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Originally Posted by dan_j_b
I took the motor apart today and I think I found the source of the noise. My guess is I either didn't get all the main caps tight enough or a wrist pin clip came out. Luckily, although the spark plug was smashed flat, the valves and head otherwise looks perfect. So I guess I am shopping around for a short block assembly. Anyone have one in the Sioux Falls area?
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I reckon you got it on the first guess, except I think you meant 'rod caps.' In the pictures, the cap bolt on the right looks fatigued where it broke. The threads on the left cap bolt are a bit chewed up, but no apparent stress marks. On the rod's crank bearing race, both sides of the race are broken off, but one side looks like a fatigue failure while the other side appears to be yield failure.
I speculate that one nut worked it's way off the [left] rod bolt. The [right] side of the bearing race held on as long as it could, flexing back and forth, which caused the wear marks on the threads of the [left] rod bolt which no longer had a nut to stop movement. The [right] rod bolt and [right] half of the bearing race flexed until the metal fatigued and failed, leaving the clean break characteristic of a brittle alloy. The bearing race of the rod, being aluminum, probably failed right away, but was held together by the rod bolt until it also failed in the only place it could flex: where the bearing race had previously failed. When the [right] side failed, the cap simply slid off the [left] bolt, which appears to be pressed in, and functions more like a stud. In this scenario, the other half of the broken bearing race was sheared off on the next revolution of the crankshaft as the remaining cap struck the stationary rod, still protruding from the bottom of the cylinder. This explains the bent rod and deep scarring of the [left] failure point. Alternatively, the cap did not simply slide off, but rather came off in a twisting motion, causing the [left] half of the bearing race to fail instantly. The bent rod and scarring would have happened on subsequent passes of the crankshaft.
...at least that's what
I think happened. Did you see anything that reinforces or diminishes my theory?
Anyway, sorry about the luck. Sometimes dotting your T's and crossing your I's can be as dramatic as throwing a monkey wrench into the works.