Measure Headset Bearings

Headset bearings are notoriously difficult to identify,  because they do not follow a conventional ISO or JIS standards for numbering. Different headset and bike manufacturers use their own codes/numbers for the same bearings. 

If you are lucky, your bearings will have their codes or measurements written on the outside. If not, to know what bearings you have. You will need to measure them. Vernier callipers are the best way, but you can get a rough estimate using a ruler too. 

A bearings measurement goes as follows: 

Internal Diameter x External Diameter x Height -  Internal Chamfer / External Chamfer

MH-P03K would be = 30.15x41x6.5 36/45

The measurements are in MM

 

Inside Diameter
Measurement of the internal diameter is a bit trickier to ensure being centred:

• Open the jaws of the vernier calliper and position the flat inner edges of the calliper against the widest portion inside the bearing.
• Ensure that the calliper is held square to the flat edge of the inner bearing race to obtain an accurate measurement.
• Gently close the jaws of the vernier calliper until they snugly fit inside the bearing.

 

Outside Diameter
Measurement of the outside diameter of the bearing is straightforward; 

• Position the jaws on the outside of the bearing
• Ensure that the calliper is held square to the flat edge of the outer bearing race for accurate results.
• Carefully close the jaws of the vernier calliper until they make contact with the outer surface of the bearing.

 

Height
Measuring the height of the bearing is relatively straightforward:

• Close the jaws of the vernier calliper.
• Gently clamp the calliper over the bearing, making sure it holds the bearing securely in place.

 

Chamfer Angles
Headset bearings are often chamfered on the inner and outer races, usually  Unchamfered - 0deg, 36 deg or 45 deg 

Chamfer angles may not always be the same on the inner and outer races. Combinations of 36 degrees and 45 degrees are common

Testing for 36 or 45 degree chamfer angles is straightforward, and you just need a flat rectangular bit of plastic or metal. A credit card is ideal 

•  Insert the credit card into the bearing and ensure that both edges touch fully on the internal chamfer. 
• If it does not touch fully and contact is only made on a section of the chamfer (usually the widest point), you can see a gap on each chamfer,  it will likely be 36 degrees.
• If it does touch perfectly, the card sits flush to the chamfers, it is likely 45 degrees.