Tightening the headset on a bicycle is necessary for safe handling and improved turning abilities. The headset refers to the set of bearings located on the front of the bicycle frame between the top tube and where the forks go through the frame. These bearings allow the fork to turn smoothly. To determine if your headset is loose, simply stand next to the bike and lock the front brake. Roll the bicycle forward and backward. If you hear or feel a clunking noise, the headset is loose and needs to be tightened.
Tightening a Loose Bicycle Headset
Remove the headset cover using the 5mm hex key. This will expose the front fork steering tube which is inserted into the frame.
Loosen the stem clamp bolts using the same hex key. There are usually two and they're located on the rear of the stem. The stem is what connects the handlebar to the forks. These bolts do not need to be removed, just loosened.
Replace the headset cap and tighten until snug. This will pull the steer tube into the stem.
Lock the front brake and rock the bike forward and backward to check the headset's tightness. If it's still making noise, tighten 1/4 of a turn and re-test.
Tighten the stem bolts one at a time.
Check the turning range once every bolt is tightened to make sure the front wheel turns freely.
- Sheldon Brown: Servicing Bicycle Headsets
- The Bicycle Repair Book; Rob Van der Plas