Actually, the earlier head has a straight thread, same as a a 5/8" bolt, I believe, not a tapered pipe thread. The adapter is for a later model head with a 1/2" tapered pipe thread in it, to make it fit the sensing bulb for a mechanical gauge. I have seen people plug the small gauge hole and drill and tap the head for the proper pipe thread, but it is a tricky operation and the hole has to be drilled exactly right. Another approach is to put a later model upper thermostat housing with a threaded hole in it on the engine, and put the electrical sensor there, then use a short bolt and a copper disc to plug the original gauge hole.
Jerry
