THREAD STANDARDS

Threaded things - nuts, bolts, holes, etc. - can be a source of puzzlement and frustration to owners of British motorcycles. The problem arises, not so much from the variety of thread types used in the motorcycle industry over the years, but management's failure to adequately reinvest profits into engineering and tooling. The ultimate effect of this policy was the industry's collapse; it also resulted in a mixture of old and new thread standards being used on the same model. The only way to be sure of what type of thread you are dealing with is to measure it.

Thread standards commonly used on British cycles include: British Standard Fine (BSF), Whitworth (BSW), Cycle, Unified Fine (UNF) and Unified Course (UNC), plus one or more pipe threads. Though there may be a difference in thread angle between systems - Whitworth is 55 degrees while UNF is 60 degrees - usually knowing the diameter and number of threads per inch will allow you to determine what thread you have or can use. Often two different systems that use the same TPI 's per diameter are, for all practical purposes, interchangeable.

Cycle, in 26 TPI, was commonly used when a fine thread was required for adjustment or security reasons. Nuts, bolts and studs with Whitworth threads are commonly found on older models (early Commandos). Nuts and bolts were often changed to UNF, a standard commonly used in the U.S., on later production models ( 72 and later Commandos). Tapped holes and their matching bolts/studs may or may not have been changed, barrel base studs on 750 Commandos went from BSW to UNF but the rocker spindle end cover bolts didn't.

THREAD CHART
Diameter UNF BSF BSW UNC Cycle Notes
1/8"     40      
3/16" 32 32 24 24 32 UNC & BSW interchangeable. Use UNF for Cycle.
1/4" 28 26 20 20 26 UNC & BSW interchangeable. Use BSF for Cycle.
5/16" 24 22 18 18 26 UNC & BSW interchangeable.
3/8" 24 20 16 16 26 UNC & BSW interchangeable.
7/16" 20 18 14 14 20 & 26  
1/2" 20 16 12 13 20 & 26  
9/16" 18 16 12 12 20 & 26  
5/8" 18 14 11 11 20 & 26  

The length of hex and cap head bolts and screws is measured from under the head to the far end of the bolt or screw. Flat head bolts and screws are measured from the top of the bolt head to the far end. Screws by the way are threaded right up to the head. If there is a space between the end of the thread and the underside of the head it's a bolt, not a screw.

Nyloc nuts are in common use, but they are supposed to be replaced after every use. Less common here are the Staytite nuts offered with our stainless hardware. These have a stainless locking ring built into the top of the nut, hold better than nylocs and may be reused dozens of times.

Great Britain went metric several years ago so don't be surprised to find today's aftermarket parts supplied with metric dimensions and hardware. Items that have to thread into existing holes will have the original thread style, but things like Lockheed racing calipers and the mounting bolts on our fiberglass seats use metric hardware. Of course Metric nuts and bolts come in both course and fine versions, and are not always the same from country to country.

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