Blown 670, tell me what you think...

[coldwater]

The piston skirt disintegrated on this 670 rotax... yet the cylinder wall looks like its in great shape.... no scoring etc. Also if you look at the picture of the head... the cylinder head above the cracked piston is grey while the other is brown with oil residue. Is this from heat? Anyone have an idea of what might have caused this?

 

[Rave]

Wow!! I have done a lot of things to a lot of 670s but have never seen that. Looks like a catastrophic piston failure. No signs of detonation?

 

[arcticcatwill]

i would try and pull the whole motor apart, go through everthing, make sure you find every piece of the piston, probly need boring. emery cloth on heads, that what i did on a yz 250 dirt bike i had that did the same deal, and i never had a probem with it.

 

[sledsrock]

How many miles on the piston? Looks like a classic broken piston skirt to me. Skirts crack over time and will do exactly like you have if not caught in time. Pull the motor and go through it all, look over everything well, mine cracked the case when the piece of skirt squeezed between the lower case and crank.

 

[SylvanLaker]

worn out piston = large piston to cylinder clearance = skirt failure, common in high mile engines...

 

[NoSecondChances]

worn out piston = large piston to cylinder clearance = skirt failure, common in high mile engines...

Exactly what I was thinking. Pea in a coffee can analogy.