The best possible news...

A shattered piston ring is the proximal cause of my engine failure.
When I started to tear down my ailing engine and think about causes, the very best possible outcome seemed to be a broken ring.  Given that all the components in the head measured within spec, and the cylinder wall seems unblemished, I was beginning to wonder if i would take the piston out and find it undamaged.  That would have left only the cooling system and the dreaded governor as possible culprits.

The thermostat came out of the head with some difficulty, and it was sufficiently encrusted to make me suspicious that a clogged thermostat and clogged hoses were part of my problem (the hoses were pretty crusty when I got them off).  Surprisingly, it still worked when put in a pan of water, but that doesn't mean it was entirely functional in the running engine.  Inspection of the cooling ports in both the block and the head suggests that the rest of the system is unoccluded, but a blockage at the thermostat could certainly result in an overheated engine. The instrument panel for the 1GM10 has only an idiot light for both water temperature and oil pressure, and it may well be that the water temperature warning circuit is not functional.

The governor on this engine had been tampered with in the past -- a lead-sealed wire locks an adjustment to the injection limiter in the governor, and the manuals say that it's factory settings should never be changed. For reasons unknown to me, that seal was broken in the past, and I spent a frustrating year learning the consequences.  I even learned to adjust it, though it is very fiddly... one extreme of a quarter turn of the injection limiter renders the engine unstartable. At the other extreme, it smokes excessively... not unlike what I observed on Puget Sound 10 days ago.

Sliding the piston out today brought to light a shattered oil ring, the most delicate and complicated of the three rings.  This was excellent news.  Unhappily, the piston itself is also damaged and will need to be replaced.  Still, these are all problems I have a good chance to fix on my own, without having the cylinder bored, or even honing it (I need to get my hands on a cylinder gauge to confirm the cylinder is within spec).

This leaves the question of what triggered the damage to the ring (and piston), and my only two hypotheses are (1) karmic failure, or (2) undetected overheating of the engine due to a transitory water blockage, or something more persistent (partial blockage of the thermostat such as I observed when I disassembled the engine head).  I won't know whether all is right within the engine until I can manage to get the necessary parts and get the whole bloody mess back together.  Just at the moment, I'm hopeful.

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