This is a subject that no one seems to talk about much about but was a big concern for me. So, for what it's worth, this is the way I drill holes when installing fittings. I'm sure I got the idea from listening to others but I can't remember who. What I do is make a drill jig using a piece of hardwood 2-inches thick with a nice smooth face. I use a drill press to drill a vertical hole in the wood just big enough to press-fit in a 2-inch long piece of stainless steel (SS) tubing with an INSIDE DIAMETER the same size as the hole in your fitting (Picture 1). Make sure there is enough flat wood area around the hole so you can clamp or hold it flush to the structural member your attaching the fitting to. Now using brass tubing I stack several pieces inside the SS tubing to reduce the inside diameter down to the pilot hole size (picture 2). The first would have an outside diameter (OD) the same as the inside diameter (ID) of the SS tubing the next would have the OD the same as the previous ID and so on until you get down the pilot hole size you like (Picture 2). Brass works the best as it is thin walled. The cool part of this is the tubing acts as an alignment pin for the drill bit by using the hole in the fitting. For example, drilling the holes for the tail wheel fittings I needed drill a hole for an AN3-17, this is a 3/16th inch hole. I used SS tubing with a 3/16th-inch ID (1/4-inch OD by the way) pressed into the wood block. Then I inserted two 3-inch long pieces of brass tubing bringing the ID down to 1/8th-inch (Picture 1 and 2). I then clamped the fitting where it needed to be, stuck a 6" long 1/8th-inch drill bit in my hand held drill. Letting the brass tubing protrude from the SS tubing I stick the brass tube in the hole where it will key its self in the hole (Picture 3). Then I press the wood jig against the fitting and drilled (Picture 4). Removed one brass sleeve, drilled with a 5/32nd drill bit next (or what ever the next ID is), removed the last brass sleeve and finally drilled with a 3/16th reamer or bit. Perfect holes all the time. Because the previous hole is so close the to final size it seems to follow along keeping it centered. Picture 5 is the fitting installed. The holes are so good that it take a bit of effort to get a bolt in there. I believe this works better then using a centering drill bit because the wood block ensures a perpendicular hole and the tubing keeps the bit centered. But best of all its cheap. | 
