Can someone confirm the thread pitch on those various brass plugs as found around the inlet manifold and the small outlet of the water pump? The ones that are around 5/8" (0.625")? I can’t tell if they’re 18 or 20tpi (or something else?) using my Starrett gauge. One of the major suppliers identifies them as 5/8-18 UNF but I’m suspicious of that. I happen to have a die in that size but when I try to fit one of those plugs in to it, it won’t fit – unless forced. I ask here because one of the threaded plug holes on my inlet manifold (XK120) has slightly messed up threads from my having removed the plugs as part of an engine tear down. I believe I can still save the threads but only if I can be assured of using the right tap.