Damn you. You "made me" go find my post ... from 3+ yrs ago....

Quoting from: http://www.unbrako.com/docs/Fastener%20Facts.pdf

METHODS for CONTROLLING TIGHTNESS
Six methods can be used to control tightness of a threaded fastener.
In order of increasing accuracy - and increasing cost - they are:

Method................Accuracy...........Relative Costs
Feel....................+/-35%..............1
Torque wrench......+/-25%..............1-1/2
Turn-of-the-nut....+/-15%..............3
PLI washers..........+/-10%..............7
Bolt elongation.......+/-3-5%............15
Strain gages..........+/-1%.............20

But this table only tells part of the story. The less the accuracy of
the tightening method the greater must be the minimum yield strength
of the fastener in order to compensate for preload variations and to
insure a minimum clamping force. This can be done only be using a
fastener material of greater strength or increasing fastener size -
both of which will increase the cost of the fastener.

The decision as to which tightening method to use depends primarily on
the criticality of the joint. Generally, the more critical the joint
the more need for higher tightening accuracy and the greater the cost
for obtaining the right clamping force. It should always be noted that
the method selected will almost always lie between the two extremes,
for few applications will allow the high inaccuracy of the "feel"
method, while the high cost, highly accurate strain gages are used
almost entirely in the laboratory. The other methods
may be summarized as follows:

TORQUE WRENCH

This is the least expensive of the accepted methods of controlling
fastener preload but is the least accurate, chiefly because the
reading on dial is affected by the friction to be overcome.

TURN-OF-THE-NUT

In this method, the nut or bolt is turned a predetermined number of
degrees after all play has been removed from the joint.
How much to turn the nut or bolt cannot be calculated (because of the
"rubberiness" of the joint) but must be developed by tests for each
joint. It eliminates the friction factor; however, its accuracy is
affected by the care of the workman in measuring the angle the
nut or bolt is turned.

PRELOAD INDICATING WASHERS

Preload indicating (PLI) washers utilize compression of an inner ring
between two flat washers with an outer indicating washer for control.
As the load increases, the inner ring, which is higher than the outer
one, is squeezed down and enlarged in diameter until the
outer washer binds against two flat washers. When the inner ring has
been flattened to this point, the correct clamping force has been
reached. The PLI washer is sensitive only to axial load.

BOLT ELONGATION

In this method, a bolt from the lot is loaded in a tensile machine
with the same nut as used in the application. The distance from the
nut face to the underside of the bolt head is measured, and a plot is
made of bolt elongation in relation to induced load. In the
application, the fastener is tightened and the bolt elongation is
measured until the required preload as determined from the plot has
been achieved.