Ron,
That is an odd one, just went and measured 2 cases I.D. = @ 1.940"
Cranks @ 1.885" so running clearance is .065" total or @ .032" per side

Ronald Olson wrote:
Ron you are right about the mix 2 low and 1 high is as lean as a 929 usually should be set. That piston looks to me like the ring rotated and the end got caught in the exhaust port and tore the top edge of the piston off it just looks broke more than melted and isn't that the end of the ring I see on the exhaust side. Had one do the same thing. The piston ring locator pin was somehow pressed in too far and the ring rotated on mine. Similar damage. ?????

That very well could have happened Dan. I took the pix right after I pulled the engine apart so I really didn't touch anything. The RCMK didn't have much runtime on it as you can see but I was looking more at the burn pattern on the piston. It was running a little hot on the previous run so I was suspecting a lean condition. The pipe was sizzling when we brought it in. I figured at fisrt as that might be typical as my nitro burners would do that.

I agree with what Daniel is saying...sure looks like the ring snagged in the exhaust port. The extra heat you were seeing was most likely generated from that extra friction as the ring was rotated out of position...until it finally snagged and all ended!

Marc and Dan, it looks like you guys were right on the money! I never really looked at he piston so here's a shot of the side. I can see where the ring was pulled down on the upstroke at the exhaust port which very well could have broken off part of the top of the piston plus cracked the skirt down to and then around the wrist pin boss. After taking this shot I looked inside the sleeve again and it feels worse than I thought that it was at first. Time to send Bruce some more money......

Just voicing opinions ..... don't mistake damage caused during the failure with diagnosing what caused it in the first place

Marc wrote:
being the INTAKE side of piston is intact .. easy to see if this explanation is even possible ?
* take a piece of the ring and place it OVER the top of index pin, unless the pin is in so far as to allow ring while over it to be flush with side of piston ... that's not it ?

Scott, the piston is still here in front of me, actually where it's been for a long time now. The last shot was taken shortly before posting the last time. I see what looks like a piece of the pin where there is one on its replacement so it very well could have spun the ring in the groove. This engine was never torn down until this incident as a side note. There are no scrapes anywhere near where the pin is/was located at.

I just went down and looked at the replacement piston and noticed the the pin on the bad piston isn't centered in ther ring groove but a couple hundred thousandths up into the top of the piston, enough to allow the ring to rotate.

Scott wrote:
Obviously for the ring to rotate, there has to be issues with the pin...either it's sunk in, or gone altogether. Or in this case, as Ron has noted, it seems like it may have been off center in the groove leaving enough room for it to rotate. I think it's clear I wasn't suggesting that it creeped over a pin that was in place as it should be.