Engine Alignment on Sterndrives and Inboards: What to Check and When

• New or worse vibration, often felt in the deck or the wheel
• A growl, rumble or grinding from the transom area, sometimes louder when turning (sterndrive gimbal bearing or U-joints)
• A burning rubber smell from the engine compartment, or an engine that revs up while the boat stops pushing (a failing sterndrive coupler)
• On inboards, a shaft that visibly wobbles, a shaft seal that suddenly drips more, or a cutless bearing that wears out early
• Anything that started after an engine or mount replacement, a repower, a hard grounding or striking something

Write down when it happens (rpm, in gear or neutral, turning or straight). Your mechanic will use it.

Boat on its trailer (chocked) or securely in its slip. Engine off, key out, battery switch off. For sterndrives on the trailer, trim the drive down. Let a hot engine cool before reaching around the exhaust.

BoatUS and Boating Magazine both say props cause a lot of vibration. Look for bent, chipped or dinged blades and fishing line behind the prop, and check that the prop is tight (see the propeller guide). Boating Magazine notes that a bent blade tip can also mean a bent shaft. Fix any prop damage before you chase alignment.

Alignment depends on the mounts, and a failed mount lets its corner of the engine drop. Boating Magazine describes a damaged mount lowering one corner and opening the coupling gap on the opposite side. With a flashlight, look at every mount and its bracket for:

• Cracked, torn or separated rubber, or rubber swollen by oil or fuel
• Rust where rubber meets metal, a bent stud, or a bracket sagging low on its stud
• Disturbed grime, rubber crumbs or fiberglass dust around the base, which show movement (BoatUS tip)
• Loose mounting bolts, or lag bolts that spin without tightening, which can mean rotten wood in the stringers (see the transom and stringer rot guide)

BoatUS also suggests watching one mount at a time against the others: one that shakes more than its neighbors is suspect. On twin-engine boats, compare port and starboard.

BoatUS describes a "backdown test" that marine diesel pros use to stress the mounts: with the boat securely tied to a strong dock, run in reverse at moderate rpm while someone safely watches the engine, which should stay tight with little movement. Only do this if your dock lines and cleats are up to it, keep everyone clear of belts and moving parts, and stop if anything looks wrong. A shifting engine means mount trouble, and mount trouble means alignment trouble.

With the engine off, battery switch off and the drive in neutral, have a helper turn the prop slowly by hand while you listen at the transom inside the boat (or use a mechanic's stethoscope). A rough, grinding or clicking feel or sound can point to the gimbal bearing or the U-joints. Also look for water tracks at the transom and water in the bilge, which can mean worn bellows (see the bellows and gimbal guide).

BoatUS and Boating Magazine both describe pulling the drive as the time to inspect the bellows, U-joints and gimbal bearing, check the engine-to-drive alignment and grease the splines. A technician slides an alignment tool through the gimbal bearing into the coupler and moves the engine on its mounts until the tool slides freely. Boating Magazine quotes a repower installer who said Mercury wants the tool to slide in and out with fingertip effort, and that bad alignment can destroy a new coupler within about 40 hours. Ask your shop to check alignment and grease the coupler splines every time the drive is off for bellows, U-joints or a gimbal bearing.

DIY owners often ask about cheap or home-machined alignment bars. If a tool binds near the end, do not force it or start shimming: first make sure it is the right tool for your exact drive and transom assembly, then look for a mount problem. A common finding is one rear mount sitting lower than the other, or a front mount adjusting nut that has turned on its own because its locking tab was never bent back. Mount designs and washer stacks changed over the years on the same drive family, so check against the service manual for your transom serial number, or have a shop do it.

Mercury's FAQ ties coupler life to two things: alignment and lubrication with the spline grease it specifies. The coupler is usually greased through a fitting or with the drive off; your maintenance schedule says how often and how. On many engines this is a dealer job.

BoatUS suggests having a helper turn the shaft slowly by hand (engine off, transmission in neutral) while you watch it from several angles where it is exposed; a bent shaft or a misaligned coupling can show as a wobble. Run a finger around the shaft where it enters the stuffing box or shaft seal: a worn groove points to long-term misalignment or a worn bearing.

BoatUS: grab the prop and the shaft near the strut and push hard up and down and side to side. Noticeable movement inside the cutless bearing means it is worn. There should be no fore-and-aft movement at all; if there is, suspect a loose prop, a loose coupling or transmission wear. A cutless bearing that keeps wearing out is a classic sign of misalignment (see the cutless bearing guide).

Without unbolting anything, look at the coupling bolts (all present and tight, with safety wire or lock nuts if fitted) and at any flexible coupling or damper disc between the transmission and shaft for cracks, tears or distortion. BoatUS suggests removing such inserts to inspect them; that is part of the pro check.

For reference, so you know what to ask for: the mechanic unbolts the coupling, brings the two faces together and measures the gap all the way around with feeler gauges, then raises, lowers or shifts the engine on its adjustable mounts until the faces are parallel, and rechecks before bolting up. Boating Magazine recommends doing this with the boat floating, because a hull can change shape slightly between the trailer or jack stands and the water. BoatUS adds a quick clue: if the shaft turns much more easily once disconnected from the transmission, misalignment is likely. The exact tolerance comes from your engine or transmission maker.

• After replacing engine mounts, a coupler, a gimbal bearing, a shaft or a strut
• After a repower or any time the engine has been lifted
• After a hard grounding or hitting a submerged object
• When the warning signs in step 1 appear
• At the interval in your engine's maintenance schedule (Mercury describes it as a periodic check)
• On inboards, after launching a boat that was stored ashore, once it has settled in the water