[William Martin]

Wow, that’s pretty messed up! It looks like some little bit of metal or something else hard got in there and danced around for a while. I’ve had that happen before, it was a small piece of the big end bearing cage that did the deed. Not so lucky with the cylinder though… could you have swallowed a tiny rock thrown up from the track surface? Doesn’t seem likely, with the filter inside the airbox. Take a close look at the ring…anything missing? Good luck, hope you figure it out before next rebuild hits the track!

Bill

[Matt Martin]

Thanks, William.

There was about a 5mm section of the ring missing.  The locating pin was still in place, however.

I’m trying to figure out if the damage came from the hunk of piston/ring that departed, or maybe debris from elsewhere.  The big end seems all there, and I do run an airbox and filter.

I’ve read a few reports of coolant getting into the cylinder and thermally shocking the piston and leading to a crack, but i didn’t see any signs of where coolant might come through.

 

 

[Glenn L Riggs]

I have had where I put a new piston in an the top ringland just cracks off was running when I shut it off was down on compression so thats why I pulled it an found a piece missing an the head all pitted from bouncing around

 

[Clark Gaynor Sr.]

I’m no expert but that really looks like detonation.  Exhaust side of piston missing, more damage on edges, etc.  Too much ignition advance, low octane fuel, too lean, ?  I don’t think anything ran through it, like a piece of metal.

There are folks on here who will know for sure.

Good luck,

Clark Sr.

[Matt Martin]

Thanks for the insight.

 

Upon rebuilding, i have double and triple checked the timing to make sure i don’t repeat the mistakes of previous.

 

More photos:

 

 

 

[William Martin]

Did you check the ring end gap with the ring in the cylinder by itself, no piston? It looks like you may have snagged the ring in the exhaust port & broke off a piece. I see some sign of trauma just above the exhaust port opening in that cylinder…I always worry about that a little bit with these oval port monsters, there is no center bridge to keep the ring out of the port. I know it works really well “most of the time”, but if the ring end gap is not there, the ring has to expand somewhere when it gets hot.

Bill

[Matt Martin]

Bill,

I did, the gap was set to about 0.015″ with a feeler gauge.

• This reply was modified 8 years, 2 months ago by Matt Martin.

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