Louis Ohland wrote:

Preparing to run coolant on my X3, I pulled the coolant plug out,
expecting an NPT thread. It isn't. It's a straight thread, Major
diameter is @ .620, pitch diameter is @ .577.

The Grizzly manual does not even show the plug, which is at the far left
rear corner.

Anyone with the answer?

Out of curiosity, have you tried emailing Sieg in China? Believe it or
not, the larger reputable Chinese manufacturers like Sieg do have people
who know english and respond to emails.