I'm not sure exactly what this thing is called, but it's what the coolant temp sensor screws into. The neck that the hose on the bottom slips over has a crack and is leaking water. Unfortunately, I didn't figure this out until I finished installing a new water pump and putting everything back together, so I still have to tear everything back down and pull it off. It's raining right now, so I figured I'd try to find out where I can get one while I'm taking a break.