[George Miller]

It was caused by heat, but that does not mean it was hot on the temp gauge. It means it was hot on that cylinder wall. I have seen it many times. Number 3 runs hotter than 1,2 any way. It could happen if you were running it faster than normal on a hot day, or pulling a long hill, spark retarded to much, or to lean which will make it run hot inside the engine. Or all of the above.