How to Check for Worn Outboard Motor Mounts

A loose prop nut, a spun rubber hub or fishing line around the prop shaft can all clunk or vibrate. With the key out, remove the prop (use a block of wood between a blade and the anti-ventilation plate so it cannot turn). Look for line wrapped behind it, a cracked or slipping hub, and worn splines. Put it back with the correct washers and nut, tightened to the spec in your manual. Mercury's owner FAQ says to recheck prop tightness after the first 20 hours and never to run with a loose prop. If the clunk goes away, you are done. See the propeller guide for details.

Mercury calls the transom bolts some of the most important fasteners on a powerboat and recommends checking them before every use. Signs of looseness: worn paint or bare metal where the bracket rubs the bolts, or any movement between the clamp bracket and the transom (or the jack plate). Small motors: tighten the clamp screws by hand. Bolted motors: hold the bolt head with a box-end wrench and torque the nut to the spec in your operator's manual. A loose transom bolt feels a lot like a bad mount, so always rule this out. If the transom itself flexes, see the transom rot guide.

Play in the steering can feel like a wobbly motor. Have a helper turn the wheel gently back and forth while you watch the steering arm, the link rod, the cable end or the hydraulic cylinder and the tilt tube nut. Any clicking or movement between parts before the motor starts to turn points to steering wear, not mounts. See the hydraulic steering or steering cable guides.

Boat on the trailer, motor trimmed all the way down (vertical), engine off, key out. Grab the lower unit near the skeg and push it firmly side to side, then fore and aft, while a helper watches the bracket area with a flashlight (or film it with a phone). Note where the movement happens:

• Between the clamp bracket and the transom: loose transom bolts or a soft transom (step 2).
• Between the swivel bracket and the clamp bracket: worn tilt tube bushings or swivel parts.
• Between the midsection and the swivel bracket, with the rubber flexing or a gap opening up: worn or broken motor mounts.
• On a pontoon, the whole engine pod or bracket moving against the deck frame: pontoon owners report rattles traced to cracked braces where the engine pod bolts to the crossmembers, from years of vibration and trailering. Look at the pod's mounting braces and welds with a flashlight. A cracked pod brace means the motor comes off and an aluminum welder repairs or reinforces it; do not run the boat hard until then.

Some give is normal: the mounts are rubber on purpose, and the motor should spring back smoothly. A dull clunk, free sloppy movement, or a gap that opens and closes is not normal. There is no owner-friendly "inches of play" spec; a dealer compares your motor to the service manual.

On many outboards the lower mounts are visible (or partly visible) where the midsection meets the bottom of the swivel bracket; some sit behind a small cover or the lower cowl pan. Your manual or a parts diagram shows where they are. With a flashlight and mirror, look for:

• Cracked, torn or crumbling rubber, or rubber pulling away from its metal plate
• Rubber that is swollen, soft or sticky (oil or fuel soaks into it, as BoatUS notes for engine mounts in general)
• Rust or white corrosion where the rubber is bonded to metal
• A shiny worn spot or a clean "halo" in the grime, which shows something has been moving
• Loose, missing or broken mount bolts or covers

Do not pull covers or cowl pans off unless your manual shows it is a simple owner job.

With the steering centered, sight down the boat: the motor should point straight back. Look at the gap between the cowl and the swivel bracket on both sides. A motor that sits crooked, sags to one side, or has rub marks on the cowl or bracket may have a collapsed or broken mount.

This test needs cooling water, and with the prop on it must be done in the water, never on a trailer. Tie the boat securely to a solid dock with clear water behind it. (If you run the engine on a trailer for other checks, use external flush muffs or a test tank with the prop removed; the built-in flush port on most outboards is for flushing with the engine off, unless your manual allows running on it.) At idle, have a helper shift slowly from neutral to forward, back to neutral, then to reverse, while you watch the motor from the dock or the stern.

• A big lurch and clunk of the whole motor against the bracket points to the mounts.
• A hard clunk with little visible movement is more likely inside: shift cable adjustment, linkage or gears.
• In reverse, worn mounts often show more, because the thrust pulls the motor the other way.

Keep hands, lines and people well away from the prop.

Mount problems usually show most at idle and when starting or stopping, and the motor may smooth out as rpm rises. Vibration that grows with speed points more to the prop, a bent shaft or the trim angle. A rough-running engine (a bad plug or injector) also shakes the boat at idle; if it runs rough, see the engine troubleshooting guides before blaming the mounts.

Worn mounts let the powerhead move more, which can upset the shift linkage. A cable that is out of adjustment also causes hard, noisy shifts. Check the adjustment as described in the shift cable guide. If shifting gets smooth again, the mounts may still be fine.

Dry swivel and tilt tube bushings can feel stiff, squeak or knock. Grease every fitting on the bracket and steering with the grease your manual specifies (see the outboard grease points guide), then repeat the wiggle test.

If the checks point to the mounts, have a dealer confirm and quote the job. On most outboards, replacing mounts means supporting the motor's weight with a hoist, separating the midsection from the swivel bracket, and on many models removing the steering arm, the lower cowl pan or even lifting the powerhead. The bolts need the correct torque and thread-locker, and the parts are matched to your model and serial number. Doing it wrong can leave the motor poorly secured to the boat. Many shops replace worn upper and lower mounts together. Ask the shop to check the shift adjustment and grease points afterward.

If the mounts are worn but the motor is still firmly attached, run gently: avoid hard shifting, high speed in rough water and big wakes. Trailer it with the support your engine maker recommends so it does not flop on the road. If the motor moves a lot, a mount is broken, or anything looks cracked, do not run it until a shop has looked at it.