OK a couple of weeks ago I picked a 95 ZR 580 with a blown motor. The guy told me that it kept blowing head gaskets because it had a cracked crank case. Well finally got around to tearing the motor apart and found a broken piston. The so called cracks in the case are not there from what I can see. Both cylinder had 120 psi before I pulled the motor apart. There is a little damage to the bottom half of the case that has been welded( looks like pin holes). I trying to figure out why piston skirts break. I'm thinking that it might have been over reved, The belt looks a little thin. Any suggestions. I haven't taken the clutch apart to see if it is a clutch problem. I just want to figure out why this happened, before I spend a bunch of money on rebuilding the motor just to have it scatter again. I does look like the pistons are the parts unlimited ones from Dennis Kirk. Any ideas on which are the best to use for average trail riding(stock cat pistons, SPI, Wiseco ?) I hope to have a pic below. Thanks, Mike [attachment=123344:40600160.JPG]
MHipke
Broken Piston Skirt
OK a couple of weeks ago I picked a 95 ZR 580 with a blown motor. The guy told me that it kept blowing head gaskets because it had a cracked crank case. Well finally got around to tearing the motor apart and found a broken piston. The so called cracks in the case are not there from what I can see. Both cylinder had 120 psi before I pulled the motor apart. There is a little damage to the bottom half of the case that has been welded( looks like pin holes). I trying to figure out why piston skirts break. I'm thinking that it might have been over reved, The belt looks a little thin. Any suggestions. I haven't taken the clutch apart to see if it is a clutch problem. I just want to figure out why this happened, before I spend a bunch of money on rebuilding the motor just to have it scatter again. I does look like the pistons are the parts unlimited ones from Dennis Kirk. Any ideas on which are the best to use for average trail riding(stock cat pistons, SPI, Wiseco ?) I hope to have a pic below. Thanks, Mike [attachment=123344:40600160.JPG]
Attachments
if it was on the intake side it could have been from some snow getting in the air box, hot aluminum and cold snow usually ends up being broken aluminum, I have seen it happen before.
Could just be metal fatigue. Once when I rebuilt my engine I found all three oem pistons had hairline cracks in the exact same spots. look at your other pistons with a magnifying glass to make sure it is not your problem.
I Would definitely check the cylinders for out-of-roundness, take them to your machinest and have him check with a micrometer, if they are ovaled out or the piston skirts are worn (or both) there will be piston "slap" and that will eventually break the skirt off, then usually the peices will get between the crank and the crankcase and put holes in it.
Thanks for the input guys. I just don't want to go and spend a bunch of money to rebuild and have it do it all over again. According to the guy I got the sled from Its been apart more in the last 4 years that together. I really like this site because someone can always point out something that I forget to look over or forget about. Thanks again, Mike 
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