RtGent is precisely correct here - and this is one of the most important things to realise about threaded fasteners. You cannot thread two things on the same fastener (unless its a locknut type set-up in which case you must tighten the two parts against each other to secure them). Such a design is what engineers refer to as
over-constrained and if you try something like that - you are doomed to have an unreliable joint. Think of it like a 4-legged stool - you can NEVER get all four legs to sit on the floor with
exactly the same load on each and so the stool will always rock
a little - whereas a 3-legged stool will always be stable.
The other key thing about threaded fasteners is that for them to work properly, the fastener (bolt, stud, etc.) must be loaded only in
tension (i.e. forces along the length of the screw or bolt) and never in
shear (forces at right angles to the axis). A tension-loaded fastener will squeeze the two parts being joined hard enough to cause sufficient friction between them - to resist any shear loads.
Imagine for example, that you installed a brake disc on a wheel and you did
not tighten up the bolts beyond finger-tight. There would be little or no friction between the disc and the wheel hub - and so applying the brakes would cause those bolts to be loaded in
shear to resist the braking forces (and they would almost certainly simply bust off with disastrous results). Alternatively, if you torque the disk mounting bolts up tight, they will squeeze the disc and hub together hard enough that there will be sufficient friction between them to resist the braking torque - and the bolts won't even "know" that you are applying the brakes.
THAT is why highly loaded critical fasteners such as brake caliper and disc mounting bolts, cylinder head studs etc. -
must be torqued to a specific value (usually measured in
ft-lb,
in-lb, N-m or
kg-m). That is done partly to keep them from coming loose, but mainly to ensure that enough friction is created to resist shear forces or, in the case of cylinder head studs, the axial fatigue loads caused by internal combustion pressure. The other reason for specific torque values and tightening patterns in bolt or stud groupings (such as the wheel studs and bolts or the studs on something like a cylinder head for example) is to ensure that all of the fasteners are tightened to the same degree and thus, the frictional forces are spread evenly across the available surface area and no single fastener is over-loaded and that the parts being joined are evenly loaded with no gaps for internal pressures to escape.
So guys:
1) buy yourself a good torque wrench and an accurate set of sockets (I like the "click-type" torque wrenches);
2) check out the proper torque in the manual and set the wrench to that value;
3) follow the tightening patterns in the manual to the letter.
.....and all should be well as you
"screw around" with your bikes.
Cheers,
Pete