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Tunnel Light Stem - What thread?

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I've just bought a new (as in just manufactured) tunnel light. I have attached a picture of the mounting stem of the light and I'm trying to establish what size/thread it is. It comes with one thin nut but I want to put a second, locking nut on it. 


The diameter of the stem is 12.3mm (just bigger than M12) and there are 26 threads per inch.


Can anyone suggest a possible thread?








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Most likely  1/2 inch British Brass.  Often called British Standard Brass, except there is no standard!  Usually in 1/16 diameter steps from 1/8 in up to about 1 inch. Always  26TPI. Threadx angle is 55 deg, Whitworth form


I doubt you will buy a nut that size.  Tracy Tools will supply a tap.  Tapping drill is 11.8 mm or 11.9 mm


Could possibly be 1/2"  British Standard Cycle.  That is also 26 tpi, but the thread form is 60 deg Unified.  Again Tracy Tools should have a tap.  Tapping drill is 12.0 mm.


A BSC nut will probably fit on a BSB thread, but not vice versa.



PS There is an ISO metric standard fine thread which is 12.00 diameter and 1.00 pitch.  That is close to what you have but your is a bit larger than I would expect even allowing for manufacturing tolerances.


Edited by BEngo
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I think it is quite likely to be a BS brass thread which has a 55 degree Whitworth thread form. There is also a standard British cycle thread (1/2" BSC - 1/2 inch by 26tpi) but cycle threads use a 60 degree thread form. A long established cycle shop may have a suitable nut in their parts bin. This was the size that was used in days gone by for cycle front lamp brackets.

Specialist on-line suppliers that I have used include:

D. Middleton and Son


Apologies I realise that much of what I have written is a duplication of what BEngo wrote - should have read through the whole topic!.

For what its worth, I have found that one of the best suppliers of proper British standard fasteners is this firm in the USA - they supplied the studs for our K3:

British Tools and Fasteners

Edited by NB Alnwick
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