How’s that work? Finding the right Fred

By Hutch

We talk about our old nails in terms of aesthetics and ergonomics, power and torque, character and looking like it’s doing 100mph standing still; but when it comes down to fixing and keeping the things running it’s more tolerances, limits and fits, bearings, nuts and bolts.

The mechanics of a bolted joint are a complex combination, taking in metallurgy, metrology, stress, strain, coefficients of friction and other factors, but usually when trying to fix your bike what you really want to know is – what thread is this bolt or nut so I can get another or clean this one up in some way.

Identifying a thread requires only a few tools, reference data and a dose of common sense. You need to find the major diameter, pitch and thread angle.

Thread angle is very difficult to measure at home, particularly on smaller diameter threads, without larger pieces of equipment (like a shadowgraph machine) and can mostly be overlooked for ID purposes. The majority of threads are either 55 or 60 degrees and occasionally 47.5 degrees for BA (British Association threads).

Pitch is the distance between two adjacent peaks, or troughs on a thread – this is not entirely true but good enough for identification purposes if not in metrology terms.

Pitch is expressed in either distance (Metric, BA) or TPI (Threads Per Inch) for imperial threads and is easily measured with Pitch or TPI gauges.

Read more in the June 2019 issue of CBG – on sale now!

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