Doh, if theres no way to sort it, i'd drain coolant, tae thermo housing off, get the nozzle of a hoover in the opening behind the thermo housing to suck out any "swarf" and drill it out carefully, if you use the right size drill bit you should get away with running the right size tap through it to get the thread out. (thats assuming the temp sender unit can be viewed through the opening the thermo housing leaves? if not im sure the fragments of metal it leaves wont do any harm? pisser

P.s. Also assuming a drill can be fitted in that small space maybe a short "devil" cheepo electric drill?