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place the tip of a finger in the spark plug hole to feel pressure or suction (if you are working by yourself, you can rig up a baloon to part of an old spark plug that screws into the hole too)
rotate crankshaft (with wrench) until you feel pressure stop & it starts to suck, look at valves, both exhaust & intake should be closed.
reverse rotate the crankshaft until it starts to suck again....when you feel the point that it does not suck or push air out, the piston is at top dead center, this will also allow you to verify if the timing marks on the crankshaft are correct/true.
That is a very unique way of finding TDC.
I've always done it the hard way and removed the cylinder head to do a visual check :\
But that was on little SOHC 4 bangers
pull #1 plug.. pop cap off of dizzy. #1 tdcc will bee #1 piston all the way up.. the balancer mark lined up with the f mark on timing cover and the rotor bug facing the #1 termnal on the dizz cap...if your valve cover is off.. both valves should be opened and u should be able to wiggle the rockers a lil bit.