One problem in tapping is in holding the tap perpendicular to the work piece as you rotate it. If it gets started crooked, it can't straighten out without bunging up the threads. If the hole you are tapping is on a piece (as opposed, for example, the engine case) it can be helpful to hold the piece under a drill press, put the tap in the chuck, and turn the chuck by hand while lowering the tap into the hole, which assuming your setup is accurate, will give you a straight, square thread. Absolutely, positively, whatever you do, do NOT force the tap. If it gets hard to turn, back it out, lubricate, and gently re-engage. You haven't had real fun until you've broken off a tap in a blind hole.