Knowing the correct sizes of the nuts and bolts needed to attach the water pump can mean the difference between a 30-minute and a four-hour project. In some cases, customers may have brought in a vehicle missing some of these pieces, so you will not have the previous parts for comparison purposes. For most water pumps, you will need bolts with a 5/16-inch thread width, and the width of the head may vary from 13 mm to 15 mm. Depending on the location of the missing bolts, you will need approximately eight bolts ranging in length from 1 1/4 to 2 1/4 inches.