All skateboard bearings are the same size, so you don't need to worry about size when selecting bearings. However, the ABEC rating denotes the quality of materials and precision used in manufacturing the bearing. Higher rated bearings are higher quality.
Skateboard bearings are used to mount the skateboard wheels to the skateboard axle. Skateboard bearings are all the same size and will fit any skateboard wheel that Warehouse Skateboards sells. While they do not vary in size, they do vary in quality. The rule of thumb tends to be that the more expensive the skateboard bearings, the higher the quality. When you choose to purchase inexpensive bearings, they tend to be made with cheaper materials, so when you come to a sudden stop, your board may not perform as well. The more expensive skateboard bearings are actually made with better sealing, more inner ball bearings and higher-quality metals.
Most bearings are measured by an ABEC rating. The higher the ABEC rating, the more accurate and precise the bearing will be. This rating system includes grades 1,3,5,7 and 9. The closer you get to an ABEC 9 rating, the faster and less friction you will have when skateboarding.
ABEC 1 skateboard bearings tend to be the least expensive, most crude and the least precise.
ABEC 3 skateboard bearings tend to be inexpensive and do not roll very smoothly or fast.
ABEC 5 skateboard bearings are the norm in skateboarding. With this rating you get a good amount of speed at an average cost.
ABEC 7 skateboard bearings are very fast and smooth and tend to be slightly more expensive.
ABEC 9 and higher skateboard bearings are extremely fast and are used more for downhill skating and skaters that want to go insanely fast.
Many companies do not use the ABEC rating scale, such as Bones Bearings. Bones Bearings use their own rating system known as Skate Rated. These bearings do not follow the ABEC ratings because they are superior in quality and the ABEC rating ignores many quality factors in which Bones Bearings prides itself.