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Concertina Hold Down Bolt Threads

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In replacing the chamois on the ends of a Jeffries, I discovered how poorly aligned the holes are that are drilled in the little pieces of brass that the end bolts screw into. I think these parts are called "sets"? Does anybody know the thread sizes for end bolts on vintage Jeffries and Crabbs for instance. And are they the same as Wheatstone threads? And finally, are there any sources for suitably sized taps and dies?




Ross Schlabach

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Like a lot of things that are out of sight the misalignment of Jeffries captive nuts only matters if it doesn't work ie. has broken through the side of the plate or partially or completely demolished one of the screws. They may have been drilled after the bellows was completed and the driller would not have been able to see the captive nuts.


There is no easy answer to your thread questions. Early concertinas predate standardised threads. Factories would make their own taps and dies to a pattern they invented, and would be the sole source of supplies. In the middle 19th century a fellow called Joseph Whitworth came up with the first standardised thread but factories did not immediately change to it and it was a little large for this application.


Consequently Jeffries threads are not the same as Wheatstone threads. As to whether Jeffries and Crabb threads are the same, I don't know, but it would be a very interesting thing to know in light of the debate as to whether Crabb made the early Jeffries.


This means there are no purchasable taps and dies for old concertina threads. However any thread can be described in terms of its overall diameter, the depth of thread cuts, the threads per inch, the angle of the cut and the shape of the top of it. Consequently a good machinist could measure these things and recreate a screw from an old one using a threadcutting lathe and I have seen Richard Evans do this for my own Jeffries. They can also create taps and dies.


If you want to replace the captive nut as well as the screw then any reasonably similar size would do, 7BA, M2.5, UNC 4-40 come to mind, and taps and dies are available for these.

Edited by Chris Ghent
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If you replace the captive nuts, irrespective of thread used, then you need to jig drill through the action box assembly, to set the bolt positioning. risking damage to the action boxes, and resulting in the same hole positions as before, or the bolts will not line up and engage with the new nut bores.


Leave well enough alone, if not broken, no fix needed



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