The big three ....DORKS
They are all idiots. Simply measure it and tell them "I want a (insert size here) freeze plug" that eliminates their brain altogether.
They can't even do that. They'll have to convert to millidamnmetric and back again.
Years ago, when the "box stores" were getting popular, and not long after I'd "left the industry" after 15 years, the Kats heater popped out of a 351W I had.
So I went into ??? and he says "I can't look up application, I need a size."
So I said "you have a Kats heater catalogue?"
"Yes."
"Look up a heater."
He says "it says part no. XXXX no size. "
I reached over the cat. rack UPSIDE DOWN, and flipped to the back of the catalog, and told him, "if you look back here somewhere is a size chart."
GAUWUAUWD he was ticked off. But I DID get a freeze plug.