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abec 9 Bearings

ABEC scale - WikipediaThe ABEC scale is an industry accepted standard for the tolerances of a ball bearing.  There are five classes from largest to smallest tolerances: 1, 3, 5, 7, 9Bullseye Bearings ABEC 9 Skateboard  The Bullseye ABEC 9 Bearing. Bearings are circular pieces of steel with flat sides that contain eight lubricated balls, their purpose is to keep wheels running 
Abec vs. Skate Rated™ - Support - Bones BearingsIn an application like this, an ABEC - 7 or 9 bearing rating may be appropriate. However, a skateboard with 54mm wheels turning 20,000 RPM will be traveling