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 [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. 

In particular, the ABEC rating says nothing about how well a bearing handles axial (side-to-side) loads, which are severe in most skateboard applications. Many companies do not show the ABEC rating, such as Bones Bearings, which makes bearings specifically for skateboarding, often marketed as "Skate Rated". Each bearing usually contains 7 steel or ceramic ball bearings, although other configurations are used as well.

The 2 Different Covers on Bearings have quite an impact on the care and Maintenance as well as the longevity of the Bearing. If you have a Sealed nsk Bearing that would mean the Bearing Cover is Rubber or Neoprene and will keep the Elements out of your Bearings, which can make your bearings last longer, however this will create Drag on the inner raceway if there is any Contact with the Raceways.

All Wheels require 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 koyo 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.

KTR 200 40x65

KTR 200 90x130

SN515