Yikes, that is a lot of number crunching.
If the threads fail in the FA head I will simply Helicoil with 3/8 UNC but to the full depth of the hole.
Choice of 3/8 UNC is a good one as my numbers show it has the largest thread tensile and shear stress areas of thread forms available.
Of the various inserts available, Heli-Coil inserts win hands down, as shown below.
Cylinder head bolts are subjected to fatigue. Fatigue strength of a bolted connection in a semi-rigid aluminum block material (esp. 7075 alloy) has been shown to be essentially identical for assemblies with and without Heli-Coil inserts,
proving that the Heli-Coil inserts’ flexible coils do allow the excellent ductility characteristics of the aluminum parent material to be transferred to the bolt. Good ductility is one factor in improving fatigue life.
Rigid solid bushings, unlike the flexible Heli-Coil inserts, cannot distribute the load over the entire length of the engaged threads, so bolt fatigue life is reduced.
In semi-ductile aluminum alloys such as rolled 6061 and wrought 7075, the number ot effective furns (pitches) to achieve full strength of the parent material is 6, as for steel. This is seen in the figures of the report (see p.8). 6 * 16% ~ 96%. In ductile alloys such as A356 and RR53,
the number of turns required to achieve full parent material strength will be greater, but unlikely more than 10. For a 3/8" UNC threaded joint, this translates to an effective treaded hole length of 15.9 mm.
See enclosed extract of Stanley Technical Bulletin 71-2.
- Knut