Here's why i never use clamps on any of my exhaust. Whenever i do exhaust on one of my cars i get it welded because clamps usually don't clamp evenly around a pipe and if you get it too tight it will bend the pipe and cause an even worse leak. It takes quite a bit of skill to be able to put together exhaust with clamps and not have it leak. Once you start cross threading something and it's hard to wrench it in, it's a good idea to get something to clean the threads out like a thread chaser or even a wire brush to get out all the little crud from the ripped up threads. Try getting a thread chaser that will fit your sensor bung and clearing out the threads so that your sensor will go in properly. You can leave it the way it is, but in the future you may have to replace it for some reason and find that the next one won't even go into the pipe. Good luck.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT