Two bearings and one spacer are required to mount one wheel to a truck. For the assembly of four wheels you need eight bearings and four spacers. The spacers prevent damage to the bearings due to laterally acting forces. Most bearings have an inside diameter of 8mm and therefore fit onto any conventional longboard or skateboard truck. Bearings are often marked "ABEC" (ABEC 3, 5, 7, 9, etc.). This designation describes the manufacturing tolerances for the production of industrial bearings. In the skateboard area, these gradations are rather less relevant. More important than the highest possible number behind "ABEC" is the quality and durabilty of the Bearings. Bones Bearings for example are skate rated and don't use the abec rating.