Thread Engagement
The intention of this article is to provide a basic knowledge of the thread engagement between a bolt and a cut tapped hole. It generally pertains to the standard steel bolts most commonly used on manufacturing machinery. Thread engagement here refers to three things, Percentage of Thread Engagement, Length of Thread Engagement, and Minimum Length of Thread Engagement.

Percentage of Thread EngagementTapped Hole Image refers to the cross sectional amount of thread available in a tapped hole. It is dependant upon the drill size used to drill the hole. The larger the drill size, the smaller the percentage of thread and the weaker the thread. The image on the right represents two threads in a tapped hole. Imagine the drill cut oversize at the beginning of the hole so the first thread has less percentage of thread engagement than the second thread.

The Inch & Metric Tap Drill Size Table presented on this website specifies drill sizes for approximately 75% of thread engagement. The drill must be sharp and the geometry properly ground for it to drill the specified size. If not, it could drill an oversize hole with the end result of less percent of thread. This may or may not be a problem, according to the Machinery's Handbook 23 Revised Edition on page 1653, "Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread."

Length of Thread Engagement refers to the distance a bolt extends into the threaded hole. More specifically, it is the axial distance over which an external thread is in contact with an internal thread. In other words, the length of full thread engagement. Corner breaks, chamfers, and threads not fully formed either on the bolt or the tapped hole do not count in that length.

One of the main guidelines in designing fastener joints is that the length of thread engagement should be long enough so that the bolt will break rather than strip the threads in the tapped hole if the joint is overloaded. It's generally easier and more cost effective to replace a broken bolt than to have to repair or replace the part with the tapped hole. This condition where the bolt breaks before the tapped hole threads fail is here referred to as the Minimum Length of Thread Engagement.

Minimum Length of Thread Engagement:  As long as the threads meet all of the proper thread class specifications, the length of engagement required to make sure the bolt breaks rather than strip the threads depends upon the strength of the material that the bolt and the threaded hole are made of. If a new bolt is used with a new properly graded nut for that bolt, the length of thread engagement is usually less than one bolt diameter. If the bolt is screwed into a cut tapped hole with equal strength materials, a length of engagement of one bolt diameter is normally strong enough. However, it is common for the bolt to be the stronger of the two materials. In that case, it is advised that the length of engagement be greater than one bolt diameter. Take low carbon steel for example.

Low Carbon Steel is a mild steel commonly used in building manufacturing equipment. It's used for things like machine bases and is welded together to form the structure. The most common rule-of-thumb for minimum length of thread engagement with low carbon steel is 1 1/2 times the diameter of the bolt.

Gray Cast Iron is another common material seen on manufacturing equipment. Gray cast iron is not ductile, or flexible, it's brittle!  Brittle materials throw another complication into the equation due to inaccuracies in thread spacing.

Theoretically all the threads in engagement should share the load. In actuality, due to inaccuracies in thread spacing associated with how ductile the materials are, the first pair of threads take the greatest share of the load. Since gray cast iron is brittle, if the first thread breaks then that load is passed on the the next thread and so on. It's not-as-common but the most common minimum length of thread engagement rule-of-thumb for gray cast iron is also 1 1/2 times the diameter of the bolt.

Accurately calculating the required length of thread engagement is a complex problem. The formulas for that require information related to the strength of the material of threads that meet proper thread class specifications. In the case of older equipment that has been maintained multiple times, there isn't always optimum thread quality and the strength of the material that the threads are cut into may be an unknown. In that case of the unknown, a common rule-of-thumb may be worth considering. After all, would it still be common in these modern times if it didn't work?
Note: Much effort has been taken to make sure this information is accurate. However, due to human error, the user agrees to accept full responsibility to verify that the information provided meets their requirements before using it.