TM K9 with K9b cylinder. It was the first race since a full rebuild. Kart ran well in practice, and the 25min race. I get home and pull the head to find this. I forgot to get a shot of the cylinder head, but it was pretty dinged up, however the cylinder itself was still perfect (luckily!). It seems to me that it was running quite rich (for break-in).
I’m leaning towards water getting into the combustion chamber, but couldn’t find any obvious signs. There were no crushed o-rings, no obvious trace of water coming in through the crankcase. I feel like i’m at a loss here.The timing is set to 1.5mm or (0.060″) BTDC. Squish was 0.042″
Fuel was C12 mixed 20:1
I went up 2 sizes on the main jet, and took it easy for the first 10 minutes – I tried to follow the swedetech break-in guide as well as I could on track
here are some photos of the carnage https://photos.app.goo.gl/69SWcpAGyFhIMzHx1
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!
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.
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
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.
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.
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.