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I'll take a stab at this one ...

Hi Don,

First, while I have done some work on small engines, I am a newbie to all this (so take my comments for what it's worth), I'm in learning mode as well when it comes to aircraft engines.

Your exhaust valve looks fine to me, however photo #4 shows a roughened edge at the top of the cylinder (you should take a photo of the top of the cylinder). My guess is that detonation may have caused that.



Cause? Bad batch of mogas that had lower octane rating than the 93 you thought you were buying? LOP at higher than 75%?

When you say that #1 did not peak, was the EGT still rising as you leaned the engine, or had it peaked much earlier and you did not notice? That cylinder may have been running lean all along (though you did say that the injector was squeaky clean).

I'm sure more knowledgeable people like Mahlon will chime in tomorrow and provide more pertinent questions/answers for you.

I'd start here:

http://www.sacskyranch.com/eng220.htm

Are you sure that the EGT probe is OK??

He scores!

That jug is scored. It has run overtemp to the point where the piston has expanded into the cyl. wall. I only know one way for it to happen unless your piston was over dimension. The valves maybe look to have a leak pattern too. Did you have any valve leakeage @ compression check?

Was the scuffing on the top, bottom, or both?

Eric