I need to make a piston stop for a big block so I can check TDC with the heads on the engine. How many degrees before and after TDC would be best for the piston to hit the stop at? I'm a little concerned that the plug enters the head at about 30 degrees to the top of the piston so I could bend the piston stop if I'm not careful.

Doesnt matter, t can be anywhere.. Just as long as its positive. I always just put the degree wheel on rotate it until it hits. Write that number down rotate it back the other way until hits again.. now you have the number with simple math!

HI Mick
I bored a plug out, rounded a 8mm nut welded it inside the top, ran a piece of threaded bar through it and fitted a lock nut.
I bent one because the bar was protruding too far so i fitted another threaded bar and screwed it through only about 1/2" then turned the motor over and that worked ok without bending. Also, i radiused the end to stop it marking the piston.
Mick