Adjusting Boat Engine Mounts: A Step-By-Step Guide

Adjusting boat engine mounts is a complex task that requires careful attention to detail and a good understanding of engine mechanics. The process involves making precise measurements, using appropriate tools, and following specific procedures to ensure the engine is correctly aligned and secured. While it can be a challenging task, with the right knowledge and equipment, boat owners can often adjust their engine mounts themselves, avoiding the need for costly professional repairs. However, it's important to recognise the limitations of one's skills and the specific requirements of one's boat, as some engines may be more difficult to access and work on than others.

Signs of failing engine mounts

Engine mounts are essential for boats as they secure and stabilise the engine to the chassis, reducing vibration and noise. When engine mounts go bad, it's usually easy to tell as there are several symptoms to watch out for. Here are the signs of failing engine mounts:

Excessive Noise

The first sign of a failing engine mount is often the notorious clunking or banging noises that occur when parts of the engine make an impact with other components. This can be alarming, sometimes sounding like the engine is about to break loose and fall out of the vehicle. However, it is unlikely that the engine will go anywhere, even with one broken mount.

Violent Vibrations

Increased vibrations while driving is another common symptom of a bad engine mount. The properties that reduce vibration are diminished when a mount becomes damaged, leading to intense vibrations that can cause further damage to the engine and surrounding parts.

Unnecessary Engine Movement

Failing engine mounts can cause the engine to lurch forward, especially when you first accelerate. The engine may also move more when the transmission shifts gears, resulting in a noticeable jolt. The engine may not run smoothly, and you may notice that it tilts, sags, or appears misaligned.

Physical Damage

Upon inspection, you may notice physical damage to the surface of the engine mounts, such as corrosion, warping, or cracks. The rubber component may also start to flake and crack, reducing the effectiveness of the mounts in supporting the engine.

Rough Idle

A failing engine mount can cause the engine to jump or idle roughly while the vehicle is stationary. This can feel similar to a failing sensor or a bad spark plug, but upon closer examination, it becomes clear that the idle itself is not acting erratically.

Removing the old engine mounts

• Assess the condition of the engine mounts: Before beginning any work, it is important to confirm that the engine mounts are, in fact, worn out and need replacement. Look for signs such as rubber dust or residue around the mounts and excessive engine vibration.
• Gather the necessary tools and materials: You will need basic tools such as wrenches, a ratchet, sockets, and extensions. Additionally, new engine mounts, matching hardware, and a car jack will be required.
• Prepare the work area: Ensure you have easy access to the engine and prop shaft. Remove any surrounding components that may obstruct your work.
• Support the engine: Use wooden blocks or a car jack to securely support the engine and prevent it from moving during the removal process. This step is crucial for safety and stability.
• Loosen and remove the transmission output flange: Carefully loosen and remove the nuts and bolts that hold the transmission output flange connected to the propeller shaft flange. This will allow you to access the engine mounts more easily.
• Measure and note the current mounting height: Before removing the old engine mounts, use a measuring tool to determine the distance between the mounting base and the top or bottom of the mounting plate on the engine. Take measurements for each individual mount, as they may vary.
• Remove the old engine mounts one by one: Start by loosening the nuts and bolts securing the engine mount to the engine and the engine bed. Use socket extensions if necessary to reach tight spaces. Pay attention to the position of the enlarged mounting hole or slot on the old mount, as this will be important for installing the new mount.
• Clean and inspect the engine bed: Once the old mount is removed, clean the engine bed thoroughly to remove any rust, debris, or residue. Inspect the area for any signs of water, oil, or coolant leaks, and address them accordingly before installing the new mount.
• Repeat the process for each mount: Work on one mount at a time to ensure stability and accuracy. Remove the old mount, clean and prepare the engine bed, and get ready to install the new mount.
• Dispose of the old engine mounts properly: Engine mounts contain materials that may be harmful to the environment, so make sure to dispose of them at a designated facility or recycling centre.

Remember to work slowly and carefully, especially when supporting and lifting the engine. Always put safety first and, if possible, have an assistant to help you during the removal process.

Installing the new engine mounts

Before installing the new engine mounts, it is important to ensure that the engine bracket is not positioned too high on the stud. The engine bracket should be supported by the adjusting stud and sit as low as possible. The higher the bracket sits, the more stressed the stud becomes from the propeller's thrust, risking a break. Over-extending can lead to broken studs or failure due to thrust as the polymer core gets deformed. If the bracket ends up too high, you can shim the engine mount from the bottom using a non-compressible material such as steel, aluminium, fibreglass or polyurethane.

The next step is to support the engine mounts properly. Engines mounted on weaker stringers can lead to transmission failures, as the most damaging part of the misalignment is taken by the transmission coupling and output shaft bearings. If the engine mounts can't be installed directly on the stringer, build heavy-duty brackets that don't flex.

Use the correct hardware when securing the engine mounting systems to the stringer. Stainless steel hardware doesn't have the tensile strength and rigidity needed for most installations. Instead, use grade eight mild steel fasteners to handle the expected loads, and thick, heavy-duty washers.

It is also important to ensure that each engine mount carries the appropriate load. Unevenly loaded engine mounts can cause more vibration, defeating the purpose of an isolation mounting system. The ideal goal is to balance the weight of the engine/gearbox evenly across all engine mounts.

Before installing the new engine mounts, ensure that the mating surfaces are clean and free of debris, rust or paint. Even a thin layer of paint can cause a loss of tension under heavy compression.

Now you are ready to install the new engine mounts. Install them parallel to the engine's centreline, ensuring that the engine bed/bracket is parallel to both planes. The mounting surface must be parallel with the engine crankshaft centreline. If the engine mounts are not installed parallel, they will become "point loaded", leading to poor performance or premature failure.

Adjusting the engine height

Before beginning the adjustment process, it is essential to measure the height of each mount above the engine bed. Use a digital caliper or a measuring tool to determine the distance between the mounting base and the top or bottom of the mounting plate on the engine. Make a sketch of the engine and record these dimensions. Repeat the measurements for each mount, as they may vary slightly. Mark the engine bed and the mounting arm with a felt-tip marker to indicate the height.

Once you have the measurements, the next step is to carefully raise the engine off the mounts. This can be done using the adjusting nuts on each mount, along with wooden blocks of various thicknesses to support the engine. Be cautious and plan each step to avoid any accidents. It is recommended to replace one mount at a time to maintain stability.

Before removing the old mounts, pay attention to the position of the enlarged mounting hole or slot at one end of the mount. This detail is important for installing the new mounts. Clean the engine bed thoroughly, removing any rust or debris that has built up.

Now, you can install the new engine mounts. Start by placing the new mount in the corresponding position, ensuring it aligns with the mark you made earlier. Install all the bed bolts in each mount, but refrain from tightening them just yet, as you will need some flexibility for the next step.

After installing all the new mounts, lower the engine onto them gently. Allow the engine to rest for a day or so to give the mounts time to "settle." The elastomer material in the mounts will compress slightly during this period.

Finally, use your digital calipers to adjust the engine height and restore it to its original measurement. Refer to your sketch and recorded dimensions to achieve the correct height. Clean both faces of the shaft coupling to ensure an accurate alignment. Reconnect the halves of the shaft coupling, making sure to observe the witness marks to maintain their original orientation. Install the coupling bolt and nut loosely, just enough to hold them in place.

Now, you can fine-tune the alignment. Use a feeler gauge to align the engine horizontally and vertically. The rule of thumb is to achieve an alignment of less than one thousandth of an inch per inch of coupling diameter. For example, for a four-inch diameter coupling, aim for an alignment within 0.004 thousandths of an inch. Repeat the vertical and horizontal alignments several times, as each adjustment will impact the other. Take your time and make adjustments in small increments to achieve the perfect alignment.

Once you are satisfied with the alignment, tighten all the engine mount bolts securely. Double-check the alignment with the feeler gauge, clean the shaft coupling faces, and replace the coupling bolts. Don't forget to clean the threads of the bolts with acetone and apply a drop of blue Loctite before tightening them.

Now, you can start the engine and test its performance. Run the engine in gear over a wide range of revolutions, paying attention to any vibrations or unusual sounds. Shut down the engine and feel the coupling to ensure it is not hot, indicating a proper alignment.

Checking for leaks

Leaks are a common problem for boats, and they can be challenging to find and fix. Here are some tips and techniques for checking and dealing with leaks in your boat.

Common Sources of Leaks

First, it's important to understand the common sources of leaks in boats. These include:

• Hull fittings: Check for leaks around thru-hulls, rudder ports, shaft logs, strut fasteners, trim tabs, and swim-platform mountings.
• Seacocks, valves, hoses, and running gear: These are usually straightforward to fix, but proper reinstallation is crucial.
• Engine cooling system: Leaks here will only appear when the engine is running.
• Fuel system: Distinguish between low-pressure and high-pressure leaks. High-pressure diesel can be dangerous and requires PPE.
• Water leaks: Check the temperature and taste of the water to narrow down the source.

Finding the Leak

Finding the leak is often the most challenging part. Here are some techniques:

• Dry the boat completely and relaunch it: Look for the first signs of water ingress.
• Use water or air pressure tests: Fill the boat with water or use air pressure to identify the leak, but be careful not to overload the hull or trailer.
• Use light: Go at night with a light inside the boat, or use a flashlight to inspect for leaks.
• Check prime suspects: Anytime something penetrates the hull, like the bung, motor mounts, or bilge pump outlet, is a likely source of leaks.
• Dry the bilge and identify the direction of the leak: This will help narrow down the potential entry points.
• Haul the boat and inspect the outside of the hull: Look for water trickling out, especially around thru-hulls and other fittings.

Fixing the Leak

Once you've found the leak, here are some general tips for fixing it:

• Replace gaskets and seals: Modern gaskets and seals are much more effective than those from 30 years ago.
• Install a drip tray: This will capture liquids and alert you to leaks.
• Clean the engine regularly: A clean engine makes it easier to trace the source of leaks.
• Address oil leaks promptly: Oil leaks can lead to expensive or terminal engine damage.
• Use new O-rings and sealing washers: When replacing fuel filters, always use new sealing components and smear clean fuel on the mating surfaces.
• Be wary of overtightening: Avoid overtightening bleed screws and fuel filter connections, as this can strip the threads.
• Cut an access hole if necessary: If the leak is in an inaccessible space, you may need to cut an access hole and install an inspection port.

Remember, dealing with leaks promptly will help prevent bigger problems down the line and keep your boat running smoothly.

Frequently asked questions