Read about this in Chuck Lampmans' book. Since I am rebuilding my Packard engine, thought I'd do this to the cylinder heads--grind the area in the combustion chamber immediately around the plug tip down just enough so as to be able to use 3/8 reach plugs instead of 7/16". Has anyone here done this, and was it beneficial? What plug number do you use, then? Thanks for any help on this subject.