We are frequently asked by end-users to provide a pulley tap to produce a longer thread length than what is on the tap, but in all cases the standard pulley tap has a shank diameter that is greater than the tap major diameter, and therefore cannot be used to produce longer thread lengths without altering the shank diameter.
They are indeed great for producing threads where reach is required to get to where the tap begins to thread, but will not normally produce longer lengths of thread.
Hand taps; spiral point, spiral flute, and form taps are all commonly made to USCTI Table 302, or 302A in the United States.
All of these taps have shanks that are larger than the major diameter of the thread itself up to and including 3/8.
Anything over 3/8 will have a shank diameter that is smaller than the thread major diameter, and therefore can be used for producing threads that are longer than the standard tap thread length.
The truth of the matter is that all pulley taps intentionally have shanks that are bigger than the thread major diameter on the tap thread, because they were originally designed to thread screw holes in pulleys, and the larger shanks were used to guide the tap thru the pulley and into the center of the pulley to be threaded.
Extension taps (4 and 6 inch long) also are nothing but hand, or spiral pointed taps with a longer shank than normal. They also are made for reach.
The shanks on all standard extension taps follow the same rules per USCTI table 302.
They are supplied with the same size shanks as all other taps of the same sizes, taps 3/8 or smaller must be ordered special to have shanks smaller than the tap major diameter.