Bicycle Headset Introduction
The headset is a system of ball bearings, bearing races, spacers, seals, and nuts or bolts that attaches the fork to the bike's head tube.
Click this link for a headset parts list and detailed photos.
The main problem with headsets is they tend to come loose. A loose headset can make a bike wobble under speed and will ruin itself, so test your headset often.
Headsets may be threaded or threadless.
Threaded headsets are found on bikes with threaded forks: older bikes, cheaper new bikes, cruisers, and BMX bikes. The fork steering tube is threaded. The top bearing race is threaded onto the fork, cupping the top ball bearings. Then a locknut is threaded onto the fork, usually with lock washer in between. The fine adjustments to tension on the bearings are made with the top bearing race, and then the locknut is forced against the top bearing race to hold everything together. The stem is placed into the fork steering tube and held in place by a long chock nut.
Threaded Headset and Stem
Threadless headsets are found on most newer good quality bikes. The fork steering tube is not threaded. The top bearing race is fitted onto the fork and held in place by a plastic compression ring. The stem and spacers are slipped on the fork. A top cap is placed on the stem, and a compression bolt is placed through it and into a star nut that has been forced into the interior of the fork steering tube. The fine adjustments to tension on the bearings are made with the compression bolt. Then the stem is tightened onto the fork steering tube.
When first introduced, threadless headsets were called aheadsets.
Threadless Headset and Stem
Threadless headsets work better. They come loose less often and are lighter and stiffer. They are also much easier to adjust.
Headset Sizes
Just like forks and stems, headsets come in sizes. On decent quality adult bikes, the sizes are 1", 1 1/8", and 1 1/4". Headset size is measured by the diameter of the fork steering tube. The headset size must match the fork size.
This section is divided into subsections: