Ask the Fastener Distributor | What is the Ideal Thread Engagement?

Your fastener distributor offers advice on all sorts of issues and problems related to fastener use, including this common question. What is the ideal thread engagement for any given application? While many answers are straightforward and direct, this one depends on the application and standards that apply.
What is Thread Engagement?
Thread engagement specifies the amount of threads engagement within the joint, and is often stated as a percentage of the total threads. Alternatively, some standards specify the number of threads that must extend beyond the outside surface of the nut.

What Amount of Thread Engagement is Recommended?
In the past, industrial fastener experts have recommended that bolts extend at least two threads beyond the outside surface of the nut. This provides a minimum amount of strength and structure within any industrial joint. However, according to the federal government standards, the “two-thread rule” is not always necessary.
According to the Federal Highway Administration guidelines for signs, lights and traffic signals, a minimum of 75 percent of the thread length must be engaged. No specifications are given for the amount of threads extending beyond the nut.
The rules change yet again when contractors and designers are involved in structural applications. As per the requirements stated in ANSI/ASME B18.2.6, the bolt length must extend to the outer nut surface or beyond, with full thread engagement along the bolt. Are you confused yet?
In simple terms, the specifications for thread engagement are dependant on the application. If you are working with non-structural components, such as road signs and lighting, a 75 percent engagement will do just fine. If, however, you are working with a structural application, consulting with the project engineer about the applicable standards for thread engagement becomes important.
Tell your fastener distributor about the specific application, and they will be able to direct you to the proper bolt length. Inadequate thread engagement may lead to loose joints and fastener failure. Avoid using generalities and find out the specifics about your application, choosing a bolt length and design that delivers the proper amount of thread engagement and strength.