Each skateboard wheel is mounted on its axle via two bearings. With few exceptions, the bearings are the industrial standard "608" size, with a bore of 8 mm, an outer diameter of 22 mm, and a width of 7 mm. These are usually made of steel, though silicon nitride, a high-tech ceramic, is sometimes used. Many "roller skate bearings" are graded according to the ABEC scale. The scale starts with ABEC1 as the lowest, 3, 5, 7, 9. It is a common misconception that the higher ABECs are better for skateboarding, as the ABEC rating only measures tolerances, which do not necessarily apply to skateboards. 

Bearing performance is determined on how well maintained the bearings are. Maintenance on bearings include periodically cleaning and lubricating them. Bearings that are kept unmaintained have their performance greatly lowered and will soon need to be replaced. Bearing cleaning kits are commonly available on the market. Example kits include the Bones Bearings Cleaning Unit. The ABEC rating does not determine how fast or how durable a bearing used for skateboarding will be. 

All Wheels require skf Bearings in order to roll well. Almost all wheels including Rollerskates, Rollerblades, Scooters and Skateboards require 2 Bearings - One on the Inside and One on the Outside. Most Wheels have an inner Spacing between the Bearings of .400 on Average for Skateboards and Blades or .325 and .285 for Indoor Rollerskates. Rollerskates can also use a Different Bearing Size (which can also date back to the 60's and 70's on Early Skateboards) you can find imprinted on the cover of a precision Bearing. If you have an 8MM Axle the bearing Number is 608. If you have a 7MM Axle the Bearing Number is a 627.

Each skateboard wheel is mounted on its axle via two bearings. With few exceptions, the nachi bearings are the industrial standard "608" size, with a bore of 8 mm, an outer diameter of 22 mm, and a width of 7 mm. These are usually made of steel, though silicon nitride, a high-tech ceramic, is sometimes used. Many skate bearings are graded according to the ABEC scale. The scale starts with ABEC1 as the lowest, 3, 5, 7, 9. It is a common misconception that the higher ABECs are better for skateboarding, as the ABEC rating only measures tolerances, which do not necessarily apply to skateboards. 

H205

H211

H214