Accessories Bicycles Parts Specials Tools

Servicing Bicycle Headsets

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by Sheldon "Steering" Brown
revised by John "Balanced" Allen
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Also see the Headset entry in my Bicycle Glossary and my Article on Handlebar Height.

 Headset anatomy

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.

Some newer headsets have slip-fit parts instead --- more about that later.

There are two common headset types: threaded and threadless.

Threaded Headset

Note wrench flats on locknut
and top threaded race.
Threaded Headset

Threadless Headset

Note pinch bolts on stem.
and no wrench flats.
Threadless Headset


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:

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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:

1 Inch threaded headset
(common traditional size)

(7/8" Quill is noticeably smaller
than a U.S. Quarter)

1 Inch Threaded

1 1/8 Inch threaded headset
(rare size, mostly 1990s bikes)

(1" Quill is a teeny bit larger
than a U.S. Quarter)

1 1/8 Inch Threaded

Other Dimensions

Other dimensions also matter when selecting a replacement headset.

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.

Standardized Headset Identification System

New bicycle designs and the increasing popularity of cartridge bearings have led to several new headset dimensional standards. The Standardized Headset Identification System (SHIS) has been developed by manufacturers to identify each of the dimensions that affect the fit of a headset to the bicycle frame and fork, using a compact code.

An upper or lower headset is usually interchangeable with another that has the same code: that is, they will fit the same frame and fork. The code does not indicate that mixing and matching parts within an upper or lower bearing assembly is possible. (One assembly might, for example, have cartridge bearings, and the other, cup and cone bearings.)

The SHIS code works for headsets in common use today. It fails for a couple of older threaded headset standards which have dimensions a fraction of a millimeter different from current standards, but these are rare.

Here's an example of a SHIS code:

ZS44/28.6 | ZS44/30

The first half of the code (before the vertical bar) describes the upper headset. The second half describes the lower headset, as follows:

The first two letters: headset assembly type

The first two letters of the code ("ZS" in the example) describe the headset assembly type. This can be different for the top and bottom assemblies.

Examples of headset types
External-cup (EC) lower headset. Semi-integrated (ZS) top headset -- note gap between bearing cup and cone just above the head tube. Integrated (IS) headset, top and bottom. The bearing cone sits directly above the top of the head tube. No visible lower-headset parts.
semi-integrated headset Integrated headset


Two-digit number: head tube inner diameter

The number which follows the two letters (as in ZS44) gives the inner diameter of the head tube bore where the cup or bearing cartridge is inserted into it, rounded down to the nearest whole millimeter. Sometimes the diameter at the bottom is larger than at the top. Examples:

After the forward slash: fork steerer dimensions

In each half of the code, the number following the forward slash, for example

ZS44/28.6 | ZS44/30

is the fork steerer outside diameter for the upper headset, and the fork crown race or bearing cartridge inside diameter for the lower headset, rounded to a whole number. If the steerer is threaded at other than the usual 24 TPI, this is also indicated, with the threading. For example, the code for a Nottingham Raleigh headset is

EC30/25.4-26tpi | EC30/26

The old Italian standard used a 26.5mm crown race, so the rounding could lead to some confusion -- but the Italian standard is obsolete, and an Italian fork can be milled down to 26.4 to take a modern ISO headset.

A threadless top race is slip fit onto the fork steerer. Even in an integrated headset whose cartridge bearings slip-fit directly into the head tube, the fork crown race is press fit. The cartridges, in turn, slip fit onto the crown race. Tapered contact surfaces between the cartridges and races prevent looseness.

Optional: stack height

Stack height (height of the upper or lower headset outside the head tube) is usually unimportant with threadless headsets, but it may optionally be listed for the upper and lower headset, as follows.

ZS44/28.6 H16 | ZS44/30 H 13.7

Also see the Park Tools page about the Standardized Headset Identification System

Tables of Headset dimensions

The tables below give the dimensions of common headset and steerer sizes, and some which are not so common. SHIS terms are given for threadless headsets, with many more types and sizes, so the codes really matter.

Threadless Headsets/Steerers
Frame Cup/
Crown race
21.15 mm
EC32 32.7 mm 26.4 mm Used mainly on bicycles with one-piece cranks. BMX/O.P.C
(25.4 mm)
IS38 38.0 mm 26.4 mm Slip fit, 36° angular contact at bottom of bore.
EC30 30.2 mm 26.4 mm This is the ISO standard 1" size.
1 1/8"
(28.6 mm)
EC34 34.0 mm 30.0 mm Most newer mountain bikes with external cups use this size.
IS41 41.0 mm Integrated, angular contact, slip fit.
ZS41 41.3 mm Low profile, press fit. May have cartridge or cup-and-cone bearings.
  41.6 mm Integrated, angular contact. Obsolete.
IS42 41.8 mm Integrated angular contact; Campagnolo Hiddenset
  41.9-42.0 mm Microtech integrated, press fit, square-bottomed recesses. Obsolete
ZS44 44.0 mm Retainer/loose balls or cartridge, which is slip fit into pressed cup
  44.0 mm Integrated, 36° angular contact. Obsolete
IS47 47.0 mm Integrated lower only, slip fit.
1 1/4"
(31.8 mm)
EC37 37.0 mm 33.0 mm Mainly used on tandems
(38.1 mm)
49.7 mm 39.8 mm OnePointFive standard for downhill and freeride applications. Press fit
52 mm
IS52 52 mm ? Integrated lower headset. Slip fit.
56 mm
56 mm ? Internal or external, press fit.


Threaded Headsets/Steerers
in order of frame cup outside diameter, crown race inside diameter, threads per inch, and steerer O.D.--
the dimensions which affect headset compatibility with frame and fork.
Crown race seat of fork can be milled smaller, frame bearing seats milled larger.

Standard Frame Cup
Outside Diameter
Crown race
Inside diameter
Steerer O.D in mm Threads
Per inch
Stem diameter,
Steerer I.D.
1" J.I.S.
30.0 mm 27.0 mm 25.4 mm 24 22.2 mm (7/8") Older or lower-quality bicycles from Asia. Can be milled to ISO standard.
1" Raleigh
30.0 mm 27.0 mm 25.4 mm 26 22.2 mm (7/8") Proprietary size used on Raleighs made in Nottingham, England
1" ISO Standard
30.2 mm 26.4 mm
25.4 mm
24 22.2 mm (7/8") This is the standard 1" size.
1" Professional/
30.2 mm 26.4 mm
25.4 mm 24 22.2 mm (7/8") Obsolete. Threads are cut at 55 degrees, but an ISO headset can be used.
1" Italian 30.2 mm 26.5 mm,
27.0 mm
25.4 mm 24 22.2 mm (7/8") Obsolete. Threads are cut at 55 degrees, but an ISO or J.I.S. top headset can be used. Fork can be milled to ISO.
French 30.2 mm 26.5 mm,
27.0 mm
25.0 mm 25.4
(1 mm)
22.0 mm Obsolete. French steerers usually have a flat filed on the back, rather than a grooved keyway as with other threaded systems. The bottom headset parts wear faster; you can replace them with ISO parts and use the old top headset.
East German (DDR) 30.6 mm 26.8 mm
(1 1/8")
26 mm 26 22.0 mm Very similar to or the same as Austrian. 0.2 mm discrepancy may result from measuring the I.D. of the head tube (press fit). See page on German site.
30.8 mm 26.7 mm 26 mm 25.4
(1 mm)
22.0 mm These are found on bicycles sold by Sears, made by Steyr in Austria. Higher quality Austrian bikes use English/ISO.
BMX/ O.P.C. bikes 32.6 mm
26.4 mm (varies) 25.4 mm 24 21.15 mm
Used mainly on bicycles with one-piece cranks, also some early mountain bikes.
French Tandem Let us know, if you do! 28 mm Let us now, if you do! 25.4
(1 mm)
22.0 mm Obsolete and rare.
1 1/8" 34.0 mm 30.0 mm 28.6 mm 26 25.4 mm (1") "Oversized" (This size is more often used for threadless systems.)
Moulton 30.2 mm top,
36.4 bottom
33.0 mm 25.4 mm 24 22.2 mm
Moulton Mark III and earlier. Top headset is standard ISO.
1 1/4" 37.0 mm 33.0 mm 31.8 mm 26 28.6 mm (1 1/8") Mainly used on tandems

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Mix and match?

It is a Very Bad Idea to install a threadless headset so the handlebar stem is supported by the threads of a threaded fork. The threaded section is not made to resist the stress from the stem and could break off, along with the stem and handlebars.

As already mentioned, it is at best risky to try to mix and match parts of a top or bottom headset. Even with the same SHIS code and similar appearance, there is no guarantee that internal parts will fit each other.

It is perfectly OK though to use a top and bottom headset of different makes, models and types, as long as the fit to the frame and fork is correct. For example, when replacing a threaded front fork with a threadless one, you could keep the bottom headset, if it fits and is in good condition.

You could cut off the threaded section of a fork steerer to use the fork with a threadless headset on a frame with a short head tube, though the fork's geometry may not produce optimal handling.

You can install a threadless headset on a threaded fork, if the fork steerer is tall enough and you use a quill stem. The threads should end above the headset top race. Use two locknuts tightened against each other to hold down the threadless top race, plus spacers as needed. In most cases, you'll have to grind away the inner lip of a top nut so you can thread it down past the top of the fork to serve as the second locknut. The Raleigh Twenty fork in the photo below already came with a second locknut for its original Nylon-sleeve-bearing top headset, and so no grinding was needed. More about this mod...

Raleigh Twenty with threadless top headset




Shakiness indicates that the headset is adjusted too loose. The easiest way to check for this is to apply the front brake and push the handlebars back and forth, front to back. If you feel a "clunk" it usually means the headset is too loose. You can determine that the looseness is in the headset and not the brake by resting a finger across the gap between the races of the top or bottom headset assembly. If the bicycle is in the work stand with the front wheel off, you can just push and pull on the forkends.


Stiffness results when the headset is adjusted too tight. Check for this by lifting the front of the frame so that the front wheel comes off the ground. Normally, the wheel and handlebar will flop to one side or the other by their own weight when you do this. If the front wheel and handlebars are off the bicycle and you turn the fork, you may feel a light, smooth, even drag from bearing seals -- this is normal. Roughness or uneven drag indicates problems.

Shakiness AND Stiffness

Sometimes you will find both of these faults at the same time. This indicates a more serious problem, not just an adjustment issue. You may find that the headset is loose when the fork is facing straight forward, but gets tight when you turn the handlebars to one side or the other, or that it simply can't be adjusted. Possible causes:

"Indexed Steering" ("Brinelling")

All of the other ball bearings on a bike spend their lives rotating round and round, but headsets spend most of their time pointing straight ahead. Sometimes you will see that the bearing balls have created a neat row of little dents in the bearing surface of the lower frame cup and/or the crown race. This creates the effect commonly known as "indexed steering" where the balls sort of "snap" into place, and the fork tends to stick in the straight-forward position.

brinell led

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

The dents in the races resemble the dents created by the Brinell hardness test procedure, and as a result, this type of damage is sometimes called "Brinelling." This is a bit of a misnomer, because the Brinell hardness test involves basically hammering a bearing ball into a metal sample with one good hard whack. Some folks assume that the dents in headset races also result from impact, but actually that is not commonly the case. Instead, it has to do with lubricant breakdown. Jobst Brandt has an article on this site describing the process in detail.

Ideally, a "Brinelled" headset should be replaced, but you can often cure the problem by replacing the retainer with loose balls. You use the same size balls, but can usually fit more of them into the races if you leave the retainer out. As a result, the greater number of balls, closer together, no longer all line up with the dimples in the races.

A headset race can, on the other hand, become indented due to overload in a crash. The dents then will be deeper in the part of the race that took the impact. Usually, in this case, the fork will be bent too. The indented race(s) and the fork need to be replaced.

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The adjustable race is secured by either a:

Adjustment (Threaded)

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.

Adjustment (Threadless)

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

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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.

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. An integrated (IS) threadless headset with cartridge bearings does not have head-tube races, but instead, the cartridges install directly into recesses in the head tube of the frame.

This threaded headset has bearing balls in cages (retainers), though generic loose bearing balls can be installed as replacements in most headsets which were supplied originally with retainers. This threadless headset has cartridge bearings. Only exact replacement cartridges will work. Cartridge bearings are mostly to be found in newer, threadless headsets.
Threaded headset, exploded view Threadless headset, exploded view
Images by keithonearth on Wikimedia Commons, reproduced under Gnu free documentation license

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 workstand.

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:


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.


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.

Spoke Divider

Spoke Divider

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