Pulley Offset: What It Is and Why It Matters

WHAT IS PULLEY OFFSET? ANATOMY OF A PULLEY.

The first thing before we get into what pulley offset is, is to understand the anatomy of a pulley so that we are all on the same page with terminology.

CRANK PULLEY

The most important part of the crank pulley is the Pilot Index. This is what centers the pulley on the balancer and carries the load of the accessories and keeps everything hubcentric. Offset is the measurement from the face of the balancer to the center of the grooves. The Bolt Circle is a measurement of the diameter of a circle that would intersect the center of all the bolt holes.

WATER PUMP PULLEY

Most water pump pulleys we deal with have a 5/8" pilot diameter but not all. Again Bolt Circle is an important measurement — it is the diameter of a circle that would intersect the center of all the bolt holes. Since most water pumps are a 4 bolt pattern this can easily be found measuring center to center on opposing bolt holes (measuring across the pilot). For Offset we measure from the mounting surface backward toward the motor.

POWER STEERING PULLEY

Power steering pulleys are a little bit different in the fact that there is a ton of crossover between engines. Mostly because of the Saginaw pump being such a popular choice among hot-rodders and adapting it from where it is most commonly seen in GM vehicles to Ford and Chrysler applications. Knowing the Pilot Shaft is the first of two critical measurements that are important because there are press fit and keyway styles and different diameters. The most common being 5/8" keyway and 3/4" press-fit in Saginaw pumps, 11/16" press-fit in Ford and 5/8" press-fit in Federal Chrysler pumps. The second critical measurement is the Offset which we measure from the nose of the pulley to the center of the groove.