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I have an old Elswick Hopper, and would like to know the size/thread of the rod brake eye bolt nut?

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    It could be anything. A modern reproduction bike is likely to have metric threads, but this bike could be imperial/sae or whitworth or some weird custom and proprietary thread. Do you have the bolt? If so, you might be able to measure it.
    – Criggie
    Oct 24 at 11:07
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I'll steal Criggie's comment and expand on it.

The best way to know the diameter/thread size is to measure it. The most likely thread standards for this bike are Imperial, SAE and Whitworth. You are on a fishing trip so use any tools you have first. It is probably not metric but it's good to check.

You can get a pitch gauge that covers Metric, Imperial, SAE and Whitworth threads

Information taken from "Thread Type Guide: NPT, BSP, JIS, SAE, Metric"
Tools Needed:
Caliper –
A caliper is a useful tool that measures the outside diameter of a male thread and inside diameter of a female thread. Using a caliper will give you the most accurate and precise measurements, but a straight steel ruler is a good alternative.

Pitch Gauge –
A pitch gauge measures the threads per inch. For metric threads, this tool measures the distance between the threads.

Things to measure:
Pitch Size
The next step in identifying your thread type is to determine the pitch size. A thread’s pitch size is the number of threads per inch or the distance between threads on metric thread types. While a ruler could be used to calculate the pitch size, a pitch gauge is highly recommended as pitch sizes can be very similar.

Thread Diameter
After you’ve figured out the pitch size, you’ll need to determine the thread diameter. Using the caliper again, measure the outside diameter on a male thread and inside diameter on a female thread.

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