Thread engagement chart
How much thread engagement a bolt needs to reach its full strength, based on the material it threads into. Too few threads and they strip before the bolt does; the softer the tapped material, the more engagement is required.
Minimum engagement by tapped material
| Tapped material | Minimum engagement |
|---|---|
| Steel (equal strength) | 1.0 x diameter |
| Cast iron | 1.5 x diameter |
| Aluminum, brass, bronze | 2.0 x diameter |
| Magnesium, soft alloys | 2.5 x diameter |
| Plastic | 2.5 to 3.0 x diameter |
Engagement length examples
| Bolt size | In steel (1x) | In aluminum (2x) |
|---|---|---|
| 1/4 in | 0.25 in | 0.50 in |
| 3/8 in | 0.38 in | 0.75 in |
| 1/2 in | 0.50 in | 1.00 in |
| 5/8 in | 0.63 in | 1.25 in |
| 3/4 in | 0.75 in | 1.50 in |
| M6 | 6 mm | 12 mm |
| M10 | 10 mm | 20 mm |
| M12 | 12 mm | 24 mm |
The aim is for the threads to be stronger than the bolt, so the bolt breaks before the threads strip. In steel of similar strength to the bolt, about one diameter of engagement does this. Softer tapped materials spread the same load over weaker threads, so they need proportionally more length, up to two or three diameters.
Setting torque or sizing the thread?
See the Bolt Torque Calculator for tightening values, or the Thread Pitch Chart for thread counts.
Why engagement depends on material
A bolt threaded into steel of equal strength needs only about one diameter of engagement, because the threads are as strong as the bolt. Thread the same bolt into aluminum, which is far weaker, and the threads would strip at a lower load, so you need roughly twice the length to spread the load over enough material to match the bolt strength.
Tapped holes vs nuts
A standard nut is proportioned to develop the full strength of its bolt, so engagement is rarely a concern with nuts. The question arises with tapped holes in a part, especially soft castings, where too shallow a hole lets the threads pull out. When in doubt, deepen the hole or add a thread insert.
FAQ
How much thread engagement does a bolt need?
About one bolt diameter in steel of similar strength, and roughly two diameters in aluminum or other soft metals, so the threads do not strip before the bolt fails.
How deep should a tapped hole be in aluminum?
Around two times the bolt diameter of full thread, plus a little extra for the tap point, so the softer aluminum threads can carry the full bolt load.
What happens with too little thread engagement?
The threads strip out at a load below the bolt strength, so the joint fails early. Increasing engagement length or using a thread insert fixes it.