How to seal temp sending unit in block?

Was bleeding air out off 4.0 cooling system and stripped temp sensor on replacement. Bought another and stripped it also. Bought another, used some RTV, it leaked. Inspection of original reveals only about two rows of threads contacting and some kind of gunk used to seal. Threads inside head look kinda of worn, but how could brass threads do that to iron threads. Anyway what kind a of sealant would work best. Should the sensor be able to go in further, only a 1/4 of the threads are making contact, apparently causing my sealing problems.

can't you just clean up the threads? as I sit here and type, I can't for the life of me think of the name of the tool... must be early alzhiemers... :anon:

anyway, don't feel bad... I just bought me a 88 cherokee that had overheating problems, I changed the thermostat, and the idiot before me used a bolt that was too LONG for the hole... I wondered why it had two washers... well needless to say, I threaded the bolt on in the dark with out the washers... ended up pushing the bolt thru....water would leak out through the bolt hole, I filled the hole with gasket maker and threaded the bolt on tight... NO leaks... but still makes me mad.

First -- the connection to the block through the threads is part of the circuit - so RTV or other film-based sealants are off the list (if they work then the sender won't).

The threads are NPT (should show exactly which on the catalog look-up page -- if not take the sensor to the hardware store and find a NPT nipple that fits, I'd gues, but I'd be wrong) -- you should be able to locate a tap easily enough to chase the threads in the head -- they will corrode over time - especially if it's leaked before... A litle owkr there will restore and allow better sealing and part retention.

If you feel that you MUST seal (again - should not really be necessary with clean threads) try a liquid teflon product -- on NPT they will tend to drive out of the contacting area (just a few threads as you observed) so the electrical part will finction, while still leaving enough sealant in the outer threads to provide a little additiona sealing help.

Try some white teflon plumbers tape,wrap threads and install.All hardware stores carry it ,or any multi store(WalMart,etc.......)with plumbing section.Thats what I used on my sending unit and also oil pressure unit.

Ditto on the white teflon tape.