A bottoming tap will clean up the threads closer to the bottom of the hole, it's not as tapered at the end as a regular tap. See a picture here, http://en.wikipedia.org/wiki/Tap_and_die. I don't know what size you need but if you measure the outside diameter of the threads that will be the M size. Count 10 threads, measure the length in mm then divide by 10. For example an M10 x 1.25 bolt is 10 mm in diameter (thread OD) with 1.25 threads per mm. If you can't find a bottoming tap at the hardware store McMaster-Carr has them, http://www.mcmaster.com/#standard-taps/=og51q1. See metric high speed hand taps. It sounds like you cross threaded the one hole by installing the bolt crooked, I did the same thing when installling headers and had to retap the hole. Check to make sure that nothing is in that hole by inserting a smaller bolt or wire, marking how far it went in then compare to another hole. If the depth is the same all you need to do is clean up the thread.

Sorry to bother you Fast one... I saw in a thread about a fussy header bolt not going in all the way you mentioned some advice about a bottom tap? I am ignorant to what why and how with self tapping clean up. I just installed headers and all the bolts went in fine except for the second from the back drivers side. It only went in half way then locks up. I took THAT bolt and tried in another location on drivers side header and it went right in.. Seems it's just this tap that has something keeping me out... Do you know what size tap I would need to get to try and clean it out? Hopefully I can drive to a hardware store for one. Thanks for your time...

Taryn