Headset Bearing - 52 x 40 x 7 x 45/45

The inner diameter is the opening in the centre of the bearing through which the axle passes: the hub axle for a wheel bearing, the crank spindle for a bottom bracket bearing and the steerer tube for a headset bearing. Measure the opening in the centre of the bearing and choose the matching size.

The outer diameter is the overall size of the bearing, measured from outside to outside of the outer bearing ring. This size determines how the bearing fits into its housing: the hub, the bottom bracket shell or the head tube. Measure the outer ring of the bearing and choose the matching size.

The thickness is the width of the bearing. Measure the distance between the two flat sides of the bearing and choose the matching size.

The angulation (bearing angle) of a headset bearing refers to the angle at which a bearing ring is ground, and to the angles at which the bearing makes contact with the crown race or compression ring on the steerer tube (inner ring of the bearing) and with the frame (outer ring of the bearing). Use the same angulation as the bearing you are replacing.

The values at a glance:

• 36/45 — an angle of 36° on the inner ring of the bearing and an angle of 45° on the outer ring of the bearing.
• 45/45 — an angle of 45° on both the inner and outer ring of the bearing.