Originally Posted by
graywolf
That 26tpi stuff is not custom, it is just the old Whitworth thread that the British used from about 1700 or so (Actually, 1841, IIRC), until after WWII.
This is a common misconception, because Whitworth tools fit the nut and bolt hex sizes of 26 tpi fasteners.
26tpi is BSC British Standard Cycle. In most sizes this is a rather fine thread, intended to avoid loosening from vibration on cycles, motorized or not. Whitworth is a much coarser thread, similar to UNC with 55 degree threads vs, the 60 degree threads of the BSC.