What causes a cracked piston?

[Blk88GT]

Background:

My wifes 99 XC 500 grenaded a rod, sawed the block in half and destroyed a cylinder. There was no indication what caused the failure. Everyone I showed the parts to could only come up with "fatigue".

I rebuilt the motor with new cases, cylinders, rods, crank, bearings, EVERYTHING you could imagine. I used all Polaris gaskets, seals and SPI moly coated pistons (0.50) The shop who supplied the parts set up the ring gap.

The sled ran great all last year. Compression was good on both cylinders. I broke the motor in and sold it to my father in law. He put on 300 miles on top of the 200 I put on. The sled always seemed to be leaking gas during this entire time. Not a lot, but enough. It would be hard to start in the morning at the cabin.

I took the carbs apart tonight and found a small chunk of rubber fuel line stuck in the needle hanging from the float. We fired the sled back up and decided to do a compression check as it wouldn't hold and idle all that well.

PTO: 55
MAG: 115

Obviously something was wrong, so we tore the motor down. We found that the PTO piston had a horizontal crack in it (parallel to the crankshaft), right through the crown and down through the top of the wrist pin hole. The only thing holding it together was the bottom of the wrist pin!!!

I can't get the pictures up here since my cell phone company has deemed my phone "too old" for picture messaging, but it's really pretty interesting.

Does anyone have any idea what could have caused this? Wash looked good, the cylinders look fine, etc.

I appreciate any feedback.

 

[theultrarider]

Was it the pto carb that had the chunk of rubber in it? If so, you probably hydro locked that cylinder. Had enough liquid fuel in that cylinder that when you tried to start it, it wouldn't fit in the squish area and cracked the piston. It does happen. That or detonation, but that doesn't sound as likely in what you have described.

 

[m8magicandmystery]

Was it the pto carb that had the chunk of rubber in it? If so, you probably hydro locked that cylinder. Had enough liquid fuel in that cylinder that when you tried to start it, it wouldn't fit in the squish area and cracked the piston. It does happen. That or detonation, but that doesn't sound as likely in what you have described.[/QUOTE

i think you nailed it..

 

[spring snow hero]

I would agree that the fuel must have done it........assuming it has not been overheated.

at least you caught it at a $80 fix.

 

[Blk88GT]

Yes, it was the PTO carb that had the chunk of rubber in it. Now that I think about it, I think you're right. Thank god we found it before it took out the motor for a second time. I went through the carbs when the motor first blew, but I must have missed it. That would have been an expensive fix.

Thanks guys.

 

[Sammy]

Just a heads up on this. If it did hydraulic i would check runout on the crank and phazing as well. I have seen bent rods due to this and would have a very close look at the bottom end.

 

[trench digger]

i had the piston in my 05 yfz450 quad crack like that and then 3 weeks later the same thing happened to my 05 yz250 dirtbike. called yamaha and all they could tell me is that i must of had to many hours on them and that i should change them out more often. then a friend of mine bought a brand new kx250f and only had it for a month and the piston cracked. im thinking that the quality of materials is going down or the piston manufacurers are trying to save some $$$ and not making the domes as thick.

 

[sledkiller1]

piston cracking is mostly do to piston slop.rebuilds can be bad bores.also not letting engine warm up enough "piston expantion".then you have faulty pistons.the worst is the damage from the factory till it finally makes it into a motor "have you ever dropped anything".

 

[theultrarider]

Definentely check your clearances closely when you rebuild it this time. You may have too much clearance and piston slop could be an issue. It may even have been just a bad piston, but it is really a big coinsidence that you had a needle and seat leaking on the same cylinder that crack a piston. I think you found your problem.

 

[m8magicandmystery]

the crack on his piston was not on the skirt etc..so i don't think clearence is the issue.. i think has the problem solved...but sure don't hurt to check