An air Gap? Yes, it's called the internal clearance. Ball and roller bearings un-mounted have an internal clearance which is actually a gap of air. Once the bearing is mounted onto a shaft some of the air gap is gone. A bearing works better with a small amount of clearance. Internal clearance in un-mounted bearings can be felt and measured. An application determines how much internal clearance should be in each bearing. There are 4 types of clearance C2, CN, C3, and C4. C2 clearance is for slow applications. CN is the universal standard clearance. C3 is for high speeds and is standard in America and C4 is high speeds and hot applications.