You need two spanners to adjust the bearings. The most common size, for 11/8-inch headsets, is 36 mm. Tou may also come across 1-inch headsets, which need a 32 mm spanner, and even the rare 11/2-inch headsets, which need a 40 mm spanner. The adjustable nut is quite narrow, so you will need a special narrow headset spanner. The top nut is wider, so use an adjustable spanner if you only have one headset spanner.
To check your headset, pick the bike up by the handlebars and turn the steering. The bars should turn easily and smoothly, with no effort. You should not be able to feel any notches. Drop the bike back onto the ground again and turn the bars 90 degrees so that the wheel points off to one side. Hold on the front brake to stop the wheel rolling and rock the bike gently backwards and forwards – in the direction the frame points, not the direction the wheel points. The wheel might flex and you may feel the tyre giving a bit, but you should not feel or hear any knocking or play. Sometimes it helps to hold around the cups, above and below, while you rock – you shouldn’t feel any movement.
The top of the fork steerer tube is threaded and held into the frame with two big nuts. The lower of these has a bearing surface on the bottom in which the top set of bearings runs. Tightening the nut draws the fork up into the frame, squashing the bearing surfaces closer together and eliminating play between fork and frame. Loosening this nut increases the space the bearings sit in, allowing them to turn more smoothly. The correct adjustment is found by turning this nut to a position that eliminates play while still allowing the forks to rotate freely. Once you’ve found this magic position, the top nut can be locked down onto the adjusting nut, holding it firmly in position so that it doesn’t rattle loose as you ride along. Once bearings have been correctly adjusted and the top nut firmly locked down, they should not work loose over time, so they will not need frequent readjustment. However, the bearings often settle a little bit after servicing, so they will often need readjusting. If you find yourself having to readjust your bearings often, check that the threads on the forks and the headset are in good condition. The threads will suffer if the headset is ridden loose, when both nuts will rub constantly over the fork threads.
Remove the stem, then the top locknut. Have a look at the threads inside the nut. They should be crisp and distinct, with sharp edges. The fork threads should be the same. Unscrew the lower adjusting nut and check the threads on it, as well as the fork threads that were concealed by the adjusting nut. If the threads are slightly damaged, reassemble the headset with Loctite on the threads to prevent the nuts from working loose. A new top locking nut will also help. However, if either the fork threads have been badly worn or the nut has worn grooves in the surface of the fork, the fork should be replaced immediately.
Step 1: Hold the adjusting nut still with one spanner and undo the top nut a couple of turns with the other. The two will be firmly locked together, so you have to be firm with the spanners to get them moving. Once the top nut is loose, use the spanner to adjust the position of the adjusting nut – tighten clockwise to eliminate play in the headset, loosen anticlockwise to allow the bars to rotate freely.
Step 2: Ideally, you are looking for the place where the adjusting nut is as loose as possible, without allowing the forks to rock in the frame. Turn the adjustable cup clockwise to eliminate rocking – anticlockwise to allow the headset to turn more freely. Test by holding the front brake on and rocking the bike forwards.
Step 3: Once you’ve found the right place, hold the bottom nut still with the spanner to maintain the adjustment, and lock the top nut firmly down onto it. Test the adjustment again – you often find that locking down the top nut changes the adjustment, and so you have to repeat the procedure. Take care as you do this not to overtighten the adjusting cup – if you wedge it down onto the bearing surface, you damage the bearings.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Threaded headsets”