If ya really want to know, exactly, break all the porcelain out of a sparkplug, and slide a 1/4" rod thru the center untill it extends about 1 1/4" past the plug nose then weld it. Dont put it in yet---remove Number 1 plug (front of engine drivers side), stick your finger over plug hole, and bump the engine with starter (coil wire disconnected) untill you feel it trying to blow your finger off the plug hole. Stop---at that point it is coming up on compression on number 1. Remove the rest of the plugs, screw the "piston stop" which you made into #1 plug hole, put a 5/8" socket on the front crank bolt that holds the harmonic balancer , and turn it by hand in the same direction that it would normally turn when the engine was running. When it won't turn any farther because the piston has contacted the piston stop, scribe a line on the balancer directly in line with the timing pointer or some other mark that isn't going to move. Then rotate the engine in the opposite direction untill it stops against the piston stop again, and scribe another mark on the harmonic balancer, in line with the same pointer. Get out your tape measure and wrap it around the harmonic balancer between the marks. Exactly 1/2 way between the 2 scribed lines is your exact dead center, in relation to the pointer that was used as a stationary witness mark. Then quickly, before you forget, remove the piston stop and put it away---if you forget and fire your engine with it installed, your engine is toast..