A form tap has no flutes or cutting edges. It creates the threads by displacing the metal into the shape (basically makes a thread that is strong like a forging). Because it displaces it requires a slightly larger tap drill. Since it has no flutes (some have a reliefe for fluid to escape blind holes), it is a much stronger body tap which is a great thing in the really small stuff.
Balax makes some great form taps
http://metalworking.mscdirect.com/CG...PMPXNO=1907339
PM me your addy and I'll send you a small tap drill chart that goes down to 0-80 and shows form tap drills. One of the tooling places I buy from sends a chart with every order, I have a few around the shop, could spare one.
Tap drill for a cut type tap 0-80 is listed as 3/64 which is .0469", the #60 is .0400" so you're right in there. If it were a forming tap it would need to be drilled with a #54 which is .0550" And form taps a a bit more particular about using the proper size tap drill due to the way they work.