Bolts, Threads & Fasteners

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Mar 7, 2005
What is the general theory behind coarse threaded bolts and fine threaded bolts/nuts?

Are coarse threads used in high load environments and fine threaded fasteners used in high vibration environments?

When re-installing my headsteady bracket on my 850 MkII I noticed that the 3 bolts that secure it to the head use only flat washers. Would stainless steel spring washers be useful or is that introducing an unnecessary component? Or is Loctite a better option?

Getting loctite on the middle bolt and not make a mess would be challenge.

Anything in conact with the engine is in a vibrating environment!

Because a bolt with a fine thread has a larger cross section than one with a coarse thread, the fine threaded bolt will withstand a higher tensile load before shearing. Bolts used in aircraft engines are all fine threads.
If the thread is in a cast material such as cast aluminium or cast iron it is better to use a coarse thread as there is less chance of the thread stripping in the component.
Unfortunately if a thread has a fine pitch it is easier to overtorque it and snap it off. (the lower helix gives you a greater advantage when tightening the nut.)
Basic engineering rules say to use coarse threads in softer materials and fine threads in harder materials. Fine threads can take a higher load. If threaded into aluminum or cast iron, coarse threads are less likely to strip or come loose.

Of course, convenience and availability over-rule engineering, which is why the 26 TPI "Cycle" threads are so common on Nortons. There were a lot of screw-cutting machines that only had one setting!
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