What Is This Shimming That I Read About on an Outdrive?

When something doesn’t quite fit, or needs a little more space, width or thickness – even if it’s within the factory-mandated tolerance – you “shim it up”: you add a washer or spacer to ensure the fit is perfect. That’s the story on shims in the outdrive. Although their presence is part of the design, they're available in various sizes to hold the gears exactly in the relationship wherein the teeth mesh properly.

  1. The Outdrive

    • The outdrive -- or sterndrive -- is a slightly modified lower unit from an outboard motor. It has its own lubrication -- separate from that of the inboard motor -- and a cooling system that uses water drawn from the water in which it is immersed.

    Shims in the Upper Outdrive

    • The gears for an inboard/outboard are housed in the outdrive, the part of the drive that houses the gearbox and propeller shaft As with any piece of precision machinery, close tolerances are essential to prolong the life of the unit and to transfer power efficiently. To ensure these tolerances remain stable, various washers and spacers -- the shims -- are used. The shims in a typical outdrive include a forward-gear thrust washer, a reverse-gear thrust washer, a reverse-gear shim and a pinion shim. Even the relationship between the propeller and the motor is maintained by a shim called the propeller-shaft thrust washer.

    Shims On the Propeller Shaft

    • Several shims are used on the propeller shaft to account for minor differences in the length of the shaft, the relationship between the ring and pinion gears that drive the shaft and the placement of the propeller – almost always inexact, because of differences in propellers and mounting processes. The thrust hub that carries the propeller may be over- or undersized. The washers and spacers that hold the shaft’s position in the bearing may not fit precisely. When the shaft must be shimmed into place, the washers themselves must sometimes be shimmed so they maintain their relative positions.

    The Process of Shimming

    • While rebuilding the outdrive of a modern inboard/outboard requires specialized tools, you may attempt the project. Ideally, you'll be able to reuse the shims that were already in place in the outdrive. Unfortunately, shims may wear down as they fulfill their purpose of holding gears together. This means you may be forced to re-shim an outdrive because of the wear imposed on the shims by the parts they hold in place. You will need to measure the gap you need to fill with calipers and measure the size of the shims available, with an outside micrometer.