I think I may have had a “eureka” moment today…and it dovetails with fleetwin’s post above: A gasket is a gasket & a screw is a screw, right? Not so fast!
Unlike most of you, I’ve had…over the years…a few ’50s and ’60s engines, but right now I only have one. My “pieces & parts” drawers are not as extensive as yours…so all I seem to have are four (4) plugs, and they’re all different head diameters:
At this point I can’t tell you which 2 were in this engine and which were in my parts drawer. But what’s even more interesting is my gearcase: the two race holes are not the same size. Yes, they’re both 3/8-16, but the race where the washer & plug head seat are different.
Lower plug hole race: 0.546″
Upper plug hole race: 0.610″
I have only one (1) plug that fits my lower…the 0.534″
…but my 0.546 plug will not fit in my 0.546 port.
It is entirely possible that my “seal” was metal-on-metal and the washer wasn’t compressed at all. This could be the source of my leak