Technical Information ISO Pipe Tap Thread Systems
The International Standard Pipe Tap Thread System (ISO) has been derived from the original Whitworth gas and water pipe tap threads, formerly
known as BSPF (Fastening) and BSPT (Taper), these systems have been so widely used throughout Europe and the United Kingdom that they have been
metricated, whilst still retaining the Whitworth thread form. These popular thread systems are the basis for the ISO parallel “G” series and the taper “R”
series, these systems are endorsed and in agreement with the current British and Australian standards. For comparison, the pitch diameter tolerance
zones are given for both the parallel and taper systems.
“G” Fastening Parallel Pipe Threads – ISO 228, AS1722 PT2 and BS2779.
This parallel thread system has only one positive internal thread tolerance and two classes of external tolerances. This series constitutes a fine series of
fastening connecting pipe threads for general engineering purposes, the assembly tolerances on these threads are such as to make them unsuitable for
pressure tight seal by the threads themselves. For the conveying of fluids, the seal may be produced by gaskets, flanges, or “O” rings etc.
INTERNAL
THREAD
internal
thread
G
GA
EXTERNAL
INTERNAL
THREAD
THREAD
external
thread
GB
basic pitch
BASIC
PITCH
diameter
DIAMETER
G
GA
GB
EXTERNAL
THREAD
BASIC PITCH
DIAMETER
taper
parallel
TAPER
BASIC PITCH
DIAMETER
(at gauge plane) RC
PARALLEL
R
MAXIMUM
INTERNAL
PITCH
DIAMETER
MINIMUM INTERNAL
RP
PITCH DIAMETER
maximum
basic pitch
BASIC
PITCH
diameter
DIAMETER
(at gauge
guage plane)
(at
plane)
MAXIMUM
internal
INTERNAL
pitch
PITCH
diameter
DIAMETER
minimum internal
MINIMUM
INTERNAL
pitch diameter
PITCH
DIAMETER
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“R” Sealing Taper Pipe Threads – ISO 7, AS1722 PT1 and BS21. The taper rate is 1-16 on diameter.
This series is for tubes and fittings where pressure tight joints are made by threads, these threads therefore must have a full form profile (no truncations).
TAPER
PARALLEL
The series include a taper
external thread (R) for assembly
with either taper internal (Rc) or parallel internal (Rp) threads. The Rp series has a unilateral
tolerance (+/–) which normally requires a special below basic low limit tap, to allow for sizing deviations at the start of the internal thread, the size is
gauged at this position, with an Rc taper gauge. The low limit Rp tap size, allows a minimum accommodation length to be machined, with an equivalent
material saving possible. RC
R
RP
477