British standard pipe taper (BSPT) internal thread is called Rc by the International Organization for Standardization (ISO). Rc refers to an internal, tapered thread that decreases along the length of the thread at a taper rate of 3/4-inch per foot.
A 1/8-inch pipe has an outside diameter (OD) of 10.30 mm. Its internal diameter is 9.0 mm.
The thread pitch is the distance between the threads. For 1/8-inch pipe, the pitch is .907 mm between each thread and 28 threads per 25.4 mm. The thread is normally right-hand tapped. Tapping pipe for left-hand assembly, is designated as LH in the thread designation.
The thread diameter for 1/8-inch pipe is 9.728 mm. It is an internal thread and tapers so the diameter gets smaller the deeper the thread goes into the pipe.
Information for Rc and other pipe tap information is available in charts, manufacturers' and printed sources.
