Well there she is, the source of my clunk! And, the way it’s sitting in there explains why the engine wouldn’t turn past TDC as well since that thing woulda been right in the way. So far as I can tell it’s a small(ish) washer that got folded in half. You can see a few more shots of it over in my Flickr set. What remains a bit of a mystery is how it got there. I’m guessing it entered through the intake, and it’s possible it was sitting on top of the throttle blades of the carb. Tough to say if it was there when I got it, or if I managed to drop it there while I was working.
All in all, there doesn’t seem to be much (maybe any?) damage from the whole debacle. The you can see both the cylinder wall and the combustion chamber for that cylinder, and they both appear unharmed! Remember back when I described that one of the cylinders had a spark plug that had 0 gap and appeared damaged? Yup, you guessed it, that plug came from this cylinder. Makes sense, all of the steel for the cylinder wall, piston, and head are harder than the washer would have been, and the spark plug would have been a softer metal as well. Soft enough to sustain some damage from that thing flinging around in the cylinder. This also explains why it seemed fairly inconsistent, when that cylinder was not on it’s power stroke the washer would have happily just moved up and down the cylinder wall in place. Then when there was ignition in that cylinder, the pressure probably sent it flying around!
I also made another discovery while I was working. The bolt on the front of the crank that I’d been using to turn the motor over, doesn’t actually belong there! The balancer is simply pressed onto the crank with no big bolt to hold it on like you find on many other motors. It’s becoming more and more apparent to me that one or more people who’ve worked on this thing haven’t been terribly methodical, and clearly didn’t have a service manual to reference. In spite of that, the motor appears to be fairly low mileage and it quite good condition.
The plan now is to do some more inspection and measure things to see if they match the tolerances set out in the service manual. If everything checks out, I think I may just put her all back together and run her. Going to keep my fingers crossed that everything is as tight as it appears to be so I can get some miles outta this thing!