I am have a MAC O/R H pipe but it has the remnants of an O2 sensor from another mustang. I could not for the life of me remove, stuck in one of the bungs. The head is rounded off pretty good so I was thinking it may need to be drilled out? any idea? needs to come out today!

 

Did you try using PB Blaster on it? or at least WD-40? Any kind of penetrating fluid should help with the removal process. If the head is too rounded and you cant even get a wrench on it, try using vise grips. If that fails, you can either drill thru it, as you suggested, which is kinda risky, or you could shave/saw/sand down the exposed end of the sensor to where its almost flat and weld on a nut. If you don't have any easy way to get it welded on there, JB weld could possibly work. I've heard of some JB weld products that adhere pretty well in only a few hours. Anyway, good luck!

 

heating it will also do alot

 

^+1, use an oxy-acetylene torch to heat the bung up to cherry red--grab it with vise grips and spin it out. Any exhaust shop should be able to do this for pretty cheap, they deal with it every day...