The basics

Why

The headset is the pair of bearings that lets the fork turn in the frame. Too loose, it develops play: the fork knocks every time you brake, and the bearings get damaged. Too tight, steering gets stiff, "notchy" in the straight-ahead position, and wears too. Park Tool sums up the right setting: as loose as possible, with no play.

When to do it

• You feel a knock or "clunk" through the bars when braking or over bumps.
• Steering is stiff, or settles straight ahead on its own.
• After removing the stem or changing spacers.

The front brake test

Stand the bike on the ground, squeeze the front brake hard, and rest your other hand over the top of the headset, where the fork meets the frame. Rock the bike forwards and back. A knocking under your hand means play.

Then lift the front of the bike and turn the bars gently from side to side: they should turn with no tight spot or notches.

How a threadless headset works

On almost every modern bike, the fork rises through the frame on a tube, the steerer. The stem is clamped onto it by its side bolts (2). The top cap bolt (1) doesn't hold the stem: it pushes the stem down, which pushes the spacers (4), which load the bearings. It sets the play, but only while the side bolts are loose.

The top of the steerer should stop about 3 mm below the top of the stem (3). If it's level with it, the top cap can no longer preload anything: you need another spacer under the stem.

Steps (threadless)

1. Loosen the stem side bolts

Loosen the side bolts (2) a few turns, until the stem turns on the steerer with a little force.

2. Tighten the top cap bolt, an eighth of a turn at a time

Turn the top cap bolt (1) one eighth of a turn clockwise. No more: it's a fine adjustment.

3. Align and retighten the stem

Line the stem up with the front wheel, then retighten the side bolts. Tighten them alternately, a little at a time, to the torque given by the stem maker (in its manual, or often printed on the stem).

4. Test again

Repeat the front brake test and the rotation test.

• There's still play: loosen the side bolts again, then go back to step 2. Never turn the top cap bolt with the stem clamped: it has no effect.
• Steering gets stiff: back the top cap off an eighth of a turn, side bolts loose as always.

Stop at the loosest setting with no play. Check the alignment and side bolt tightness one last time.

Threaded headset (older bikes)

On an older bike, the stem slides inside the steerer and the play is set with two nuts above the frame: an adjustable race and a locknut that holds it.

1. Hold the race with one spanner and loosen the locknut about half a turn.
2. Turn the race one eighth of a turn (a quarter turn for a bigger adjustment): clockwise to tighten, anticlockwise to loosen.
3. Hold the race and retighten the locknut. Park Tool gives about 20 Nm.
4. Test again, and repeat until the bars turn with no stiffness and no play.

Check it's done right

• No more knocking in the front brake test.
• The bars turn freely, with no notches.
• The stem is in line with the wheel and doesn't twist when you grip the wheel between your knees and pull on the bars.

Common mistakes

• Turning the top cap bolt with the stem clamped: the adjustment has no effect.
• Overtightening the top cap to be sure the play is gone: steering gets stiff and wears.
• Forgetting to retighten the side bolts: the stem turns on the steerer, and you lose steering.
• Greasing the steerer: the stem slips.

What next

Log the adjustment in your logbook. If the play comes back quickly, or the steerer sits flush with the stem, add a spacer under the stem or have the headset checked at a shop.

Sources

This guide was checked against the following documents:

• Park Tool, How to Overhaul a Threadless Headset (how the top cap works, 3 mm, eighth-turn adjustment, front brake test)
• Park Tool, How to Adjust a Threaded Headset (as loose as possible with no play, notchy steering, 20 Nm)

Values vary between models: your part's manual always takes precedence.