Water pump front port plug: Dorman 65256 (M14 x 1.25)

Comes with a fiber sealing washer which I swapped out for an aluminum washer.

I tried a 1/4" NPT pipe plug but it leaked and never really tightened up against the metric threads. I'm sure with enough teflon it would seal, but the Dorman part was available from Advance for less than $4