The Park Tool headset installation page has a threadless adjustment section (scroll down 3/4 of the long page, to the title "Headset Adjustment - Threadless Type")

The idea is:
The stem's two bolts holding the steerer tube (the top part of the fork) holds the steerer in place, so the ball bearings in the headset have the correct amount of play and the fork can turn, but not be loose.

The top cap is tightened just enough while the two stem bolts are completely loose to put the right amount of pressure on the bearings. This pulls the steerer tube up, since the top cap presses down on the stem and the spacer rings. Typically, you need very little pressure on the 5mm wrench--it's just barely tight.

Then the stem is clamped tight with it's two bolts. At that point, the top cap doesn't do anything to hold the assembly together-- the stem does--a very strong connection.


A torque wrench is a good tool to have, long term, since any carbon fiber component will need to be torqued. Otherwise, it can fail suddenly. Aluminum parts aren't as critical, but it's still possible to strip threads or have a part break. I've changed stems attached to an aluminum steerer tube without a torque wrench, but now that I have one, I always use it.

You can remove the top cap now, while the stem is still tight, and you'll either see an aluminum tube or a carbon tube inside the stem. (carbon on the lightweight, more expensive forks).