Metric and Whitworth Thread Systems

Screw threads come standardized in 2 separate types:

Metric Threads

There are three types: metric coarse thread, metric fine thread, and metric pipe thread. The thread angle is 55° on Whitworth threads, while it’s 60° on metric threads.

H = 0.8660 . p
H₁= 0.6495 . p
d₂= d- H₁
d₁= d-2H₁
r = 0.1082 . p = H/8

metric ISO standard thread dimensions table

Metric Thread Angle

In the metric thread system, the thread angle is 60 degrees, and dimensions are expressed in millimeters.

Whitworth Thread Angle

In the Whitworth (inch) thread system, the thread angle is 55 degrees, and dimensions are given in inches.

Incompatibility Risk

Metric and Whitworth threads aren’t compatible with each other; a mismatch can damage the thread structure.

Whitworth (Inch) Threads

These are used more commonly in Anglo-Saxon countries (like the UK and Australia). They’re also the first standardized thread type in industrial history. There are three types: Whitworth coarse thread, Whitworth fine thread, and Whitworth pipe thread. Unlike metric threads, the thread angle is 55°.

In Whitworth threads, the peaks of the triangle forming the threads are truncated and rounded off by 1/6 of the triangle’s theoretical height.

There’s no gap between the nut’s threads and the bolt’s threads. For this reason, Whitworth threads are threads that provide a seal.

In metric threads, when the nut’s threads and the bolt’s threads engage, small gaps form at the roots of the threads. Consequently, the threads only engage each other on their flanks. In Whitworth threads, however, the nut’s threads and the bolt’s threads engage each other completely. Because of these properties, Whitworth threads are used more in pipe connections that require sealing. Metric threads, on the other hand, are only used as fastening screws.

For more on this topic, see: Metric and Whitworth Thread Tables

Related Questions

The two systems’ thread angle (60° and 55°) and unit of measurement (millimeter and inch) are different. These differences prevent the threads from seating fully into each other; if forced together, the thread structure can be damaged.

The most practical method is to measure the thread angle, or check whether the dimensions are given in millimeters or inches. To avoid confusion, the standard is usually stated on the product label or in the technical documentation.