The objective is for them to maintain this clamping force after use.
Wobbly, what makes you think the bolts “lose” their preload capacity just because they have been slightly stretched? Just because they may have been somewhat “yielded” doesn’t mean they have lost strength or load capacity. They may have gotten longer but are still carrying the load that caused them to get longer.
The “yield point” of ductile materials is generally obtained during tensile testing at a point of 0.2% of plastic offset--that is, the material has already undergone some “set” just to obtain that basic information. When the test is continued, the specimen will continue to carry more and more load at much greater amounts of strain, and the ultimate strength is always greater than the “yield” strength.
The 0.31 mm elongation added to the bolt by the 75 degree turn amounts to only 0.4% strain--just barely(0.2%) past the minimal amount used to determine the yield point.
Consider:
1) That’s what Triumph says to do.
2) When you do that, with the bolt loaded up to that degree, both they and you are satisfied. The bolt is supporting the desired load in a slightly yielded form.
3) Removing the bolt won’t alter its form.
4) Reinstalling the bolt returns it to the same load condition that it was satisfactorily sustaining previously--so what’s the problem?
Based on the two make-up methods it seems that Triumph are shooting for about 6000 lb preload for the bolts and are using something like 75 ksi yield material to limit it to that neighborhood but insure that it is at least there with the 75 degree turn method.