How to Properly Tighten the Headset on Your Mountain Bike
To tighten the headset on a mountain bike, first loosen the stem bolts and remove the top cap. Use a hex key to tighten the bolt in the center of the top cap, then reassemble the top cap and stem, tightening the stem bolts gradually and evenly. Check that the headset is tight by gripping the front brake and rocking the bike back and forth – the handlebars should not move independently of the front wheel. Adjust as necessary and recheck until the headset is tight.