How many threads should protrude through a nut in an optimal joint?
The generally accepted answer is 1 to 3 threads. Most externally threaded fastener blanks are manufactured with a header point prior to thread rolling, which leaves the first 1 to 2 threads undersized for ease of assembly. To ensure full load carrying capability for a nut and bolt combination, this rule makes sure we have fully formed threads throughout the entire thickness of the nut.
One notable exception to this is for nuts with a locking feature at the top – or toplock nuts. This includes all nylon insert nuts. For these, it is best to have a minimum of 3 threads protruding through the nut to ensure that the locking feature is engaged on a fully formed external thread. Anything less could compromise the locking affect.
Is there a rule for the maximum number of threads protruding through a nut?
Too many threads is a waste of material, adds unnecessary weight and can be a hazard or cause interference with other components. However, functionally there is no downside to having too much thread protrusion.
When selecting fastener lengths, be conscious of the standard length increments. Metric fasteners are generally available in 5mm length increments up to 70mm and 10mm increments beyond this. Inch fasteners have similar standards. When choosing your fastener lengths, it is best to select the shortest fastener that will consistently give you 1-3 threads protruding through the nut.
For questions, please contact our Bossard engineering team at ProvenProductivity@Bossard.com.