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H1

Pipe Threads: Nominal Sizes and Dia.

【英】困ったときの知恵袋詳細
Cutting Taps Pipe Threads Standard (Screw Threads, Center Holes)
Major diameters of Unified threads, such as 1/4-20 UNC, 1/2-13 UNC, are expressed in inches. In the case of 1/4-20 UNC, I understand its basic major diameter is calculated on 1/4 of 1 inch (25.4mm), it equals to 6.35mm. However, the basic major diameter of external pipe threads G1/4-19 or the old standard PF1/4-19 is 13.157 mm or 0.518 inch that doesn't equal to 1/4 inch. Can you explain the difference of the major diameter of 1/4 UNC and 1/4 PF?
It is impossible to calculate the basic major diameter of pipe threads based on inch (25.4 mm). Perhaps there is a historical reasons to it. Unfortunately, there is no other way to determine the basic major diameter of pipe threads except to check the individual values on the standard of pipe taps.

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