by LROBBINS » 07 Dec 2021, 11:38
Although U.S. and metric taps are both in sets of 3, the two are a little bit different. In metric sets, the first tap, marked by a single ring, is both tapered and a bit smaller diameter. The second tap, marked by two rings, has a shorter taper and is slightly larger diameter. The third tap has very little taper (but still some) and threads to final diameter. The advantage of the metric scheme is that the drill size doesn't need to exactly match the thread throat diameter so you need fewer different drill sizes (though, for example, a 3.2 mm drill, not in most simple sets, is the correct one for a 4 mm thread). The disadvantage is that, in harder materials, you have to use all 3 taps one after the other.
U.S. sets are taper, plug and bottoming and differ only in the amount of taper, but all have the same final thread diameter. The advantage of the U.S. scheme is that you can use only one tap - taper for through holes, plug where there's enough bottom clearance, or bottom when there's no bottom clearance. The disadvantage is that you need a pretty complete set of odd-sized drills that match each particular thread - hence there are fractional-sized, number-sized and letter-sized drills in a full machinist's set.
This leaves out pipe taps, that are also different in metric and U.S. series although plumbing fittings are both given in inches, but the are inches of an ideal, laminar flow tube not of the actual tube or pipe we use. Metric pipe fittings, however, are straight thread and need copious sealant or teflon tape to seal them. U.S. pipe fittings have a pretty marked taper so need almost no sealant in order to get a leak-free connection.
With helicoil kits you really don't have to worry about this. The supplied drill is the right size for the supplied tap which is a bottoming style. If you break a drill bit, however, don't be surprised if it isn't a common 1mm or 0.5 mm bit, but something a few tenths of a mm different. The helicoil will screw in fairly easily, but it gets tightly pressed into the thread when the bolt is screwed in. Once in and after having a bolt inserted it's pretty hard to get them back out, with or without using loctite.