I've used the liquid method on rotary engines that have no well-defined TDC (as in a true dead spot where the piston stops and reverses direction). You have to work quickly because there may be a tiny bit of leakage past the rings. There's also air that gets trapped, and the oil sticks to the tubing, and all sorts of things conspire to make you less than 100% certain about accuracy.

The bolt method seems like it would be very accurate and easy, provided the spark plug hole gives a good angle to the piston. Do this method only when your relative is in another state. Otherwise he WILL use the starter to crank the engine while the bolt is in there. Even if he has to hook up the battery to do it.

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1987 W201 190D