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Servicing
Bicycle Headsets
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The "headset" is the bearing assembly that connects the front fork to the frame, and permits the fork to turn for steering and balancing.Older bikes used "threaded" forks and "threaded" headsets.
Many newer bikes (since the 1990s) use "threadless" forks and "threadless" headsets.
The bicycle frame doesn't generally 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:
Threaded vs Threadless
There are two different systems for attaching and adjusting the adjustable race:
- Traditional threaded headsets fit forks with threaded steerers. The adjustable race screws on to the steerer, and a locknut screws on after the adjustable race to secure it. There is normally a keyed washer between the adjustable race and the locknut for extra security.
In addition to the keyed washer, there may also be one or more spacer washers.
Threaded Headset
Note wrench flats on locknut and top threaded race.
Threadless Headset
Note pinch bolts on stem.
The nominal size of all headsets is based on the outside diameter of the steerer. This is a source of confusion, because the steerer is not visible on an assembled bicycle. The steerer is enclosed inside of 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 used 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 1/8 Inch threaded headset (rare size, mostly 1990s bikes)
(1" Quill is a teeny bit larger than a U.S. Quarter)
Threaded Headsets Size
Steerer O.D.Stem diameter
Steerer I.D.Crown race
Inside diameterFrame Cup
Outside DiameterThreads
Per inchNotes BMX/ O.P.C. bikes .833"
(21.15 mm)26.4 mm 32.7 mm 24 Used mainly on bicycles with one-piece cranks, also some early mountain bikes. French 25 mm 22 mm 26.5 mm,
27.0 mm30.2 mm 25.4
(1 mm)Obsolete. French steerers usually have a flat filed on the back, rather than a grooved keyway as with other threaded systems. 1" ISO Standard
(25.4 mm)7/8"(22.2 mm) 26.4 mm 30.2 mm 24 This is the standard 1" size. 1" Italian (25.4 mm) 7/8"(22.2 mm) 26.5 mm,
27.0 mm30.2 mm 24 Obsolete. Threads are cut at 55 degrees, but ISO or J.I.S. headsets can be used. 1" J.I.S.
(25.4 mm)7/8"(22.2 mm) 27.0 mm 30.0 mm 24 Older or lower-quality bicycles from Asia 1" Raleigh
(25.4 mm)7/8"(22.2 mm) 26.4 mm 30.2 mm 26 Proprietary size used on Raleighs made in Nottingham, England Austrian
(26 mm)22 mm 26.7 mm 30.8 mm 25.4
(1 mm)Higher quality Austrian bikes use English/ISO French Tandem 28 mm 22 mm 25.4
(1 mm)Obsolete and rare. 1 1/8" (28.6 mm) 1" (25.4 mm) 30.0 mm 34.0 mm 26 "Oversized" (This size is more often used for threadless systems.) 1 1/4" (31.8 mm) 1 1/8" (28.6 mm) 33.0 mm 37.0 mm 26 Mainly used on tandems Threadless Headset Sizing
Threadless Headsets Size/Stem diameter
Steerer O.D.Crown race
Inside diameterFrame Cup
Outside DiameterNotes BMX/ 26.4 mm 32.7 mm Used mainly on bicycles with one-piece cranks. 1" ISO Standard (25.4 mm) 26.4 mm 30.2 mm This is the standard 1" size. 1 1/8" (28.6 mm) 30.0 mm 34.0 mm Most newer mountain bikes use this size. 1 1/4" (31.8 mm) 33.0 mm 37.0 mm Mainly used on tandems 1.5" (38.1 mm) 39.8 mm 49.6 mm Proposed OnePointFive standard for downhill and freeride applications.
A conventional threaded headset consists of four races plus associated parts:
- The crown race, which is pressed on to the bottom of the steerer, just above the crown.
- The lower head race is pressed into the bottom of the head tube.
- The upper head race is pressed into the top of the head tube.
- The adjustable race attaches to the steerer.
The adjustable race is secured by either a:
- Keyed washer and lock nut, for Threaded headsets or a:
- Star fangled nut and cap, or compression rings in the case of some threadless headsets.
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 to be installed inside the steerer.
This type of headset must be used with a special handlebar stem that clamps on to 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 starnut 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.)
- "Diatech" threadsets from Dia Compe 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 stop for the front brake, if the bicycle has a rigid fork and conventional cantilever 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 lower collar downward, taking up any slack in the headset bearings.
- The unfortunately extinct YST "GeForce" headset had a collar which slips over the steerer, and clamped to it with a binder bolt near the top of the collar. The outside of this collar was threaded, and the upper bearing race screwed onto these threads. The upper race assembly was fitted with a binder bolt that clamps it tight onto these threads when the desired adjustment has been attained.
The Ge Force was the only threadless headset which doesn't use the handlebar stem as part of the adjustment. These headsets could be used with any stem, either external clamping or internal expander/wedge type.
Symptoms:
Shakiness
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.Stiffness
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 handlbars will flop to one side or the other by their own weight when you do this.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 handlbars to one side or the other.
- The fork steerer may have been bent in a crash. Generally, this is not reparable, and you would need to replace the fork.
- One or more of the bearing races may be installed slightly crooked. This usually is fixible by having the frame's head tube and the fork's crown machined or "faced" to ensure that the races install with correct alignment. This requires expensive special tools that only better bike shops will have.
- One or more of the bearing races may be loose where they attach to the frame/fork. This will cause looseness as the race wiggles back and forth, even though the bearings may have been tighened up sufficiently to eliminate bearing play. This is most often related to the fork crown race.
If the crown race is loose, sometimes you can fix it by using a hammer and centerpunch to create a series of "craters" in the side of the surface that locates the crown race. The raised edges of the craters may be just enough to remove the slop.
Alternately, you may need to go to a different size headset. In particular, if you are workign with a J.I.S. frame and fork, it can be re-machined to work with an ISO headset. Since J.I.S. head tube races are smaller, and J.I.S. crown races are larger, there is material available to be machined away to upgrade to ISO.
- You may have installed a bearing retainer upside down. This is not uncommon. There is no real standardization as to which way the retainer faces, so it's easy to err. However, if you take a careful look at the races of your particular headset, you will see that if you install the retainer upside down, the retainer can come into contact with one of the races. Just flip it over. The retainer must not be in contact with anything but the bearing balls.
"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.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 descrbing 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.
Overhaul
Headsets are pretty durable, and hardly ever need to be serviced if you have 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.
If you have a work stand available, it helps to turn the bike upside down once the stem has been removed.
If the bearing balls are held in retainers (most are) it is usually best to clean and re-use the retainers. There's little standardization among different makes/models of headsets, so finding a replacement retainer may be difficult or impossible.
Threaded:
Unscrew the lock nut, remove any washers, then unsrew 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.
If the headset uses loose bearing balls, it's generally better to replace them...they're cheap enough! 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 part way 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 lots of grease!
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 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 and complete assebly/adjustment.
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.
Replacement/Installation
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 looseley, 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 would be 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 work stand 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 them remove the race.
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