How Thread Engagement Works
A bolt in a tapped hole can fail two ways. The bolt body can break. Or the threads in the hole can strip. Which one happens first depends on how many threads are engaged. The more threads engaged, the harder to strip.
The 1D Rule
A simple rule says engagement length should equal the bolt diameter. For a 1/2 inch bolt, engage 1/2 inch of threads. This works when the bolt and tapped material are the same strength. It does not work when the tapped material is softer.
Soft-Material Threading
A steel bolt into aluminum needs about two times the diameter of engagement. Aluminum has about 40 percent the tensile strength of steel, so you need more threads to carry the load. This calculator uses FED-STD-H28/2B to find the exact length. See our threading standards guide for more on thread types and specifications.
Pro tip: If your tapped material is weak, use a threaded insert like a Heli-Coil or Key-Sert. A hardened steel insert in aluminum gives you the full bolt strength at 1D engagement.
Pull-Out Force
Pull-out force is the load at which the threads strip. It depends on the thread shear area and the shear strength of the tapped material. Shear strength is about 60 percent of tensile strength for most metals. This calculator shows pull-out force so you can compare it to the bolt strength.
When to Use This Calculator
Use this calculator any time you thread into a material that is not the same as the bolt. Examples: steel into aluminum, steel into plastic, titanium into titanium with a different grade. The calculator shows the minimum depth to reach full bolt strength.