These DGX thumbwheel nuts are milled from a solid block of Delrin® thermoplastic. Delrin is lightweight and wear-resistant, with excellent dimensional stability. The outside surface is knurled to enable easier gripping and no washer is needed, so they can be easily adjusted or removed without a tool. Sold as a pair, the imperial 5/16-18 threads fit almost every brand of doubles bolts found in North America including DGX, Dive Rite plus the Highland bolts for 8-inch cylinders.
In the European Union and most other parts of the world the common metric bolt sizes are M8 x 1.5 or M8 x 1.25 and with force the imperial 5/16-18 threaded thumbwheels will screw down the metric bolts, but the threads will eventually strip creating an unsafe condition.