When I put on my headers last year i was only able to get the 02 sensor half way in. (thought it was all the way in) well anyways i was under there today and notices i could have screwed it in anohter 3/8 about. so i scred it in some but after a while i couldnt get it in the last 2 theads. i can still spin the washer around. I noticed the car runs better now so should i attemp to put it all the way in. I dont want to crack it off. Any advise would be good. is it nessesary t ohave it all the way up?

 

The gasket is what seals it, although I would guess yours has some rust sealing it in place for now. I would see if you can clean up the threads and get it to thread in all the way.