[russcc]

It appears both NGK and Autolite use the same method for measuring "reach". It is likely standard across all mfgs. It is from the base where the gasket rests to the end of the shell, (the part that surrounds the insulator & electrode.
So the question is what plug has a 1/2", or .500. ?
I can live with the plug location over the exhaust valve, although it is not ideal.. With a 1007B cam, the exhaust valve lift at it's zenith is .345.. So from the top of the valve to the plug seat in the installed head is .610. he thread depth in the Navarro head is .500. So with a .065 gasket, the reach is.435, which exposes about 1 thread which I will chamfer out. That leaves about .055 ckearance between the valve and electrode tip. Thank you Fordbarners.