Any time you install/re-install an O2 censor run a tap/thread chaser into the bung. New or used headers, it doesn't matter ..chase the threads..especially on coated headers (ceramic,chromed etc.) They need to have this done.
I speak from experience. That would be the hardest (but most enduring) way to learn something.
on sale right now
[ame="http://www.amazon.com/Lisle-20200-Thread-Chaser/dp/B0009OMYCO/ref=sr_1_1?s=automotive&ie=UTF8&qid=1421175566&sr= 1-1&keywords=Lisle+20200+Thread+Chaser"]Amazon.com: Lisle 20200 Thread Chaser: Automotive[/ame]