This is an excerpt from Essential Bicycle Maintenance & Repair by Daimeon Shanks.
In the threadless-style headset, the steerer tube is smooth and passes through the entire headset. A threadless-style stem is clamped directly to the steerer tube, and adjustments are made using an expanding bolt and top cap that loads the headset bearings. Also, instead of using a threaded bearing cap to achieve bearing load, the threadless headset uses a top cap that pushes the stem (and stem spacers) against the top race of the headset to load the bearings.
Much of the installation and adjustment procedures used on a threadless headset are exactly the same as with a threaded system. That's because the basic principle of top-loaded bearings is the same between the two; they just go about it in different ways.
Threadless Headset Installation
Threadless headsets are much more common on road bicycles these days and may or may not have bearing cups. Installing headset bearing cups requires the use of a special headset press tool. If you don't have the correct tool, do not attempt to install cups using any other method.
1) Install Bearing Cups
Before you begin, note that many modern bicycles do away with external bearing cups altogether and use an integrated cup system. If your bicycle has this style, simply apply grease to the integrated cups and set the bearings into the frame by hand. Integrated threadless headset bearings come in two popular sizes (the International Standard and the Campagnolo Standard) and are differentiated by having different bearing contact angles. Refer to your bicycle's instructions to make sure that you have the correct bearings.
If you do not have integrated bearing cups, follow these steps:
• Lightly grease the head tube of the frame where the headset cups will contact. If you have a titanium frame, use an antiseize compound.
• Slide the top cup onto the upper race of the headset press, and insert the headset press through the top of the head tube.
• Slide the bottom cup onto the lower portion of the headset press, and install the lower race of the headset press. Hold the bottom of the headset press with a wrench so that the tool does not spin as you press in the headset cups. As you install the cups, make sure that the cups are straight and are not binding in the head tube.
• Stop when the cups make flush contact with the head tube. Do not overtighten because the cups can be damaged if they are pressed after they are firmly seated in the head tube.
2) Install Fork Crown Race
The fork crown race sits on the crown of the fork and is the contact point for the lower bearing assembly. Use a crown race setter and the correct attachment for your specific size and type of crown race.
• Slide the crown race down the steerer tube until it reaches the flange at the bottom.
• Slide the crown race setter over the crown race and strike the crown race in an up-and-down motion until the race is flush with the crown of the fork. Check that there are no gaps between the crown and crown race.
3) Install Lower Bearing
Apply a thin layer of grease on the fork crown race and slide the lower bearing onto the race. Liberally apply grease onto the lower bearing cup and slide the fork into the head tube from the bottom until the lower bearing is seated into the lower cup.
4) Install Upper Bearing
Liberally apply grease to the upper cup and install the upper bearing into the upper cup. Apply a thin layer of grease on top of the upper bearing.
5) Install Centering Cone and Top Cover
Threadless headsets use a centering cone (sometimes called a compression ring) that is pressed into the bearing by the top cover to achieve bearing load.
6) Install Stem and Spacers
Using headset spacers, determine the correct stem height. If the steerer tube needs to be shortened, see the instructions on cutting on page 32.
7) Insert Expansion Plug or Star Nut
The top cap presses the stem and spacers down on the top cover to achieve bearing load. To do this, the top cap is anchored to the fork using either a star nut (for use with steel, aluminum, or titanium steerer tubes) or an expansion plug (for use with carbon steerer tubes) and a long bolt that is set in the top cap.
To install a star nut, you must use a special star nut setter. This tool consists of two sleeves: the inner sleeve has a threaded post on which the star nut is threaded, and the outer sleeve slides over the steerer tube. The inner sleeve is hit with a hammer to drive in the star nut, and the outer sleeve keeps the inner sleeve straight and has a lip to stop the star nut from being inserted too far.
If you are using an expansion plug, install the plug so that the top cap has room to be adjusted both in and out. The expansion plug will have an Allen bolt that can be tightened to expand the plug in the steerer tube and hold it in place.
Read more from Essential Bicycle Maintenance & Repair by Daimeon Shanks.