With any luck I'm going to swap my clutch this weekend, and I figured I'd either recap, plug, or repurpose this vestigial port on the rear coolant neck. Searching only brings me to mentions of replacing the rubber cap with an OEM piece, or leads me to the coolant port on the side of the block.

So, the local Mazda dealer does not have a factory rubber cap in stock and can't get one by Saturday. Anyone ever used a hardware store substitute? Or know what the ID of the cap needs to be so I can give it a shot?

Alternately, I was wondering if my Autometer temp sender will fit in that port instead of the cap/plug. Anyone know the thread size and pitch of that port? Or I may just plug it with a bolt if the temp sender won't fit.