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Mine are have fallen apart and the replacement items from Rimmers look completely different. I have managed to find some online that look more consistent with the original style, but I don't know what the thread measurements are for our cars, or how to measure this myself? I see things online listed as having a thread of "8-32", "10-24", "6-40" etc but I am not sure which is appropriate or how to tell. I just wondering if anyone could help with the thread measurement, or how to measure it myself?

AFAIK there are only two types of door lock button, the earlier round one and the later square one (1980 on?). From your thread query I assume you have the earlier round one as the later square one is held on with a grub screw. Rimmers show the original as NLA but list an alternative round button:
http://www.rimmerbros.co.uk/Item--i-615888ALT

bottomtop wrote:Mine are have fallen apart and the replacement items from Rimmers look completely different. I have managed to find some online that look more consistent with the original style, but I don't know what the thread measurements are for our cars, or how to measure this myself? I see things online listed as having a thread of "8-32", "10-24", "6-40" etc but I am not sure which is appropriate or how to tell. I just wondering if anyone could help with the thread measurement, or how to measure it myself?

The thread sizes you list are ANC-American National Coarse.
Listed in size order starting with the smallest they are 6-40, 8-32, 10-24, the first number is the size no., the second one states the `threads per inch`.
The `size no.`is a bit odd to us as we expect a diameter(!)-the outside diameters that correspond to `6`, `8`, `10` are 0.138", 0164", 0.190".
I`ve just measured one of mine (the thread part) and it is 0.165" so that `looks like` size no. 8 but the thread is very very coarse about 18 t.p.i.??--so not 8-32!!
I do have to wonder if the original buttons were smooth bored and screwed on to the shaft in the manner of a self-tapping thread?

(1980 DHC,round buttons)

I hope that helps

Tap & die sets (everybody needs one to chase nuts and bolts) have metric and SAE thread measuring tools.

Thanks all for the replies. Robsport have some new old stock, so that's me taken care of for the next 35 years or so (if they last as long as the originals). FWIW it seems the same ones were fitted to the Dolomite and the 2000/2500s, based on less than scientific google images search during a moment of lateral thinking.

I have a 2500 that would be handy for comparison purposes, but my TR-7 has after market lock buttons of unknown origin, care of a previous owner......I'd ask him where he got 'em, but I'd need a crystal ball as he is sadly no longer among us.

Howard722 wrote:I do have to wonder if the original buttons were smooth bored and screwed on to the shaft in the manner of a self-tapping thread?

The NOS Items from Robsport have arrived and, indeed, they are smooth bored with no thread.