Whitworth (BSW) Hand Taps HSS (Set of  three) 3/8.16 tpi Pitch .

Tapping drill Size 7.9 mm (for information  only)

HAND TAPS (ISO 529)
These are NEW professional quality ground finish straight flute general purpose  tools which can be used for both machine or hand tapping. They are generally  the most economical tool for use on production runs, but are best on materials  that produce chips, or where the swarf breaks readily. Where deep holes are  to be tapped, in materials which produce stringy swarf, other types of taps  may be needed, especially for coarse threads.

Hand taps can be supplied in sets of three; bottom,  second and taper leads, or individually (please enquire).

BOTTOM TAPS have a chamfer (lead) of 1–2 threads, the    angle of the lead being around 18 degrees per side. They are used to produce    threads close to the bottom of blind holes.

SECOND TAPS have a lead of 3-5 threads at 8 degrees    per side. They are the most popular and can be used for through holes, or blind    holes where the thread does not need to go right to the bottom.

TAPER TAPS have a lead of 7-10 threads at 5 degrees  per side. The taper lead distributes the cutting force over a large area, and  the taper shape helps the thread to start. They can therefore be used to start  a thread prior to use of second or bottom leads, or for through holes.

IMPORTANT NOTE ON TERMINOLOGY! In the    U.K. bottom taps are often referred to as ‘plugs’. In North America second taps    are often referred to as ‘plugs’! This can easily lead to confusion. To avoid    problems when ordering it is best to use the terms bottom, second and taper.
  Another variation is that second taps are sometimes referred to as ‘intermediate’,    and taper taps can be called ‘firsts’.