Metric or Imperial Threads

Someone commented to me the other day whilst walking around a car museum in the UK that they couldn’t cope with all of the different imperial threads and the array of spanners that you need to work on the cars, metric is so much simpler.

This got me thinking.

Metric threads are easy to use as I only need a fairly simple array of taps and dies and a simple selection of spanners, but is that the only thing that matters? What about the design of the thread and the ratio of diameter to pitch? Some of the imperial threads just look better.

BA threads in the model engineering world are just exquisite, especially with the option to buy the nut and bolt with a size smaller hex size. The metric equivalents just look clumsy in comparison, but are they a better thread?

Possibly the best thread of all is the Whitworth thread, Sir Joseph Whitworth proposed this thread in 1841. The first standardised thread form. The British Standard Whitworth (BSW) thread has an angle between the thread flanks of 55° and the thread has radii at both the roots and the crests of the thread. However, this is now an old thread form.


There are a couple of close matches between metric and imperial threads. Should you ever come across 1/4″ BSF threads on old British equipment (1/4″ x 26 TPI), the nuts will screw onto a 6 x 1 mm metric bolt (25.4 TPI) with a slightly loose fit.

A 20 x 2.5mm metric thread is 10.16 TPI and so a metric nut should screw onto a 3/4 x 10 UNC bolt, once again with a slightly loose fit. Neither of these examples is a good enough fit to be used.

There is a similarity between 5/16″ x 18 UNC and 8 x 1.25 metric (16.9 TPI). In this case the metric nut will start on the 5/16″ bolt and vice versa, this will lock after a few turns. This could still be very dangerous as someone will gorilla the last few turns, damage the thread, think they have tightened the nut and leave it.

A-325 structural bolts, 3/4″-10s, are very close to 19 x 2.5mm – this is an unusual metric thread, but they are available.

About Nigel 229 Articles
Have been making models since I was around 7 years old and using a lathe from the age of 11, a self taught engineer with a passion for making model engines.

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