Hmm, I bet you regret doing that!!
I'm sure others will come up with ideas too but the first thing I would try is as simple as putting a dab of RTV silcone on the thread. Clean it up first. RTV silcone should seal the thread, withstand the heat and yet you will still be able to undo it later. The heat should not be too bad anyway seeing as how it is so close to the coolant.