According to the great Racer Brown , it can be as much as one degree of rotation( out of the 360) that the pistons hovers at TDC. The only way to be exact is with dial indicator with the head off. Using a dead stop to get exact TDC can be a challenge in that you might not add and subtract correctly with your degree wheel .

And timing can make a huge difference in performance - whether valve or ignition timing . 

Thanks everyone for the help . Great to get advice from fellow Z owners

i gutted a spark plug and installed a bolt through the center with the end ground smooth.  I then turn the crank slowly until the piston stops from the stopper.  The length of the stopper bolt should cause the piston to stop 5° - 10° from TDC.  I note the number on the degree wheel and then rotate the crank backwards until it stops again.  Again, I note the number on the degree wheel.  I rotate the degree wheel on the crank the number of degrees it takes to make the CW and CCW numbers match.  I then do it all again to double check that the degree wheel is exactly zeroed at TDC.  From there, I can start checking timing per the cam timing card.  I use the eccentric sleeves from a SBC as described in the How To Modify Your L Series book. 

And then there is this ...



https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F201984173362

I have one and it's very useful.

Ps. I LOVE the home made spark plug idea.