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Piston Skirt broke, 2nd time.

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2nd time in a row this has happened. OEM piston with about 26 hours of woods racing. Running 50:1 with amsoil dominator, Didn't snap a pic of the cylinder, looked fine, nothing caught with a fingernail. Rebuilt Bottom end about 24 hours ago, when rebuilt had a small top end noise that we never diagnosed, would be an on and off noise. Never had a problem before this. Ask any other questions and i will answer, just trying to find out if this is a common problem with Gen 3 Bikes. Bike is an 06 CR125 IMG_0778.PNG

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Send your cylinder out to be replated. The cylinder has been worn and the skirt was able to grab a hold on a port. This is common to happen.

already at the local shop getting done. just wondering if maybe it was a different occurrence. thank you

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Evertime I broke skirts was from crank failures :facepalm: wrist pin bearing with little play will do it also.  And ofcourse excessive piston clearance :naughty:

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What's seizes the rings??

 

Oil can gum up the rings, detonation might damage the ring lands, but more so on 4-st with short skirts, large exhaust ports beat the rings up when they move into the port, then get slammed back into the piston. On the road race cylinders which have huge ports you could need to dress the rings after every session.

As mentioned above piston slap probably did in the piston. Check the clearance and also check the cylinder to make sure it's not out of round.

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i dont know about that surubi, i been getting away with a start-up flip the choke off imeadiatly then give it enough throttle to keep it running for maybe 30 seconds then shut it down and get my gear on. after that restart it run it maybe another 30 seconds then head out gently no more than 1/4 throttle but dont lug it and pull a few gears easy dog around a couple min then wack it . any studder or hesatation back off and let it warm up a little more and hit it again is my weisco warm up procedure for the last 20 years.

I  forgot to say a torque plate is used on my cyl for bore and hone work so that makes it break in and tolerate abuse nicly

Edited by peaceofwork
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NOT a Common Problem with Gen 3 bikes..........

Hyperutectic cast piston and 50:1................That is what is going to happen.....

50:1.......Very bad Idea with a high RPM 125...............

Edited by Smokin CR 125R
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NOT a Common Problem with Gen 3 bikes..........
Hyperutectic cast piston and 50:1................That is what is going to happen.....
50:1.......Very bad Idea with a high RPM 125...............

Bike was not ridden as an MX'er, mostly a down low clutched along woods bike.

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Bike was not ridden as an MX'er, mostly a down low clutched along woods bike.

Since you race it and want to rely on it you should try a piston at 32:1 just to appreciate if there's any difference. For slow trials like trail riding you can get by with less oil.
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32:1... better ring seal=more compression... more power, longer engine life.

Raced small bores for years, 125's and even 200's need 32:1.

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