The headset, a common item that is often not correctly adjusted when bikes come in for service that is in fact very simple to adjust. Here’s how:
At the bottom of the picture is the headset cover. Above is a stem spacer, then the stem and finally a top cap and bolt. When all parts are installed with the two vertical stem bolts undone the top cap is gently tightened to remove all play from the headset. Play can manifest itself as a rocking/knocking sensation when applying the front brake. When correctly adjusted the two vertical stem bolts can be tightened, once done the top cap effectively becomes redundant.
Before starting undo the two vertical stem bolts. Here’s a quick walk through:
Install the headset top cap.
Tighten the headset top cap to eliminate any play. Check by applying the front brake and push forwards on the handlebar making sure you cannot feel a knocking sensation or movement. Also check that the top cap has not been over tightened by lifting the front wheel and steering from side to side. The bearings should feel free running not tight.
Tighten the two vertical stem bolts to the manufacturer’s recommended torque setting which are usually marked on the side of the stem. To double check the stem is correctly tightened stand astride the front wheel facing the back of the bike with legs bracing the front wheel. twist the handlebar from side to side to make sure the stem doesn’t move. Once done you are ready to ride.
As with all adjustments, if you are unsure please contact a bike shop.