Percentage Strength of Number of Threads Engaged

To All:

Here is some validation for the old rule of thumb that 6 threads is all you need to be engaged in the work.

Best to visit the site because they have a simple graphic showing the stress on the threads.


As nuts and bolts are not perfectly rigid, but stretch slightly under load, the distribution of stress on the threads is not uniform. In fact, on a theoretically infinitely long bolt, the first thread takes a third of the load, the first three threads take three-quarters of the load, and the first six threads take essentially the whole load. Beyond the first six threads, the remaining threads are under essentially no load at all. Therefore, a nut or bolt with six threads acts very much like an infinitely long nut or bolt (and it's a lot cheaper).

Stress on bolt threads. Note how the majority of
the stress is on the first thread to the left.
Image from Spiralock.

Thread % %Sum
1 34% 34%
2 23% 55%
3 16% 71%
4 11% 82%
5 9% 91%
6 7% 98%

There is little point in having more than six threads in anything. Nuts with National Coarse threads typically have 5 threads in them, whereas nuts with National Fine threads have about 8 threads. Nuts are usually stronger than the bolts they are on, which is to say that the bolt will usually break before the nut strips.