Bearing Size: 8mm x 22mm x 7mm (608) - Single Shield - Ceramic Ball Bearing - SET OF 4 plus 2 spacers - This is probably the best value for high-end ceramic bearings on the market. The balls are made of Silicon Nitride, which is an excellent combination of properties that leads to thermal shock resistance, ability to withstand high structural loads to high temperature, and superior wear resistance. Silicon Nitride bearings are used in the main engines of NASA’s Space Shuttles. Silicon Nitride ball bearings are harder than metal which reduces contact with the bearing track. This results in 80% less friction, 3 to 10 times longer lifetime, 80% higher speed, 60% less weight, the ability to operate with lubrication starvation, higher corrosion resistance and higher operation temperature, as compared to traditional bearings. Silicon Nitride balls weigh 79% less than tungsten carbide balls.