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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?

 

Thanks

 

 

 

 

IMG_20210518_114813.jpg

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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.

 

N

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.

N

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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