Here at Medusa, we carry a full stock of skate wheel bearings. For roller derby skates, you will need sixteen bearings, or two bearings for each wheel. 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. Call us if you are unsure of the size of bearings you need. All bearings fit all the roller skate wheels.