Engine Outdrives: Inspect Those Bellows

A friend of mine — and I swear it wasn’t me, honest — bought a nifty little used runabout not too long ago, and got a pretty good deal on it. Or so we thought. It was only eight years old, the price was on the low side of fair, and it looked pretty well kept.

About an hour into its first outing, we learned what might have contributed to the low price. Turns out the U-joint bellows inside the stern drive had split in one of the folds, and it was leaking water into the bilge.

The U-joint bellows is the flexible rubber boot that seals the U-joints, gimbal bearing, and output shaft from water intrusion.

I, er, I mean, my friend didn’t notice it on the test drive, but he didn’t run it long enough for the much water to leak in.  If it is leaking badly, you’ll hear a roaring sound coming from the back of the boat. That’s a  sure sign. It’s basically water getting into the bearing grease and rusting the bearing.

The bellows itself is the beefy rubber boot that goes from the transom assembly to the upper drive housing. It’s a bit difficult to see unless the drive is pinned to one side or another. It looks like an accordion. Oh, and it’s black, just like the rest of a MerCruiser drive. Inside, it’s reinforced with spring steel in the crests of the bellows and is clamped at both ends. The output shaft from the engine runs through it, from the gimbal housing to the where the pinion meshes with the upper gear set.

An exploded view of a Mercruiser Bravo 2 sterndrive assembly.

When bellows crack, water can seep into the boat and soak your output shaft and gimbal bearing. If you’re beached, it’s no big deal because there’s only so far your boat can sink. But let’s say your boat is docked for the evening. Then it is a big deal: Water seeps in the cracks in the boot, floods past the gimbal bearing, and drops into the bilge. Sure, the bilge pump should keep up with the flow, but the real question is how long your batteries will be able to provide power to run the pump.

As you can see, the boot is worth checking, and the factory service interval calls for replacing it every six years. Again, they’re easy enough to inspect if you pin the drives to one side, then the other. Look for cracks and other signs of dry-rotting. Use your fingers to pull apart the folds of the boot so you can see down into its valleys. Also, whenever you have the drive off for servicing, be sure to look at the boots from the inside, or have them looked at. Check for rust on the output shaft and U-joints, which is a sure sign water has intruded.

Not that I’ve ever had that happen. Honest.

- Brett Becker



  1. Hazel Owens says:

    It’s hard to resist a good deal when you see one, but sometimes there’s a reason for the price. Your tip to check for rust on the output shaft and U-joints should save someone from experiencing the same problem that you- I mean, your friend- dealt with. I like the picture of the exploded stern drive; it’s cool to see how all the parts fit together. Thanks for the article!

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