How To Measure Sealed Headset Bearing Sizes

One of the parts of the bike that gets the least attention is the headset. Its function is to offer effortless and play-free twisting of the handlebar, stem, and fork. It is located in the head tube of the frame. Professional mechanics are frequently met with bikes that squeak from the front end, have fork play, and steer awkwardly because the headset is directly exposed to their sweat. Extreme headset bearing wear or corrosion situations may result in a fork steerer that is unsafe. Although maintaining a modern sealed bearing headset doesn’t require very complex skills, deciding what size bearing to replace can be challenging. There are, in my estimation, more than 45 sizes of sealed headset bearings. Here is a helpful guide on how to take measurements. 

What do you have to measure?

Your bike may occasionally come with a branded headset with a clearly visible model number that makes finding a replacement bearing simple. In other circumstances, the bicycle maker will be able to specify the particular bearings you require. Additionally, if your headset bearings are in good shape, the numbers are frequently carved or stamped on the outer race. The numbers are small, though, and they frequently disappear permanently due to rust or being let loose. The purpose of this paper is to address that. The external diameter, internal diameter, and height are the three main dimensions of a typical sealed/cartridge bearing, like those used in many hubs and bottom brackets nowadays.  The sealed headset bearings used now differ significantly from the ones in your old bikes. The same three measurements apply to headset bearings, but you also need to know the angles of the inner and outer races. The 36 degrees and 45 degrees angles have remained popular in the bicycle business, but there are more combinations. Popular companies have long produced bearing measurement equipment, but with the correct information, a good vernier caliper, and a business card, you won’t need them. 

Measuring The Height, Outside, And Inside

Although a vernier calliper may appear like a specialised equipment, it is quite helpful to have when you need to measure to millimeter decimal points. Regrettably, a standard ruler isn’t precise enough to distinguish between a 41.8 and 42 mm bearing. Only vernier calipers can measure it to millimeter closeness. Insert the tool square into the inner flat edge of the bearing. It will give the internal diameter. Now to measure the outer diameter, adjust the tool square to the outer diameter. For measuring height clamp the vernier calipers over the bearing. 

Measuring headset bearing angles

It is the most confusing metric so much so that people purchase special tools unless they know a trick or two. Grab a rectangular business card or a similar object. Now you can use the corner of the object as a gauge. If it sits perfectly, then the bearing angle is 45 degrees, otherwise, if the object shows a gap it it 36 degrees at the race. You also use a protractor as a measuring tool. 

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