How to measure sealed headset bearing sizes
The bicycle headset is one of the most neglected components on any bike. Situated in the head tube of the frame, its role is to provide smooth and play-free turning of the handlebar, stem, and fork.
The headset sits in the line of fire of your dripping sweat and so professional mechanics are often greeted with bikes that creak from the front end, that have fork play, and that steer rough. In extreme cases that headset bearing wear or corrosion can lead to an unsafe fork steerer.
The actual process of servicing a modern sealed bearing headset isn’t a particularly advanced skill, but knowing what size bearing you need for replacement can be. By my count, there are over 45 sizes of sealed headset bearings on the market. The reality is that if you thought bottom bracket ‘standards’ were bad, you’re blissfully unaware of headsets and the various formats, fork diameters, and frame types they must fit.
This article isn’t a detailed guide to the endless number of headset variants and ‘standards’. Nor is it a step-by-step service manual. Rather this is simply a quick guide to show you how to measure a sealed headset bearing when it’s time to find a replacement and when the printed numbers have rusted away. Of course, this does assume you had the correct bearing in the headset to begin with.
What you need to measure
In some cases your bike will have a branded headset with an easily identifiable model number that makes sourcing a replacement bearing simple. In other cases the bicycle manufacturer will be able to tell you exactly what bearings you need. And if your headset bearings are in good condition, then the numbers are often printed or etched onto the outside race.
However, those numbers are small and, quite often, they get permanently hidden by corrosion or by being run loose. And that’s where this article comes in.
A normal sealed/cartridge bearing – such as those found in many modern bottom brackets and hubs – has three key measurements: external diameter, internal diameter, and height.
Modern sealed headset bearings are typically quite different to those found in your hubs and bottom brackets, and that’s because the bearings are angular to deal with off-axis loads.
A headset bearing has those same three measurements as above, but then you need to know the angle of the inner and outer races. The bicycle industry has commonly stuck with 36º and 45º angles here, but there are various combinations.
Brands such as FSA have long offered bearing measurement tools for this task, but with the right knowledge, a basic vernier caliper, and a simple business card you won’t need them.
Measuring inside, outside, and the height
A vernier caliper may seem like a specialist tool, but it’s a highly useful tool to own when you’re needing to measure to decimal points of a millimetre. Unfortunately a common ruler simply isn’t accurate enough to differentiate between a 41.8 and 42 mm bearing.
With a vernier caliper and the worn bearing in hand, you want to make note of the three major dimensions of the bearing.
Measure the widest portion of the inside of the bearing while keeping the tool square to the flat edge of the inner bearing race. This is the internal diameter.
Now measure the widest portion of the outside of the bearing while keeping the tool square to the flat edge of the outer bearing race. This is the external diameter.
And then measure the height of the bearing by simply clamping the vernier over the bearing.
We’re almost there.
Step 1: Measure the diameter of the inside race. 
Step 2: Measure the diameter of the outside race. 
Step 3: Measure the bearing height.
How to measure headset bearing angles
It’s the bearing angle that commonly confuses and trips people up, and often leads to the purchase of a specialist tool. However, there’s a clever measurement trick here that mechanic Kevin Wells of CycleZone Darwin recently shared with me. It was a real “uh, duh” moment.
To measure the angle of the inside bearing race you’ll need a rectangular business card or similarly slim and square-edged object. You simply use the corner of the card as a gauge within the bearing – if the inner race has a 45º angle then the business card will sit perfectly against both sides of the race. If it’s a 36º bearing then the card will show a gap at the race.
In this example the business card is sitting flat to the inner race angle. This bearing has a 45º inside angle. 
In this example the business card is not sitting flush to the inside race angle. Therefore this headset bearing has a 36º inside angle.
The trick for measuring the outside angle (most of which are 45º) requires you to have a spare bearing with a known 45º angle. Here you simply place the old bearing against the new and try to form a 90º angle from them. If they’re both 45º bearings then you’ll have a square corner. If one bearing (or both) is 36º then you won’t achieve a 90º angle.
Of course, there are other methods for measuring bearing angles, including the use of a protractor. Those with a keen eye may be able to spot the difference, too.
Matching the size and common headset bearing types
So you’ve now got your measurements.
You’ll often need to account for tolerance differences – it’s normal to have to round up. For example, the bearing shown in the photos was measured to have an inner diameter of 30.14 mm, an external diameter of 41 mm, and a height of 6.47 mm. The inside angle was 45º and the outside angle was also 45º. This bearing is sold as a 30.15 x 41 x 6.5 mm 45/45º headset bearing. This bearing is commonly found in Ritchey headsets and is often referred to as a MH-P03.
For another example, let’s say I measured a headset bearing from the bottom of a 1 1/2″ Cane Creek headset. I measured 39.9 mm, 52 mm and 6.48 mm for the inside race, outside race, and height respectively. Then the business card didn’t sit square on the inside, telling me the inner race was a 36º angle. Meanwhile the outside was confirmed to be 45º. Officially this is a 40 x 52 x 6.5 mm 36/45º bearing (note the inner angle is listed first).
Below is a list of a few common headset bearing sizes and common corresponding manufacturing part numbers (provided by my go-to bicycle bearing distributor in Australia, DIY-MTB):
- 30.15 x 41.5 x 6.5 mm 36/36º (aka TH-872, MR043, MR172)
- 30.15 x 41 x 6.5 mm 36/45º (aka TH-873, MR122, MR0401, MR054, MR115, or 1 1/8″ Cane Creek headset bearing)
- 40 x 51.8 x 8 mm 36/45º (aka TH-073, MR110, MR127, MR019)
- 30.15 x 41 x 6.5 mm 45/45º (aka 1 1/8” Ritchey headset bearing, MH-P03, MH-P03M)
- 30.6 x 41.8 x 8 mm 45/45º (aka 1 1/8″ Campagnolo style, TH-870)
- 34 x 46.8 x 7 mm 45/45º (aka MR082, MR100, MR168, TH-970)
- 40 x 52 x 6.5 mm 36/45º (aka Cane Creek 1.5″ lower bearing)
- 40 x 52 x 7 mm 45/45º (aka TH-070, MR128, MR170, MH-P16, MH-P16M)
For further information on headset types and servicing consult Park Tool’s resource page. The vast majority of bikes from the past decade should conform with the Standardized headset identification system (SHIS).
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