Hmmmmm.... Whitworth....

More specifically, the threads.

Note that on the linked BSC reference site you can download a basic spreadsheet containing quite a bit of thread data. I’ve not used it myself and cannot vouch for the site at all but it looks alright at a glance.

Tools

  • Pitch Gauges - Generally come in the metric, imperial, and Whitworth flavours. You’ll want at least a metric and either a Whitworth or imperial set. The only difference will be the tooth profile (55° vs 60°). Either one gets the job done.
  • Vernier/Micrometer - If you know for sure that it’s a metric thread for example, then you can get away with a ruler (although not advised). But you can’t go too wrong using a set of calipers.
  • Thread Tables! - I initially planned on reproducing a load of thread tables here, but found most of them over on the TR fastenings website in appropriate format (all except BSC -_-).

Method

The general gist is to the use the pitch gauges to identify the pitch of the thread. Note that imperial gauges will fit metric threads for certain pitches for some pitches (and vise versa), and that 60° gauges will also fit 55° (and vise versa), sometimes well enough that it’s obvious to determine determine the thread profile angle and whether it’s metric or imperial purely from these measurements.

The second step is to measure the overall diameter of the bolt or thread that you can get at. i.e, for a bolt, just put the flat part of the calipers across the thread, and for internal threads, measure the hole inside of the thread.

Then, match this measurement to the tables:

  • For external threads, you’ll match your measurement to the major diameter column.
  • For internal threads, you’ll match your measurement to the minor diameter column.

On occasion it might be helpful to note that you’re going to be measuring a thread that’s had some use and is a bit worn, and the thread has perhaps been flattened somewhat. Try to pick a clean/nicely formed part of the thread to measure and expect your measurement to be slightly undersized (for external threads on bolts), from what’s in the tables.

More threads beyond this do exist, but these are the only one’s I’ve encountered in my experience of working on old British and Japanese things.

Good luck.

Edit: 2021-02-28

I’ve just found that over on trfastenings website they have all these tables. I just prefer scrolling through them directly as opposed to clicking between pages.