any play in the rod?up&down movement?

Good luck on your rebuild! It sure feels good to successfully complete it!

1392525448766.jpg

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Cool pic! Nice to know that my jetting is visibly good. Thank you much! I'm really looking forward to having the bike back in commission for this spring/summer.

The cylinder looks pretty good as far as I can tell. Lots of cross-hatching left, a little bit of polishing going on, though. But no scoring or deep scratches! I'm gonna take the cylinder to a local shop sometime during the week and get their opinion on it since I really don't know what I'm looking at.

The crank appears to be in good shape. I'm not positive about it, though haha. There's a good amount of sideways play in the rod, but I couldn't feel any up and down play. (Is that good?) I'm tearing the bottom end apart tomorrow after church to replace a cracked trans case and will replace the crank bearings while I'm in there. So I'll be getting a good look at the crank while it's apart.

The tear down is due to experiencing a sudden super lean condition during the last ride that caused the engine to rev WAY high and wouldn't die by just cutting the spark. We had to lock up the rear brake, pop the bike into a high gear and drop the clutch to kill the dang thing. I think it was the OEM reed valve. When I pulled it off there was no gasket... The exhaust gasket was toast as well and spewing fuel, so it could be that that gasket gave way and caused the high rev.

Edited by Thinlineggman

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I believe the side to side isnt bad,but the up&down is,so its probably good. Besides,you said your replacing all the bearings anyways right?

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Could also be crank seals. Wet side(clutch side)allows it to suck tranny oil,which could cuase it to blow out the exuast port at cylinder to pipe area aswell as spooge out the muffler,even foul plugs....&flywheel side will be a airleak,similar to what your explaining. But I'd imagine as you do this rebuild,you'll replace the crankshaft seals anyways.

Edited by yz250 947

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Excuse my ignorance, but are the crank seals those rubber rings that were included in the AllBalls crank bearing kit I got? I can't see any other way the crank would need isolation from the trans, so I'm kinda assuming that's what they are haha. I also cannot find anything labeled as crank seals for my year (1997) that look different than those rings.

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Yes sir! Those are them. They call for the same seal since like 88'. I have a 98',& I screwed up the clutch side seal when I installed it the forst time&it sucked up a grip of trans oil. When I drove the seal in,it got bound up on the sleeve or snout on the crankshaft and the inner lip ripped&folded. 2nd time,I slipped that little snout off the crank,slipped it in to seal(after lubbing with grease) then drove the seal into the case. Went flawless the 2nd time!lmao

A trick learned here for removing the seals without a seal puller or messing up the cases is to drill small pilot holes at like 3&9 oclock postion on the seals,then run sheetrock screw into the pilot holes&use a clawhammer to pull them out.SUUUUPER easy this way.

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Note how the seals are installed now,be sure to re-install the same way.they should also be flush with the case when driven back into the case.

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Thanks for the tips and answers! I pulled the clutch side crank seal out with a screwdriver padded with a rag to prevent damage to the case, before looking back at this thread haha.

Is there a good way to press that bearing out? Seems like it would break if I tried to press on that center piece and I would be more screwed than I am right now....

Progress tonight was halted by not having a clutch basket stabilizer for getting that dang nut off. Are there any other special tools needed before I crack open the cases Saturday and find that I need one more tool to finish the job? We made a makeshift case puller that's more sturdy appearing than yamahas. Can't figure out how to make a clutch stabilizer without damaging splines, though so that tool is getting ordered.

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Thanks for the link! It answered my other questions, and made the job look waaaaaaaay more easy than it actually is haha.

And we tried the air impact while I tried my best to hold the clutch in place with my hand, and all that did was tear up my hand haha! Gonna be ordering the correct tool tonight. Hopefully it'll be here by Saturday.

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Wow! Sucker must really be tight! Nothing wrong with having all those tools tho! You'll use them again,this stuff never ends! Lol

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Yeah, I'm suspecting that the PO wasn't big on using torque wrenches... Came across a few seized bolts and a couple stripped screw heads. Actually had to dig out the dremel and cut a slot into a stator screw because the last guy stripped the Phillips slot! That was an unpleasant experience...

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Yeah, I'm suspecting that the PO wasn't big on using torque wrenches... Came across a few seized bolts and a couple stripped screw heads. Actually had to dig out the dremel and cut a slot into a stator screw because the last guy stripped the Phillips slot! That was an unpleasant experience...

I've replaced all the phillips screws on every Yz I have ever owned with allen heads. Just a much better design for any screw that need any sort of torque applied to seal a surface.

 

BTW: piston wash looks great. What does the underside of the piston look like? That will tell us more about the type of heat your top end is seeing.

Edited by BDubb106
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I've replaced all the phillips screws on every Yz I have ever owned with allen heads. Just a much better design for any screw that need any sort of torque applied to seal a surface.

BTW: piston wash looks great. What does the underside of the piston look like? That will tell us more about the type of heat your top end is seeing.

+1 on the Allen head mod. Phillips heads are a pain in the ass from the get go.

On a side note am I the only one who can see those horrendous hot spots on the crank wheels? When was the last time the crank was disassembled and inspected and big end replaced ?

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Bottom of the piston looks brand new! No heat marks or anything. The CL ad for this bike said new top end, but we all know how that goes haha. I guess this guy could have been telling the truth about that.

New plating on the cylinder is one thing that he certainly lied about, though! Looks like I can get another 100 hours out of it with a new piston, though. Having it measured on the CMM at work tomorrow to figure out which size piston to get.

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