Also see the Headset entry in my Bicycle Glossary and my Article on Handlebar Height.

The "headset" is the bearing assembly that connects the front fork to the frame, and permits the fork to turn for steering and balancing.

A headset consists of four races plus associated parts -- from top to bottom:

1. The adjustable race attaches to the steerer.
2. The upper head race is pressed into the top of the head tube.
3. The lower head race is pressed into the bottom of the head tube.
4. The crown race is pressed on to the bottom of the steerer, just above the crown.

"Pressed" means that the parts are attached using an interference fit between cylindrical surfaces. The inside diameter of the outer part (head tube, crown race), is very slightly smaller than the outer diameter of the part which mates with it (head race, fork crown race seat. Force must be used to assemble (or disassemble) them. When assembled they act as a single part.

There are two common headset types: threaded and threadless.

 

• Traditional threaded headsets fit forks with threaded steerers. The top race screws onto the steerer, and a locknut screws on after it to secure it. There is normally a keyed washer between the top race and the locknut for extra security.

  In addition to the keyed washer, there may also be one or more spacer washers. One of them may be extended forward to serve as the cable housing stop for a centerpull or cantilever brake or on older British bicycles, as the mounting for a headlight.

• Threadless headsets used on many newer bikes (since the 1990s) fit forks with unthreaded steerers and use a clamp or collar to secure the top race. Usually, the handlebar stem does double duty as this clamp or collar.

Most bicycle frames don't care which style headset is used, as long as it is the correct size. For any given frame, you can usually switch back and forth between threaded and threadless by changing:

• The Fork
• The Headset
• The Handlebar Stem

The nominal size of all headsets is based on the outside diameter of the fork steerer. This is a source of confusion, because the steerer is not visible on an assembled bicycle. The steerer is enclosed inside the frame's head tube.

Threaded Sizing

In the case of a bicycle with a traditional quill/expander/wedge type stem, the stem shaft will be 1/8" smaller than the steerer. Sometimes people measure the stem diameter and assume, incorrectly, that this is the size headset they have.

The vast majority of bikes with threaded headsets use the 1 inch size. A sizable minority uses the 1 1/8 inch size. Other sizes are generally quite rare. The illustrations below show how to tell the 1 inch size from the 1 1/8 inch size:

Other dimensions also matter when selecting a replacement headset.

• Outside diameter of the frame cups where they press fit into the top and bottom of the bicycle's head tube.
• Crown race inside diameter, where the crown race is press fit onto the fork crown.
• Thread pitch, for threaded headsets.
• Stack height. If the new headset is shorter, its height can be increased using spacer washers, but if the new headset is too tall, parts will extend past the top of the steerer, preventing proper assembly.
• There also are variations in the depth and shape of the sockets at the top and bottom of the head tube for the upper and lower head-tube races -- "standard", "integrated," "semi-integrated", "low profile". The integrated and semi-integrated headsets have a larger-diameter head tube with cartridge bearings recessed into it.

Unless you are very sure about all of these dimensions, take the old headset and/or the frame and fork with you when you go to buy a new headset. Common dimensions are shown in the tables below. Better bike shops can mill a fork of most frames for a smaller crown-race press-fit diameter.

Shakiness

Stiffness

Shakiness AND Stiffness

"Indexed Steering" ("Brinelling")

A "Brinelled" crown race (larger than actual size)

The adjustable race is secured by either a:

• Star fangled nut and cap, or compression rings in the case of some threadless headsets or a:
• Keyed washer and lock nut, for threaded headsets.

Adjustment is easiest if you use two wrenches. You may use large end wrenches, or special wrenches made to fit the wrench flats of the locknut and top race -- but in a pinch, you can get by with a single large adjustable wrench. There is some risk of stripping out the tab of the keyed washer or of its damaging the threads of the steerer if you use a single wrench.

1. Leave the bicycle right-side-up with the front wheel resting on the floor.
2. Loosen the locknut. Preferably, hold the top race from turning with one wrench while turning the locknut with another. If using one wrench and the top race starts to turn along with the locknut more than a tiny bit, go looking for a second wrench.
3. (Skip this step if threads and washer are greased.) Unless the threads and keyed washer are already greased, remove the locknut and keyed washer. Do not unscrew the top race, unless you are going to go on to overhaul the headset -- loose bearing balls might fall out.
4. (Skip this step if threads and washer are greased.) Grease the keyed washer and the threads of the steerer.
5. (Skip this step if threads and washer are greased.) Replace the keyed washer, and screw the locknut down most of the way, leaving it loose..
6. By hand, without using a wrench, screw down the upper head race until the bearings just begin to bind, then back it off about 1/8 turn. (Tightening the locknut will press down the upper head race, due to its slight looseness on the threads.)
7. Tighten the locknut lightly and test the bearings for free rotation with no excess play. Unless the adjustment is perfect, loosen the locknut and readjust the bearings as necessary. When the adjustment is correct, tighten the locknut fully and check again.

Older French threaded headsets use a serrated or pinned keyed washer which mates with the upper head race. With these headsets, you must back off the locknut farther so that you can rotate the upper head race.

Threadless headsets have an adjustable race that slips over an un-threaded steerer. There are three distinct types of threadless headsets which adjust differently:

• "Aheadset" style threadless headsets, the most common type, require a threaded fastener such as a Star Fangled Nut (photo at right) to be installed inside the steerer.

  This type of headset must be used with a special handlebar stem that clamps onto the outside of the steerer, either with one or more binder bolts, or other means. The stem is further secured by a plastic or metal cap which is bolted to the star nut.

  To adjust an "Aheadset"-type headset, the stem binder must be loosened, then the bolt that runs through the cap to the star nut is tightened, usually with a 5 mm Allen wrench. This presses the stem down against the tapered bushing that fits inside the adjustable cup, and takes up the slack in the system. The stem is then aligned with the front wheel and tightened with its binder bolts. Once the stem binder bolts have been tightened, the adjusting bolt that goes to the star nut is under no significant stress, and may even be removed. This makes some unusual customizations possible, for example, adding a wedge/expander-type stem to hold a second set of handlebars!

• Some "Diatech" headsets from Dia Compe and some Aheadset models use a special pair of collars, usually mounted just below the stem and above the top race. The lower collar has a beveled, conical top surface, which fits inside of a matching bevel on the inner circumference of the upper collar.

  The upper collar has a gap at one point, with a binder bolt to squeeze the gap together. (This upper collar may also include a cable housing stop for the front brake, if the bicycle has a rigid fork and conventional cantilever or centerpull brakes.)

  The handlebar stem is clamped tightly to the steerer, preventing the upper collar from moving upward. As the upper collar is compressed by the binder bolt, it squeezes the lower collar downward, taking up any slack in the headset bearings.

• The YST "GeForce" headset, unfortunately no longer made, has a collar which slips over the steerer, clamped to it with a binder bolt. The outside of this collar is threaded, and the upper bearing race screws onto these threads. The assembly is fitted with a binder bolt that clamps it tight onto these threads when the desired adjustment has been attained.

  The GeForce is the only threadless headset which doesn't use the handlebar stem as part of the adjustment. This headset could be used with any stem, either external clamping or internal expander/wedge type. It is commonly found on Bike Friday folding bicycles made in the mid-1990s.

  To adjust a GeForce headset, loosen the binder bolts in the collar and the top race; screw the collar and upper bearing race together, then back off by a fraction of a turn. Tighten the binder bolt of the collar while pressing the assembly down; turn the top race to adjust it; then tighten the binder bolt of the top race. The collar uses a 4 mm Allen wrench, and the top race, a 2.5 mm Allen wrench -- available at some bike shops and at better hardware and auto parts stores. You might take the collar/upper bearing race assembly with you to check the fit of this wrench when you buy it.

Headsets are pretty durable, and hardly ever need to be serviced if your bicycle has a front fender.

If you ride without a front fender in wet conditions, the front tire will spray filthy, gritty water right up into the lower race. (It is the lower race that supports your weight. Most headset failures occur at the lower race.)

To overhaul a headset, you first have to remove the stem. It usually also makes things easier if you remove the front fender, wheel and brake, or at least disconnect the front brake cable.

If you have a cyclecomputer pickup mounted on the fork, you'll also need to remove one end or the other of the computer wiring.

It helps also to turn the bike upside down once the stem has been removed -- easier with a workstand. Don't rest old upward-arched drop-bar brake cables on the floor.

• Threaded:

  Unscrew the lock nut, remove any washers, then unscrew the threaded bearing race. You'll be able to just lift the fork out of the frame.

• Threadless:

  Once the stem is off, all of the spacer washers should come off easily. The upper bearing race may be a snug fit to the steerer, so you might need to give the top of the steerer a rap with a mallet to break it free.

Once you have the fork out of the frame, you can see what kind of bearings the headset uses. The illustrations below show the assembly order for typical headsets with different types of bearings. A headset may or may not have contact-type seals as shown under the lower bearing of the headset at the left.

Once you have removed the fork, you can check the condition of cartridge bearings by turning them with your fingers. They should turn smoothly, though there will usually be some drag from bearing seals. Exact replacement is needed with cartridge bearings -- or else, replace the entire headset.

After cleaning a headset that uses retainers or loose bearing balls, check the bearing races for indentations and pitting.

If the headset uses bearing balls held in retainers, you could re-use them if they are still in good condition too. There's little standardization among different makes/models of headsets, so finding a replacement retainer may be difficult or impossible. The top and bottom retainer are usually identical, and exchanging them will increase wear life. It may be possible to pop the old bearing balls out of a retainer and insert new ones, taking care not to bend the retainer. You can increase the load capacity and wear life by replacing retainers (especially the bottom one) with loose bearing balls, though in some cases it's difficult to keep loose balls in place during reassembly.

If the headset had loose bearing balls, replace them...they're cheap enough! You very likely lost some anyway when you disassembled the headset. Most headsets use either the 5/32" or 3/16" size.

For re-assembly, it is very helpful to have the bike mounted upside down in a work stand.

If you lower the steerer partway into the head tube, you will be able to load up the bottom (but facing upward) head tube race with bearing balls without worrying about having them fall down through the head tube, because the steerer will fill the gap. Use enough grease to hold the bearing balls in place. Leave the race one ball short of full, so the balls won't jam up against each other when you reassemble the headset. Once all the bearing balls are in place, add lots more grease, to help seal out dirt when the bicycle is in use.

Once the balls are in place for the lower part of the headset, you can drop the fork down the rest of the way into the headset.

Grease the inside of the upper race, and get it started onto the steerer. Load it up with balls, add more grease, then slide (threadless) or screw (threaded) it "up" to take up most of the slack in the system, so the balls can't fall out.

For a threaded system, you can then turn the bike right-side up to complete assembly/adjustment and wipe off the excess grease.

For a threadless system, you need to secure the fork before turning the bike right-side up. I sometimes use an old toe strap for this, tying the fork crown to the down tube of the frame.  

Removing and re-installing the headset bearing races is not necessary for a normal overhaul, only when you are replacing the headset or installing a new fork.

If you are working with an inexpensive all-steel headset, you can often manage with basic hand tools, but if your headset has aluminum or plastic parts, you really need specialized tools made for the purpose. These tools are generally not economical for a home mechanic to purchase, given how rarely they are likely to be used.

The following assumes you're working with an all-steel headset:

Removal:

The races can usually be removed with a hammer and an old flat-blade screwdriver that you don't care about.

The head tube races usually come out pretty easily if you stick the old screwdriver down into the head tube until it hits the internal edge of the race. Tap gently once, then move the screwdriver to the opposite side of the head tube. Keep going back and forth every other blow, so that you won't be driving the race out crooked, which could damage the head tube.

The fork crown race is sometimes more challenging, depending on the construction of the fork crown and the width of the race. You may or may not be able to get a good purchase on the underside of the race.

If the race has a good overhang on both sides of the fork crown, a bench vise is often best for this. Open the vise just wide enough to permit the fork crown to fit through loosely, so that the crown race is sitting on top of the vise jaws. Use a mallet to rap on the top of the steerer, driving the fork down and away from the race. It is a good idea to thread the headset locknut onto the steerer before doing this, to prevent damage to the threads of the steerer.

Alternatively, a bicycle workstand can be used to clamp the steerer with the fork upside down. You can use the hammer and old screwdriver to drive off the race.

Some races and some fork crowns give very little to grab onto. There are special shop tools to work around this problem, so you might want to just bring the fork into your local shop and have it remove the race.

Replacement:

The press-fit races need to be seated firmly.

The head tube races can be tapped into place with a mallet and a block of soft wood that spreads the impact all the way around the margin. Use another block of wood to support the other end of the head tube, and be sure it's sitting level so you don't damage it. Bike shops have threaded tools to pull the races into place.

The fork crown race is more difficult to seat. You can tap on one side and the other, inboard next to the steerer with your old screwdriver, or you may tap on a length of pipe that just fits over the steerer. Turn the pipe around as you tap, in case the surface which rests against the crown race isn't perfectly level.

If you don't get the races firmly seated, they will seat themselves during riding, and the headset will loosen. It then only needs to be readjusted. If working on someone else's bicycle, you should be sure to get the races seated fully before delivering it. If you don't have special tools, you could go for a short, bumpy ride to seat the races, then readjust the headset.

Also see my Adventure Cyclist article about headsets.