Sometimes circumstances just erase all doubts: the engine drips oil from the stump of a connecting rod hanging out are covered with recently-molten aluminum: a sample of oil from the crankcase twinkles with tiny shards of journal and bearing metal, grit rubbing sharply between your fingers. But usually the question of whether to open the engine for inspection is somewhat more in doubt: perhaps the most common question-after a timing belt has broken or stripped-did the valves bend? Or there are mechanical noises inside, noises that could be lifters or wrist pins or piston slap, but you’re not sure. Or there is an oil or coolant leak with no obvious source, or a regular cylinder miss unexplained by fuel, spark or compression loss. What about those cases where coolant leaks into the oil, causing unknown damage to the oil pump, bearings and rings?