As ubiquitous as threaded fasteners are, I have always found it ironic that most people  (including most engineers) hold significant misconceptions about their use and installation.

Some fastener facts:

Maximum fastener performance is achieved when the fastener is installed with it's tensile preload is into it's yield range.

Accurate fastener preload can only be achieved through fastidious attention to installed conditions OR by measuring overall stretch.

A properly preloaded fastener gets no benefit from retaining devices (split washers, star washers and even (gasp) safety wire). These devices are only used to give the pilot a little extra time to find the "never properly tightened" or "over stressed in use" fastener before it fails. This is why you NEED to do nut and bolt tightness checks periodically.

Because preload and torque are related by the coefficient of friction between the mating threads and the fastener and the part land AND the coefficient of friction varies widely with surface conditions and lubrication IMHO, for most situations, calibrating a torque wrench is a like calibrating a hammer.

Most (99%) of all fasteners out there are crap. Their thread form, material properties, surface coatings, residual stresses, fillet radii, overall dimensions, surface lubrication, surface corrosion (you get the point) vary so much that using a calibrated torque wrench on them is comparable to using a micrometer on Jell-O (brand flavored gelatin). Surface lubrication alone can change to torque to preload ration by more than two to one. Besides, why would you need to torque a bolt to an exact, calibrated point when the point was selected from a table of general recommendations?

Using a calibrated torque wrench IS a good idea when you are installing $25, individually packaged  bolts into a dimensionally controlled, high performance structure.

If you want to be consistent (and you do want to be consistent) then the method I would recommend for fasteners where you don't have a manufacturers recommendation (as compared to the engine or prop fasteners) is to select the nut - bolt - washer combination you plan to use, assemble them in the part and then torque to failure. Now grab another set, install it and torque to 75% of the failed torque. Remember, everything has to be the same. You are, in effect, calibrating the torque wrench to the fastener performance. Your torque wrench only needs to be repeatable, not accurate. It seems that everything I know that is useful I learned by breaking something.

If a threaded joint is creaking then it IS moving, and movement at a threaded interface is always bad. Remove the bolt, add shims to the aft spar - gear box interface until the gap is gone and ream the spar bolt hole to the next bolt size. Be sure to chamfer the hole before installing the new, oversize bolt.

Remember, if you install a fastener in your airplane, always tighten it even if you are SURE you are going to remove it later. This way you can never install a loose fastener.

For further reading see Carroll Smith's book "Engineer to Win" chapter 8.

On a final note, I asked a builder why he wasn't using Locktite instead of safety wire on a non critical fastener. "I don't trust that glue" was his reply. Pointing to his airplane I said "Friend, now is NOT the time to loose faith in adhesives".

Regards
Brent Regan