I think that one is actually a 1/4" pipe thread. Ditto on using the petcock- - - -it's a lot easier to drain the block or do a flush by just opening the wing nut valve. BTW, that's not a good place to put the temp gauge. There's an adapter that needs to go to into the hole inthe head, and the temp gauge bulb screws into the adapter. The block thread will leak if you try to put the gauge tube in without the adapter, and the hole in the block isn't big enouth to accept it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!