How to Properly Tighten Bolts
These methods apply to numerous disciplines, since just about everything you work with (bikes, cars, etc.) use threaded fasteners of some sort (i.e. Hex bolts.) But it is a basic “handyman” skill that you need to know before working on a bicycle…
There are two considerations here: applying proper torque to each bolt, and tightening the bolts in the right order.
1. Proper Torque
Each bolt on a bicycle will have a torque specification. That is, how much force you should apply when tightening it.
This is most important on lightweight, fragile parts (like carbon handlebars,) but it is still a good habit to tighten all bolts to the recommended torque.
For suggested torque values, please see the owner’s manual for the part in question. If that is not available, Park Tool provides a torque specification chart.
2. Tightening Pattern
When there are multiple bolts in one location, like on a stem’s faceplate, bolts need to be tightened in the proper pattern. (Much like lug nuts on your car’s wheels.)
You will tightening the bolts progressively, and you do so in a “star” pattern. Progressive tightening means that you tighten bolts a little at a time – you don’t tighten them completely on the first pass. The pattern means, on a stem with four bolts, you’ll start with the top left bolt, then do the bottom right bolt, then the top right bolt, and finish with the bottom left bolt. (And then repeat from the top left.)
(With two or four bolts, you can’t use a real star pattern like you would use on a car wheel, but you can replicate that pattern as close as possible.)
So you start by threading the bolts in and tightening them just enough to hold things in place (hand tight.) Then you can tighten each to the proper torque. (Note: Both steps should be done in the star pattern.)