Thanks, guys. The adapter is basically frozen in the head. The valve cover gets in the way if you want to put a socket or box end wrench on it. Using an open end wrench to remove it would just lead to bleeding knuckles, bad language and a rounded adapter! I was able to clean it up using a 1/2" drill (I just carefully put it in and twisted it with my fingers). So I will put the sender back, install the coolant and see what happens. If it leaks, I will take it out and try a bit of RTV...
