Bearings are a small but extremely important part of your skates. Roller skates require two bearings for each wheel, or a full set of 16 bearings. These can be put in with a bearing tool or inserted with a bearing press. The majority of skates use 8mm bearings (the diameter of the inside of the bearing and the size of the skate axle on your skate trucks) but some use 7mm bearings. All bearings fit all roller skate wheels.