Larry,
I will have to agree with you...I have never seen a pipe thread that small. 1/8" pipe is VERY common and probably what you have. The next size up would be 1/4" but this would be too large.
When you buzz up to your friendly ACE hardware store, take a piece of your plumbing and do a side by side to make sure.
When you chase the thread, try to clean it up but not cut any deeper. The pipe thread is a taper thread and your sending unit or whatever might bottom out before becoming tight if you go wild with the new tap. Be sure to flush all the smegs out of the hole.
Stuart
