[Mart]

PeteVS has stated the correct and classic way to accurately locate a tdc mark.

If the heads are on, as the top of the piston is not directly accessible, another method is possible, that uses the same principal.

If you thread a suitable object down the plug hole so that it can limit piston travel before it gets to tdc, the same can be achieved. A cable tie is suitable for this method. Thread the cable tie, big end first, down the plug hole so it sits sort of mid piston. Bring the piston up till the tie acts as a stop, mark the pulley. Then without moving the tie, rotate almost one full turn in the opposite direction and bring the piston up again, mark the pulley. As PeteVS says, a new mark half way between is the true TDC.

It's the same as the method Pete VS stated but using the head and a cable tie as the stop.

A few years ago I used a long length of welding wire as a "wand" to get a reassonable indication of tdc. I had about 2" of wire poked through the plug hole, and about 2' sticking out, bent, so as the piston travelled past TDC the wand rose and then fell. As there was a 12:1 ratio, the movement at the end showed TDC pretty well, Easily good enough to allow me to make a mark on the pulley and use it for setting up the engine initially. In hindsight, if I were to do it again I would stop the wand 1" short of the highest point in both directions and split the difference as in the other two methods.

Mart.