Piston Skirt broke

[MN250R]

Well , My R motor locked up racing through the trails haveing a blast in the snow. Lucky i had a friend with a 4wheel drive to pull me back. After i got home and tore the engine apart, the skirt was broke on one side and had metal chunks in the rod and crank bearings . My question is what causes the skirt to break like that? Anyone know? Im gonna do a full rebuild on it top and bottom, but was wondering if the piston was to old or what?

Thanks

[Tri-Z Pilot]

I think it has something to do with the exhaust port, if the exhaust bridge isnt relieved after a rebuild it can heat up and break the piston skirt. I am not positive, but I know it's something like that.

[hrc85250r]

I think it has something to do with the exhaust port, if the exhaust bridge isnt relieved after a rebuild it can heat up and break the piston skirt. I am not positive, but I know it's something like that.


it doesnt typically break, but it can hreat the piston up and wear a big groove in the side of the piston, also it can grind the rings down pretty good...sounds like you just had a crappy piston, unless its a stroker, which i have heard that it isnt too uncommon to break a piston skirt...

[x.system]

Once you get it tore down you better check your cases real good, broken skirts are know for tearing up case halfs when they get down in the crank journal. Piston slap is probably the reason it broke, to much wear or clearance in your sleeve.

[MN250R]

ahh, ya cause the exhaust port was really built up.
the guy i bought it from a while back was running 20.1 ratio, i wonder if that would clog it up faster

[MN250R]

i will check the case also, thanks!

[Tri-Z Pilot]

ahh, ya cause the exhaust port was really built up.
the guy i bought it from a while back was running 20.1 ratio, i wonder if that would clog it up faster
I dont think it's build up that does it, might contribute to it. From what I remember, when you bore an engine, if you dont relieve the exhaust bridge, it heats up, then extends out into the combustion chamber causing the piston to hit it.

[JLurch58]

If you need a new water cooled sleeve I got one, make ya a hell of a deal. Its from LA sleeve.

[MN250R]

The sleeve looks in good condtion, Anyone using a WSM connecting rod?
Are they as good as prox and versah? Thanks

[HaggLE]

How long was it between rebuilds? 2 strokes arent designed to go forever...
Was it the exhaust side that broke?
Is it a Honda piston or an aftermarket?
Does it vibrate when its reving? Ride somebody elses to compare. Maybe there is a balance issue that is causing it.

[MN250R]

it looks like an original piston , not sure what side broke , cause its outside in the garage, i will look though. not sure how long its been, but i would say quite a while

[Apollo]

What does it mean to relieve the exhaust bridge?

[hrc85250r]

you just have to grind and polish into it a few hundredths, so when it expands when its hot it wont hit the piston....i think honda recommends like .002 or .003 i cant remember and i dont feel like digging around in the freezing garage to find the book...lol...

[TimSr]

Piston slap is probably the reason it broke, to much wear or clearance in your sleeve.

I agree completely. You cannot tell if your cylinder is worn by looking at it. You need to have it measured with a mic. Any bike shop will do it, and most will do it free of charge.

[max]

Mine Did The Exact Same Thing. The Intake Side Skirt Shaddered. I Inspected My Crank And Case Real Well And Went With A 40 Over Piston And Bore. I Think Mine Broke From Just Normal Wear And Tear. I Had The Original Piston In My 84. You Mite Also Want To Get A New Boss Pin. Mine Was Slightly Wore. Good Luck

[mbg75]

cast pistons (such as stock) are known to crack at the skirt. this is due to too much clearance between the piston skirt and the bore as the motor wears (or improper clearance on new motors). the up and down motion of the rod and crank causes the skirt to slap forward and back. this usually can be heard as piston slap before the skirt breaks.

running a forged piston such as a wiseco usually solves the problem, as the forged piston will actually start to bend near the skirt instead of cracking.

either way you should be able to hear it before it causes damage.

[MN250R]

Thanks for all the replies, im gonna bring the cylinder up to a machine shop and have it looked at and done right the first time. :)