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piston hitting head
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Posted by: dkeen73

I just got done with the top-end rebuild, its an 86 with 86 bottom-end, I went with wiseco piston 258cc. Well anyway I put it all back together and the piston is hitting the head(stock), with the head off it goes up and down smooth, do I need a different head or a coolhead or do I have to get the head cut to fit the piston? A little help would be great, Thanks

Posted by: 86 Quad R

got a pic of the head aswell as the piston coming up outta the cylinder? how much is the piston sticking out at TDC?

Posted by: fiveOnick

Sounds like you have a later model rod in there or you have a cylinder thats been decked a bunch.

Posted by: 86 Quad R

my thoughts.

Posted by: dkeen73

It is sticking out about a 1/4 of inch or so, not much. I do have a different cylinder on it, I never thought about the differ cylinder being milled down. I will measure that and let you guys know, Thanks for your help

Posted by: punker69q

quote:
Originally posted by dkeen73
It is sticking out about a 1/4 of inch or so, not much.

!!!!!!!!

1/4 of an inch is alot! what you have is a piston made for a short rod in a long rod motor. basically, the difference is 5mm (0,1968 inch) between those rods. What you need is a cylinder spacer, probably a 5mm.

Posted by: nacs420

My little 2 stroke motor had that problume. I just added a couple extra gaskets and that fixed it for me.

Posted by: Scott-300ex

Yeh man, thats wut I thought, 1/4'' is a lot!

Yeh, maybe you have the piston for the short rod on the long rod.

Either way I'd say you have to do another top end and get a new rod or piston, but prolly piston for sure cuz it might have a flat spot on it from hittin your head, and might have to mill the head.

Posted by: dkeen73

The bottom-end is a short rod and the piston is for a short rod going by the numbers on top of the piston, it isn't actually going over top of the cylinder by a 1/4 that was a guesstimate, it not going over but by a few mm's, it didn't do any damage because I never cranked it hard I just tried to turn it over by hand and when it wouldn't turn over I removed the head. I measured the height of both cylinders and there the same...what about using 2 base gaskets, or would a different head work? When you go so big on the bore do you usually have to cut the head to fit the bigger piston width? Thanks for all your help guys...

Posted by: dkeen73

I took a couple of pics, the piston does go about a mm higher I could snap a pic fast enough tho. I also took a pic of the head too...

Posted by: dkeen73

the head gasket is still on it...

Posted by: dkeen73

head

Posted by: fiveOnick

A 250r will have at least 1mm + deck from the piston if never decked. So add that to whatever your sticking up. Its not something you should stick several gaskets under and run.

Posted by: xlr810

That looks like a flat top piston to me. You'll need to have your head cut for it to work properly, or you could go back to a 526 series piston.

Good luck.

Posted by: 86 Quad R

you sure this engine doesnt have a 4mm stroker in it. that piston is coming up outta there about the distance it would for a stroker without the required baseplate.

typically the stock piston to deck distance can vary from .050 to .080 roughly. take and measure the sroke and see what you come with.

what piston is that you have in there now?

Posted by: dkeen73

Its a wiseco(526M06750) the piston that was in it before was for an 86 short rod also, thats what stumps me I think I still have piston somewhere I'll get the number off of it too make sure...I didn't do anything to the bottom-end also...only thing changed was the top-end...

Posted by: 86 Quad R

that piston checks out to be a weisco pro light piston for an 86 250 with a 1.5 mm over bore. if possible, measure the old piston from the wrist pin to the top as well as the new one and compare.

Posted by: C-LEIGH RACING

That top pic, looks like the piston is up higher than the second one & looks like around 0.040 pop up.
You sure you put the same amount of base gaskets back on as you took off. Probably would take at least 4 base gaskets for the piston to be flat with the deck, somethings not right.
Its not impossible that the wrist pin hole could have been machined in the wrong place, say like one from a monday morning & the worker punching bottons was drunk all weekend.
Neil

Posted by: esr250r86

quote:
Originally posted by punker69q
!!!!!!!!

1/4 of an inch is alot! what you have is a piston made for a short rod in a long rod motor. basically, the difference is 5mm (0,1968 inch) between those rods. What you need is a cylinder spacer, probably a 5mm.

yeah, I would get a cylinder spacer,not quite sure what size of spacer though.