A bike headset on a threaded steerer needs to be serviced once a year. This involves taking the assembly apart, cleaning, inspecting, relubricating and reassembling. If your bike has sealed cartridge bearings you don't need to service it until it stops working correctly but you may need to take it apart once to find out what kind of bearings it has. If the bike is used in dirty conditions, gets washed a lot or has no front-mudguard underneath the fork it may need to be serviced and regreased more often.
With the bike on the ground, loosen the lock-nut.
It may be necessary to disconnect the front brake cable in order to remove the handlebar stem.
Put the bike in a stand remove the front wheel(link). Put a strap under the fork crown and around the down-tube. Unclamp and remove the handlebar stem, loosen the lock-nut, remove it along with any washers, cable hangers etc. taking note of the order in which the parts come off. Unscrew the adjustable race. Look out for rubber seals and note which way they go up.
Disassemble The Top Race
Remove the balls or rollers. They may be held in a clip or rolling free. If the balls are free count how many there are. Some may stick to the top race so make sure you don’t lose any.
Most modern bike headsets have sealed bearings. These are in the form of a ring shaped cartridge. They can’t be serviced. If they look OK, turn freely and don’t have any ‘play’, reassemble the bike headset. If they need to be replaced you need to match them exactly. Tap them out with a screw driver. Take note of any writing on the cartridges and/or take them to a bike shop. You can make a ‘press’ to push the new cartridges in using a length of threaded rod and some big washers. Make sure you press on both the inner and outer rings of the cartridges, otherwise the force may damage the new bearings. In some cases the old bearings are useful to help push the new cartridges in.
Disassemble The Bottom Races
Once you’ve undone the top you can carefully undo the strap while supporting the forks and take the bottom race apart, there may be a rubber seal, note it’s position.
If you have the bike in a stand you can turn it upside down so the forks can’t fall out.
Clean And Inspect
Clean the balls or rollers and the races with degreaser. Inspect the races for signs of uneven wear or roughness. Changing the balls for new ones of exactly the same size – take an old one to the shop as a sample – will prolong the life of the races. Balls in a clip are easier to handle but if you use loose balls you can fit more in so each one carrys less load. If the races are rough or badly worn the whole bike headset needs to be replaced.
Grease The Bottom
Fill one of the bottom, races with new grease then fit the rollers or balls. The grease holds the balls in place while you start to reassemble the bearing. If you’re replacing loose balls use the same number that came out. If you’re replacing balls in a clip, with loose balls, put in as many as will fit – in-line, along the wear-line of the race – then remove one so those that remain have room to roll.
Put The Forks In
Put the threaded steerer tube back into the headtube, take care not to disturb the balls – or roller clip – stuck in the grease. If the race had a rubber seal put it back in the right place. Strap the fork to the downtube.
Grease The Top
If you turned the bike over spin it back to the right way up. Grease one side of the top race and fit the balls or rollers.
Fit any rubber seal and reassemble the top race, any washers and screw down the lock nut finger tight. Put the handlebar stem back in the threaded steerer tube and lock roughly in place.
Check the position of the handlebars and lock in place.