If it narrows things down a bit –

60º suggests metric or American thread.

4tpi X 60º suggests American (Unified) thread although the standard 2″ UNC is of 4.5tpi.

2.1/16″ = 52.39 mm, which does not seem a logical metric size. Even if allowing for wear, 53mm is not very logical either; but it would have a metric thread anyway.

So the original lathe could be of US manufacture.

As I have stated repeatedly … it is a simple matter to take a wax impression from a thread, and [at the scale being discussed] it  should be a trivial task to distinguish between the Whitworth and Unified thread-forms

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MichaelG.i

Agreed … but he specifically states:

it is hard to get my short thread gauge in and to see at the same time.

MichaelG.

Then Nigel’s logic seems sound to me

It is most likely for  an American spec. lathe.

MichaelG.

On 17 March 2026 at 17:20 old mart Said:

There is a possibility that it is not from a lathe, some dividing heads have threads.

I will ask about it on the Home Shop Machinist forum over the other side of the pond.

That’s not a bad guess “old mart” if I recall my Brown & Sharp dividing head spindle has a large diameter very coarse thread. I will check it in the morning as I have to remove the adaptor plate which I made so I can take a chuck off the lathe and bolt it straight to the D/head.

On 16 March 2026 at 20:04 Mark Davison 1 Said:

I tried both 60 and 55 degree thread gauges.  It is definitely not 55.  I could see enough of the first thread to be sure but it is hard to get a good view of the fit with the 3rd and 4th threads to see a subtle difference between 4TPI imperial 6mm metric.

Take an impression with some modelling clay, Platicene etc, or even some bread screwed up into a dough ball. Then compare that with your thread gauge.