Thread engagement calculator
An M10 8.8 bolt needs 16.6 mm of thread in 6061-T6 aluminium to break before the thread strips (1.66 times its diameter). In Grade 250 steel plate it needs 10.5 mm, and in a block as strong as the bolt 7.44 mm, where the bolt’s own thread sets the length. Type or pick the thread below; on the Joint tab, choose the bolt class and the tapped block’s material beside the drawing to get the length, which thread strips first, and how strong a shorter thread is, with a table of every material and class for the size.
Each thread shears off along a cylinder: the bolt’s teeth at the hole’s minor diameter, the tapped block’s at the bolt’s major diameter. A softer block needs more length in proportion to how much weaker it is.
How deep should a tapped hole be?
Deep enough that the bolt breaks in tension before either thread strips: a broken bolt is obvious and easy to replace, a stripped hole in a casting is not. The method in FED-STD-H28/2B asks for a shear area of twice the bolt’s tensile stress area along each thread. The bolt’s thread reaches it at a length that depends on the size alone, because bolt and teeth are the same steel. The tapped thread needs that area scaled by the strength ratio: an 800 MPa bolt in a 260 MPa aluminium part needs about three times the shear area the part would need if it were as strong as the bolt.
The numbers are minimum lengths of full-form thread at minimum material. Leave out the tap’s lead, the bolt’s chamfered end and about half of any countersink, then add the tap’s run-out below the bolt to get the drilled depth.