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Well .. thickness of the metal insert with relation to the strength of the fastening depends entirely on what size of threading you use.
There are very good resources for calculating the force you can apply to such and such a length of threading of a certain size .. the "ballpark rule" of at least four turns of a given thread is probably not far off for high tensile materials like Hard aerospace alu alloys, steel and titanium.. Bear in mind that the tightening torque of the nuts and bolts SUBTRACT from the available strength... this is also given in various tables, some of which can be found on the internet. A lot of people seem to forget that tightening the nuts and bolts do NOT make for a stronger "joint" It just helps to keep the bolt from undoing..
The force that a given thread can hold also depends very much on the thread tolerances, and proper sizing of the holes that you thread. Too small a hole drilled in Titanium WILL break your tap, even slightly too large a hole drilled in aluminium will produce a weak thread.
The rule I've learned is: for softer alloys the thickness of the fastener has to be twice the drill size for the thread (for standard Metric threads) Metric Fine and UNF / UNEF can go down to 1 - 1,5 x for thickness.
For harder alloys the rule says: 1:1 i.e. : 5mm Metric requires 4,2 mm thickness to keep the inherent strength.
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Do not adjust our mind, theres a fault in reality :-)
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