To mount a wheel onto a truck, two bearings and one spacer are needed. For the assembly of four wheels, you will therefore need eight bearings and four spacers. The spacers prevent damage to the bearings from lateral forces. Most bearings have an inner diameter of 8mm, making them compatible with any standard longboard or skateboard truck. Bearings are often labeled with "ABEC" ratings (ABEC 3, 5, 7, 9, etc.). This designation describes the manufacturing tolerances for industrial ball bearings.