I had a new, old stock spark plug... For my 1972 Cortina that I sold back in 1986!
Thought it may be useful, so got busy with the milling machine and the lathe. And then discovered a "gotcha"
The plug hole has an internal lip at its outer side, so when I had made the "timing pin" to run as vertical as possible, it would not go in the hole. It does now, I tapered it...
My pin is just a couple of degrees off vertical, and that is insufficient to need to do trigonometry to get the "right" reading, so the pin is calibrated in 16ths, and at the top, 32nds...
The 4mm pin runs in a small brass tube, with a slot milled in the side of the spark plug to enable the tube to run at as much of an angle as possible. The tube is the "Araldited" into the spark plug body.
There is no chance that the pin could jam in the hole. Quite the opposite... As the piston comes up, it "blows" the pin out of the hole! Nuisance, but a good safety feature, so I am not tempted to drill another hole through to relieve the air pressure.
I've gotta pull my mag off... Oil is being a "Houdini" again, where the mag joins the rear of the timing cover.... Hence I made the "timing pin".
Greybeard, just saw your other post re not happy again...
Real sorry to hear that...Be careful with those £175 "diamond encrusted" cam followers!
Cheers,
Stan.