Should be an easy question but I'm doubting myself.
This fitting looks like it's taped in the oil cooler. I THINK it should be used for a transmission oil temp sensor or can be capped off. I haven't removed the original plug and it's leaking coolant from the threads so I figured rather than capping it with a rubber cap and clamp, I'd use it for tranny temp?
Or would it be used for engine coolant temp sensor?

 

If coolant is leaking from the threads, it is probably not a location that will provide an accurate indication of transmission oil temperature. Given its location in the radiator (what is often the "cool" end) it would not be an accurate indication of coolant temperature either.

It appears to be a somewhat universal radiator, intended for as many applications as it can be screwed, welded, bolted, or baling-wired into. Those potential fits could include those applications with an external expansion tank, and that fitting could be present to accommodate the small return lines from those.

 

In lieu of a cap and clamp, there appears to be sufficient material in that welded-on boss to tap the thread for a different fitting or plug, use thread sealant, and solve that leak.

 

Makes sense. No way it'd be an accurate temp anyway.
I like the tap and rethread for a good plug with some sealant idea.
Thanks!

 

If it's leaking coolant, then the plug goes into coolant; and any temp sender you put there, will measure the temp of that tank of the radiator. Either the hot tank or the cold one. Obviously the hot tank has the hot rad hose going to it, and the cold one the other. You should be able to feel the difference by hand with the engine and fan running, or use your IR temp gauge. It will not be at trans temp in any case; and if it's the cold side, which seems more likely, the temp indication wouldn't be very useful anyway.

That fitting looks like it's intended to have yerbasic worm-gear-clamp type of connection on it, if needed; most likely a heater hose.

I don't see any problem with the plug you have not being "good", butt off course it's all covered with tape, so ... You could just pop it out, and get a new one if you think it's not "good", and re-tape it, or use PTFE thread sealer if that's your preference. I don't think I'd mess with tapping the pipe threads unless they're definitely fornicated, like to the point of being visible, in some way. OTOH if it's leaking around whatever is jammed into the hose nipple that the plug then screws into, that's a different matter yet again.

 

That's a heater hose nipple. Since you don't need a heater hose at that location, just pull the plug and reseal the threads.

 

Looks like it might be a Mishimoto radiator? In any case, Mishimoto (and probably others) use that threaded port for the temp sensor on their electric fan controller. Also looks like the fitting can take a heater core hose and clamp, for plumbing configurations that use that setup. Since it sounds like you don't need this connection, I would suggest draining the radiator down below the level of that port, remove the threaded plug, clean off the sealant or teflon tape, use new thread sealant, reinstall and refill the radiator.