NPT in a single pass
A single-pass tap cuts the finished thread with one tool, without taper and bottoming taps. With tapered pipe threads that is the obvious approach, because the taper already spreads the cutting load over the threads. VÖLKEL makes 20 versions in form C from HSS-G, from NPT 1/16 x 27 to NPT 2" x 11.5.
What a single-pass tap saves
No tool change, no second start, no realignment. In repair, assembly and small batches that is the difference between one operation and three. It needs a cleanly prepared tap drill hole and stable axial guidance, because the tool gets no second attempt.
Form C and HSS-G
Form C has a short lead of two to three threads. On a tapered thread that is enough, because the taper itself ensures that each thread removes only part of the profile. HSS-G is ground high speed steel, dimensionally accurate and with a clean cutting geometry.
To depth, not to a stop
As with any tapered thread, torque rises with every turn. Cut to the standard engagement depth. Going further opens up the thread and loses the seal, which depends on the flanks wedging together.
Longer versions are under NPT machine taps, left-hand ones under NPT left hand, the British variant under Rc (BSPT). The differences are explained in the articles pipe thread types explained and NPT vs BSPT.
