Building stop to check TDC

I did something very similar. Knocked the insulator out of a used plug and ground out the insulator side a little and then punched in a 1/4 -20 bolt about 1 1/2" long in it, enough to stick out about 3/4" or so on the combustion chamber side. Then put a nut onto the combustion chamber side of the threads, tighten it down good. Then grind the nut down to the same size as the end of the plug so it can thread into the head. I've had the same tool sitting in my box for 25 years, works fine.