Outboard Engine Gaskets, Seals, Grommets & O-Rings

Boat Motor Gaskets, Seals, Grommets, and O-Rings Resources and Information

Gaskets, seals, grommets, and o-rings are necessary for performing periodic service and repairs on your outboard engine. PartsVu offers a complete selection of these essential components from top boating product suppliers, including Sierra and Coghlans. Need help servicing your outboard engine? PartsVu also occasionally publishes helpful resources like the following: 

•  Outboard Service Schedule
•  Outboard Specs & Capacities Chart
• Mercury Outboard Service Schedule
• Mercury Outboard Specs & Capacities Chart
• MerCruiser Inboard Maintenance

Looking for more marine products for your boat and outboard engine? Browse PartsVu’s products and accessories or aftermarket engine parts.

Outboard Engine Gaskets, Seals, Grommets, and O-Rings FAQs

What is an outboard motor gasket?

Your  outboard head gasket is a very important component in the performance and lifespan of your engine. Located between the cylinder head (where your spark plugs are screwed in) and the cylinder block, a head gasket is responsible for sealing these components together to withstand the compression from the engine and enable cooling water to flow between the block and the head.

Does my outboard head gasket need to be repaired?

To ensure your outboard performs its best and to extend its lifespan, preventative maintenance should always be a top priority. recommends a number of regular maintenance intervals for your outboard based on its usage – 20 hours for a new engine, every 100 hours or annually, and enhanced service at 300 hours and 500 hours.

However, one common benchmark for outboard head gasket replacement many boaters miss is replacing it every four (4) years. Depending on your usage, this timeframe may be shortened but four years is generally regarded as a “must” to change your head gasket before other problems develop.

There may be no visible signs of head gasket damage by this time, but this repair will give you peace of mind – plus, when you’re changing the head gasket, you can perform other inspections and replacements (such as anodes, engine bores, salt/sand buildup, spark plugs).

Another common sign that your  outboard head gasket is in need of repair is low pressure in the cylinders. This requires a simple compression test on each cylinder, which involves removing each spark plug, plugging in a compression gauge, and cranking your engine without giving it throttle.

What you’ll be looking for as you test each cylinder is near-identical readings between cylinders. If there’s more than 10% variance between each cylinder, it’s likely that the head gasket is the culprit.

If you notice black specks, a blackish crud, or a grey slime in the cylinder, it’s more than likely that a leaking head gasket is to blame. The reason for these substances is that combustion byproducts build up in the combustion chamber during combustion. These byproducts then either flake off (specks), build up inside the cylinder (crud), or mix with water from the adjacent cooling chamber (slime).

When idling, especially at low RPMs, you may notice that your engine runs too slow – almost to the point of completely dying. Or, it may provide a rough idle that’s not consistent, causing a jarring motion.

The reason for these events is that the spark plugs cannot create a consistent spark due to water leakage on the spark plugs. If you remove the spark plugs and notice corrosion, then water is leaking in and diluting the fuel due to a faulty head gasket.

Can you replace a head gasket yourself?

Yes, a head gasket repair is an easy job that only requires a few tools. Unlike a vehicle, where getting to the head gasket requires the removal of the engine and other components, an outboard is smaller and easy to access.

How much does it cost to replace a head gasket on a boat?

If you choose to replace your outboard head gasket yourself, you can only be on the hook for parts ($50 or less for many engine models) and tools (many of which you may already have in your toolbox).

Taking your outboard to a mechanic will include the cost of parts, but also labor. Considering labor generally costs between $70 – $120 (or more!), the total cost of just a head gasket replacement can be a few hundred dollars. Of course, the advantage of a head gasket replacement by a mechanic is that they can make other replacements, trim the heads if they’re warped, and diagnose any other developing issues that may be plaguing your outboard.

Is a head gasket worth fixing?

Yes, a head gasket is a worthwhile investment that can prolong the life of your entire outboard motor. Better yet, you can learn an essential maintenance skill and expand your knowledge base, both of which will pay you dividends in the future.

How do I service my 4-stroke outboard engine?

Use an appropriately sized screwdriver assisted by a wrench if necessary to loosen these screws which should be pretty tight. An impact driver and hammer might be needed. If the drain screw has a magnetic tip, inspect the plug after removal to see if heavy deposits exist. Some fine shavings are normal, but chunks or a large quantity of metal is cause for further inspection. Have a drain pan ready, and extra rags handy as gear lube will begin running down the skeg as soon as you remove the upper vent plug. Be sure to collect the used gaskets from both plugs after removal as these are not reusable and should be discarded.

Allow several minutes with the engine tilted all the way down to drain all the old gear oil fully. Like your engine oil, inspect the spent oil for evidence of water intrusion, other contamination, or metal. If you have purchased one of PartsVu’s handy lower unit oil change kits, you will already have new gaskets, oil, and a pump. Screw the pump into the lower drain hole, then connect it to the container of gear oil.

Begin pumping oil into the Mercury lower unit until oil begins to flow from the vent plug at the top. Stop pumping and wait a few minutes for all the air bubbles to work themselves out of the gearcase. After this brief pause, pump a few more times until clean gear oil without bubbles is flowing from the vent plug. With a fresh gasket in place, install and torque the top vent plug. Unscrew your fill hose from the lower drain plug and quickly swap in the lower drain plug with a new gasket — torque to spec.

Before you do get started on any maintenance projects, make sure to get the right parts (even for Mercruiser engines) so you don’t have to stop right in the middle.

What is an outboard leak down test?

An outboard leakdown test is a simple diagnostic checkup for your outboard – and it can even be incorporated into your regular maintenance intervals for peace of mind. 

During the test, you’ll be able to determine if there’s any loss in air pressure around your engine’s internal seals and passages. Even a pinhole leak can drastically alter your outboard’s performance, so performing a leak down test is important to determine whether the piston seals and other components are in good shape. 

Fortunately, a leak down test only requires a few common tools and some basic knowledge about your outboard. Even boaters with minimal experience can diagnose developing issues that affect the overall performance and lifespan of your boat’s engine(s). Best of all, performing this test will save you money and headaches down the road.

A outboard leak down test indicates the condition of each of the engine’s cylinders and the overall condition of the motor. Similar to a compression test, it involves spraying compressed air into each cylinder and using a pressure gauge to measure a percentage of pressure that’s lost after a short period.

Also, part of doing a leakdown test on a motor is to listen for any signs of leakage. By listening closely to each cylinder and the areas around them, you’ll better understand where leaks may be present.

The goal of a pressure test is to confirm whether the motor has any air leaks. The presence or absence of air in engines often leads to leaking of fuel-air mixtures and leakage of fuel during operation. 

Additionally, these leaks may become infiltration points for water and moisture from the aquatic environment, resulting in internal corrosion, sub-optimal performance (i.e., at idle, low RPMs, and high RPMs), and other issues.