How to Inspect Sterndrive Bellows, Gimbal Bearing, U-Joints and Shift Cable

Before anything else, look and feel along the inside of the transom around the drive opening and in the bilge below it. Water collecting there with no other source, water tracks running down from the transom openings, or a bilge pump that cycles on and off more than usual can mean a bellows leak. Note the date and how much water you find so you can compare.

With the boat on the trailer or out of the water, trim the drive fully up so the bellows are visible behind the transom plate. Turn off the battery switch so nobody can trim it while you are behind it. Do not reach under or between the drive and the transom unless the drive is supported.

Use a flashlight and mirror. Look for:

• Cracks, especially deep in the folds (BoatUS says to look in the folds especially)
• Dry, chalky, faded or hard rubber
• Pinholes, cuts or chafe marks
• Areas that feel sticky or soft

Turn the drive left and right to open up the folds you can't see straight on. Photograph what you see so you can compare next time.

Scrape off any barnacles, mussels or other growth on or next to the bellows. A BoatUS surveyor warns that shells are razor sharp and can slice the rubber as the drive steers and tilts. On boats kept in the water, this is one more reason to haul or lift the drive and look regularly.

The bellows are held on with stainless band clamps at each end. Look for loose, broken or heavily corroded clamps and for gaps where the rubber meets the housing. Do not crank the clamps tighter to "fix" a leak; that can cut the rubber.

Look at the smaller shift cable boot and the exhaust bellows if you can see them. Same signs: cracks, splits, dry rubber, loose clamps. A failing exhaust bellows often makes the boat noticeably louder. BoatUS notes the shift bellows is the smallest and the most likely to crack in its folds, so give it extra attention.

If the drive is removed for service, look inside the U-joint bellows:

• Water inside: the bellows has a hole. Water also wears out the U-joints and gimbal bearing quickly.
• Gear oil inside: usually a leaking drive shaft seal, not a bellows problem. Have it checked.
• Rust on the U-joints or yoke: water has been getting in for a while.

The gimbal bearing supports the drive shaft where it passes through the transom. Signs it is wearing:

• A growl, grinding or rumble from the back of the boat that changes with engine speed, often louder in gear
• Noise when turning
• A rough or gritty feel when a mechanic spins it with the drive off

Worn U-joints often clunk when shifting or make a clicking or knocking sound while turning. These parts fail faster once water gets past the bellows, so noise plus an older bellows is a good reason to pull the drive.

Shift through forward, neutral and reverse at the dock with the engine running and the boat secured. Hard shifting, grinding, a shifter that won't return to neutral cleanly, or a shift that "clunks" harder than usual can point to a worn shift cable, a split shift cable bellows, or an out-of-adjustment linkage. Shift cable adjustment on many drives is precise; leave it to a pro.

Mercury advises storing a sterndrive in the down position when the boat is stored with the drive installed. Keeping it tilted up for long periods can permanently deform the bellows and cause them to fail. For a boat kept in the water, Mercury gives a second reason for down: it keeps the drive's anodes in the water, where they can protect it. A BoatUS surveyor adds a winter reason: rain and snowmelt can run into the exhaust outlet at the propeller hub of a drive left up, collect inside and freeze, which can crack the housing. If the drive must stay up, cover the prop and hub with a sturdy waterproof cover. Trailer it according to your manual's instructions.

Replace the bellows if you saw any cracks, dry rot or water inside, if they are older than your manual's interval, or if age is unknown. Most mechanics replace all bellows at the same time, along with the gimbal bearing if it is rough and any worn U-joints. It is a drive-off job with special tools for bellows installation and engine alignment. You may find videos showing the exhaust bellows changed with the drive still on (it is the only one that can be). Skip that shortcut: the U-joint and shift cable bellows are the same age, and a surveyor's rule is that when one of a set of underwater parts wears out, you check and replace them all.

Whenever the drive is off, have the engine coupler splines greased and the engine alignment checked. Mercury notes that engines can drift out of alignment with time, and that misalignment or a lack of coupler lubrication causes damage that the warranty does not cover.

Write the date, what you saw and your photos into your BoatMend log. Next inspection, compare. A crack that is growing is a clear signal.