I'm threading a high silicone cast aluminum using a Guhring 1/4-28 UNF STI 3B Modified Bottoming TiN coated tap about 7/8" deep. I'm using the same type of tap (5/16-24 and 10-32) with no issues. However, with the 1/4" tap, my GO gage will not go all of the way down. I've tried chasing it several times with a new tap but no success. I'm wondering if there's anything I can try in the machining process to help cut the thread to size.
FYI: I'm circular interpolating the minor diameter to the high end of tolerance. I was worried that the drill was wondering and creating a "bent" and/or out of round hole.