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Upon removal of the left/drivers side head I found this: a smeared wear pattern over at least a third or half the cylinder on the lower side, with only a few linear scratches. At the top of the stroke, there is a very rough area between where the upper compression ring and the oil rings are located. I suspect one of the rings broke. Will find out if I live long enough to get the damn thing apart. Pulling the whole engine out won't be easy (the story of this vehicle). In spite of all the ugliness, a bore dial indicator showed less than 0.001" of taper or out-of-round! There is also a small finger-detectable swell in the wall about 2" down, about 3/4" wide. But couldn't get the dial indicator to show much of anything at this location...maybe 0.001" if that. It's located about 45° from the pin axis, roughly in line with the rough area at the top of the cylinder. Somethin' happened in that vicinity, and spread the wonderfulness to adjacent areas of the cylinder wall. Lemme know if you have guesses.
I was wondering the same. The upper area (above upper compression ring) looks the same on all cylinders IIRC, but it did occur to me that the block may have a crack or something that allowed coolant into the cylinder. Don't recall any large amounts of coolant being lost, but I suppose seepage could still do this if there is a fault in the wall. The wall is surprisingly smooth for the visual ugliness. But whereas the other cylinders still show crosshatch hone pattern (after 225,000 miles!), there's not much of that left on this one (cylinder #4, at left front of engine).
So it was caused by water/coolant in the cylinder. The head was cracked between the two valve seats. New heads ordered from Japan Engine in San Leandro CA $200 each. Have hardened seats installed, better castings ostensibly. And saved me from drilling out a lot of broken exhaust bolts, etc.
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