Gimbal Bearings and Engine Alignment: The Rumble You Shouldn't Ignore

Between the engine and the sterndrive leg there is a short driveshaft that has to turn at engine speed while the leg steers and trims around it. It does that through a pair of universal joints and a bearing pressed into the transom assembly, the gimbal bearing, which supports the shaft at the point where it passes through the hull. The gimbal bearing is out of sight behind the bellows, it gets no attention until it makes a noise, and by the time it makes a noise it has usually started to take the universal joints and the engine coupler with it. This guide covers how it fails, how to check it, and why engine alignment is the other half of the job.

How it fails

Two ways. Water is the common one. The gimbal bearing lives inside the universal joint bellows, the big rubber boot between the transom and the leg, and when that bellows cracks or its clamps let go, seawater gets to a bearing that was only ever greased, not sealed against immersion. Rust follows in weeks. Our bellows guide explains why that boot is the cheapest part that sinks boats; the gimbal bearing is the reason it is also the cheapest part that wrecks drivelines. The other failure is load. If the engine is not aligned with the gimbal bearing, every revolution flexes the driveshaft and loads the bearing sideways, and it wears out regardless of how dry it stays. A gimbal bearing that is replaced without the alignment being checked and corrected will fail again.

What it sounds like

A rumble or growl from the transom that is there at cruising speed, changes pitch or loudness when you trim the leg up and down, and often goes quiet in neutral with the engine still running. Trim changes the angle through the universal joints, which changes the load on the bearing, and that is the clue that separates a gimbal bearing from a gearbox or propeller noise. A metallic clicking or knocking at low speed in gear, worst when turning, is more likely the universal joints themselves, whose needle rollers go dry and pit; the two often fail together. Vibration through the hull that grows with speed can be either, or a propeller, and the propeller guide covers how to rule that out first.

Checking it with the leg off

The only proper check is with the sterndrive removed, which is also when the bellows are inspected, so the two jobs belong together. With the leg off, reach into the gimbal housing and turn the bearing's inner race by hand. It should spin smoothly and silently with no roughness, no notchiness and no play when you push it from side to side. Any grinding, any gritty feel, any rust staining on the race or the housing around it, and it comes out. Look at the bellows from the inside while you are there, and at the shift cable bellows and the exhaust bellows, since all three are the same age. On the engine side, pull the driveshaft out of the engine coupler and look at the splines: dry, rusty or worn splines on the shaft or the coupler mean the coupler was never greased at a service, and a coupler with chewed splines will strip in the middle of a trip.

Alignment

Alignment is checked with a bar, a machined steel shaft that slides through the gimbal bearing and into the engine coupler splines. If the engine is sitting right, the bar goes in and out with light hand pressure and no binding. If it catches, drags or will not enter the splines at all, the engine has to be moved on its mounts, up, down or across, a fraction at a time until the bar slides free. Engine mounts sag with age, so an engine that was aligned when the boat was new may be well out ten years later, and a boat that has had the engine lifted for any reason needs it checked before the leg goes back on. Volvo's SX and DP transom units and MerCruiser's Alpha and Bravo all align the same way in principle and each has its own bar. It is a workshop tool, not a thing to improvise, because a bar that is not true gives a false reading and the new bearing is doomed from the first start.

Replacing the bearing

The old bearing is pulled from the gimbal housing with a slide hammer and puller, and the new one is pressed in square to the housing using a driver that bears on the outer race only. A bearing that is cocked a few degrees on the way in is a bearing that is already misaligned, no matter what the engine does. Grease it with marine grease through the fitting on the gimbal housing where one is provided, grease the driveshaft splines and the coupler, fit new bellows and clamps, and align the engine before the leg goes on. Genuine bearings, seals and bellows kits for Volvo transom assemblies are in the Volvo Penta drives and propulsion range, with the bearings themselves listed under bearings and bushings. A complete transom unit is sometimes the better answer on an old boat where the housing itself is corroded, and we hold used transom assemblies for exactly that.

The job is a day's work for a workshop with the boat on a trailer, and it is a job for the off-season rather than the week before the holidays. If your leg is due to come off, ring 0412 532 984 and we can put the bellows, bearing and seals together for your drive before it does.