When using a piston stop in the spark plug hole be sure to loosen off the rocker nuts completely on that cylinder
so the valves do not move while using the piston stop.
Or the moving valves will hit the piston stop.
You can make one or buy one.
I make my own using a old spark plug body and a 5/16" or 3/8" steel tube. (steel fuel line)
Weld or braze the tube into the hole of the gutted spark plug body. May need a slight curve on the end. Trim the length of the probe as required. Do not weld the tube shut.
You want the stop to contact the piston before TDC.
Only turn the crank BY HAND. one direction, then the other way and mark the stop points on the balancer. Do not jamb the piston against the stop.
True TDC is right smack in the middle of the two stop points.
Either remark the balancer with a file, then paint a white line on the now TDC mark or move the tab to correct it.
Even factory balancers and tabs are fubared.
reset the rockers after you are all done. Useing the IC EO method.