The bolts that secure an outboard motor to the transom may be the most vital fasteners on any powerboat. Transom bolts secure the motor and maintain its proper alignment, and they are under a lot of stress as the thrust of the outboard pushes and pulls on the transom. For this reason it’s important to regularly check that the transom bolts are properly tightened.
Watch this video to see how Mercury pro angler Brain Latimer tightens the transom bolts to secure his Mercury Pro XS® outboard.
With regular use, transom bolts can actually stretch, not enough that you would notice with your eye, but definitely enough to allow the nut securing each bolt to loosen a bit. Mercury Marine recommends that transom bolts are inspected for tightness to the boat transom before each use of the boat. When inspecting for signs of looseness, look for loss of outboard transom bracket material or paint caused by movement between the outboard mounting fasteners and the outboard transom brackets. Also, look for signs of movement between the outboard transom brackets and the boat transom, or the lift plate/setback bracket if the boat is so equipped. If any looseness of the outboard or mounting fasteners exists, tighten the outboard mounting fasteners to the specified torque. (Ed: Above info from Merc operator manual)
Even if the bolts appear tight, it is good practice to re-torque these fasteners regularly during the season. It’s a simple task that requires a torque wrench and the correct-size socket and a box-end wrench. Check your operator’s manual for the correct torque specification for your outboard model. For most late-model Mercury outboards, the torque spec is 55 lb. ft. if the motor is mounted directly to the boat transom, or 90 lb. ft. if the motor is mounted to a jack plate or setback bracket. Use the box-end wrench to hold the bolt head and apply the torque wrench to the nut.
You can boat with confidence knowing your transom bolts are tight and secure. To access a digital Operator’s Manual with consolidated information specific to your Mercury engine, or to order a printed Owner’s Manual or Service Manual, go to Mercury Marine Owner’s Resources. A Mercury Authorized Dealer is always your ultimate resource for service and maintenance advice and products.