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7/16" X 30 tpi thread

Anybody able to identify?

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Swarf Maker05/10/2017 18:05:27
72 forum posts
4 photos

I don't want people to spend hours trawling the net or whatever, but I have not been able to identify this thread standard. It is used to adjust clutch lift on a whole series of pre-WW2 motorcycles that use the smaller of the Burman gearboxes. I am completely confident of the sizes quoted as the measured diameter is 2.5thou under 7/16" and the Moore and Wright Whitworth 30tpi gauge fits snugly along its entire length. I can readily deal with both internal and external single point screw cutting for replacement components, so it's no big deal. I am just intrigued as to whether there is a standard that applies. Also interesting that 30tpi doesn't seem to appear very frequently. Anybody know this one off-hand?

Chris Evans 605/10/2017 18:39:21
1442 forum posts

I play around with pre war bikes and do come across a few manufacture specific threads. A lot of 1920s Douglas motorcycles had 25 TPI threads and not the more usual 26 TPI cycle thread.

alan-lloyd05/10/2017 18:50:30
157 forum posts

The vintage motor cycle club will have the answer i'm sure, burman gear boxes were fitted to many old motorcycles as i'm sure you know already, good luck

David Standing 105/10/2017 19:58:03
1240 forum posts
45 photos

Prior to the BSC/CEI standard, it was a free for all and as stated above, manufacturers went with whatever suited them. 7/16 x 30 is unlikely to fall into any standard.

Swarf Maker05/10/2017 22:49:03
72 forum posts
4 photos

Thanks chaps. Good to know that I am not alone in unearthing these obscure threads. It's a bit of industrial archaeology in a way and is useful when restoring things to understand a little of the likely thinking of the day. I have come across the Douglas specials myself but such things always make me wonder, was this a ploy to keep a hold on the spares supply market or was the designer being careful to ensure that the chosen thread met exacting engineering criteria? I suspect that it's best not too think too deeply and just get on with recording such original features, reproducing them when the pain is not too great, and modifying things if that is the most appropriate course of action. For this job things will remain as designed.

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