A bicycle headset is one of those components whose inner workings you can remain blissfully unaware of, until something goes wrong. (Or until you bend your fork.)
But headsets are oh-so-critical, because they are what attach the fork and front wheel to the rest of the bike, allowing you to steer. Simply put, a bike with a straight fork and a properly adjusted headset will steer smoothly, and track straight when you take your hands off the handlebars.
And while bike technologies and standards have marched on, threaded headsets and forks – as shown above – are still by far the most common type on bikes that are on the road today. Before diving in, though, let's review relevant headset terminology and how its parts fit together.
Anatomy of a threaded headset
The top-most component is the locknut, followed by a spacer, and the top race, which controls the adjustment in the headset. Beneath that we have the first set of bearings (generally 3/16" or 5/32"), which are sitting in a bearing retainer; the black plastic ring beneath is a dust seal that fits snugly into the top race. The black metal headset cup is the final component that fits on the top of the fork; it is press-fit into the top of the bike frame's head tube.
The bottom headset cup (also inscribed with Shimano 600) is press-fit into the bottom of the head tube. Then comes the second set of caged bearings, a second dust seal, and lastly the crown race, which is press-fit onto your fork.
Press-fit pieces – a recurring theme with bicycle headsets – are finely machined components of a nearly similar diameter that are smushed together and which (generally) stick together for the duration of their lifecycle.
Adjusting a threaded headset
Since headsets in need of adjustment are encountered relatively rarely, it may take a while for you to master this skill. The basic concept is similar to adjusting the hubs on a bike wheel. We're seeking that sweet spot, where there is no mechanical play ("wobble") in the fork, but where the moving parts turn smoothly without binding.
You'll certainly need a pair of thin headset wrenches to accomplish this task. Clamping your bike's top tube to a stand – with the front end inclined forward at a 45-degree angle makes our task easier – as we can use gravity to assess how well the headset turns.
Adjusting a headset is an iterative process, where we start out with the headset a little "too tight," and gradually loosen it to find that Goldilocks adjustment. A good starting point is to tighten the top headset race until it is finger-tight, and then to apply the headset wrenches to it and the locknut, turning them against each other to lock them in place.
The headset will certainly be too tight at this point. You can verify this by turning the handlebars sideways and releasing them, which lets the weight of the front wheel and fork do the work of turning the headset for you. Initially, it won't turn very far before coming to a rest.
To loosen the headset adjustment a tiny bit, use the top headset wrench to loosen the headset locknut ever so slightly – turning it maybe 30-degrees counterclockwise – while using the bottom wrench to keep the top headset race in place. Then, hold the locknut in place while turning the headset race counterclockwise against it to fasten this now-slightly-looser adjustment in place. Check again how the headset turns; repeat the step outlined in this paragraph as needed.
Always also check for play in the fork, by grabbing the stem with one hand and the fork with the other, and trying to move it back and forth. The fork should not move laterally—even the tiniest bit. If you feel any amount of play in the fork, start over.
Replacing a threaded headset
You may need to replace a threaded headset if, despite your best attempts at adjusting it, the bike's front end still turns like this:
This behaviour is called "indexed steering;" it is an indicator of excessive wear, or past abuse. It's possible that someone rode with an overly tightened headset for a while. Individual bearings, squeezed between the headset's cups and races, have gradually carved out little dimples in the two rolling surfaces where they would prefer to rest. Engineers call this "brinelling." (I plan to keep riding it, however, because this early 80s Shimano 600 Arabesque headset is a beautifully machined component that I'm loth to replace—and hey, it still steers!)
Anyhoo, if you were to replace a threaded headset, this is how you would proceed. We'll use a 70s French Peugeot as our guinea pig; its original fork was bent, so I took the opportunity to swap out its headset as well.
Oh, and did I mention we will not be using the proper, Park Tool-approved tools for this task? Yeah, no, we won't.
So here we have a seat post that has been cut and spread open, which we will use as a wedge to drive out the headset cups. Slip it into the head tube, until it comes to rest against the bottom headset cup inside the frame.
The "tool" is now in position. Bring out the hammer.
With a mighty thwack, the bottom cup has now been driven out. I took out the post, flipped it around and stuck it in the other way, and did the same to the top cup . (Unexpectedly, both the headset cup and the "tool" shot out of the head tube on a ballistic trajectory.)
The new headset cups were tapped into place with a mallet, and an old friend was called upon: two nuts and some flat, wide washers on a length of threaded rod. The nuts were slowly tightened, gradually pushing the headset cups into the frame. It is important that both cups end up completely flush with the head tube, with no space between them and the frame. If either of the cups end up sitting slightly crooked, use the seat post to drive them out and start over.
And this is the end product. The new headset and fork have been installed, tightened, and the bike will soon be ready to be bought off Craigslist by an unsuspecting college student, who'll run it into the ground.
[Images by author]