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Converting From an Older OMC to a Mercruiser

Our Transom Hole Adaptor Plates are no longer in production.

If you have the large rubber boot you will have to fill in the transom and re-cut the hole.

This has to be done right and you should find a fiberglass person who is experienced in such a modification.

It involves removing the layers of wood across the entire width of your transom.
Then you have to make sure you have strong stringers in the right place if you are installing a vee type engine.
If you are installing an inline 4 cylinder engine you will need to have a front mount pad area.

We use a pneumatic die grinder with a carbide wheel to cut out the inner layers of glass as so we can cut and chisel out the old transom wood.

Preparation is critical.
ALL the old wood needs to be removed and you must grind and sand the areas where the sheets of glass cloth will be glued.

The cloth will have to fold to the sides of the boat and also tie into the boats floor and engine stringers.

It's a miserable job and you will have fiberglass in your skin for weeks to come.

Then we form a template out of a large sheet of door skin or cardboard to get the shape and size of the inner transom. We use this template to cut out 2 sheets of 3/4" marine plywood.

We also take another sheet of cardboard and we trace the shape of the hole to it. This can then be used later on to cut layers of cloth to fill the hole from the outside.

We also take the time to cut out 3 sheets of fine rove fiberglass matt in a way as when the glass is laid up it will be wide enough to turn and tie into the sides of the hull. This mates the hull sides to the transom for required strength.

Have other smaller squares of cloth cut out to tie into the stringers as well.

You don't want to have to be cutting cloth while your fingers are full of resin.

These sheets will have to fit up snug against the inner transom and they must spread from one side of the boat to the other.

Your transom has to end up being 2" to 2-1/8" thick.
We butter up the transom and the first sheet of wood with epoxy resin and set it in place.
We run wood screws into the wood from outside the boat to draw the sheet of wood up tight and to squish out all the excess epoxy resin.
Don't worry about the screw holes in the outside of your transom. They will be filled and fixed later on before you paint the outside of the transom.
Then we butter up the installed section of wood and the second sheet of wood and we screw it into place as well.

The epoxy must be thick enough to be squishing out around the edges and making a huge mess. Clean up the heavy drips and use this excess to coat the last piece of wood.
Make sure the wood is saturated with epoxy and lay the first sheet of cloth into place.

Work the epoxy into the cloth with your hands until there is no air and there are no air pockets. Make sure it is as neat and as flat as possible. Work the corners so the sheets adhere well to the sides of the hull as well.
Use the smaller sheets and tie the transom into the floor (about 8" is good) and into the stringers on all sides.

Now that you're done with the glasswork on the inside you need to pay attention to the outside of the boat.
Standing behind the boat and looking at the transom you will see the wood that you installed from the inside.
You will have to goo up the woods surface and lay 2 layers of fine rove mat into the hole. You might even need 3 or 4 layers depending on the hull thickness.

Let everything dry a day or two and remove the screws that pulled the wood into place.

Use a DA sander and sand the outer hole area flat.
The hole will need to be faired smooth with epoxy filling compound, let dry and resanded. This process might have to be done 2 or three times until the transom looks smooth and flat.
It MUST be smooth and flat.
Prime and paint your transom.

On the inside of the boat, prepare the newly glassed areas for a coat of paint and paint the entire bilge so it looks good. You can add colored tint to the epoxy for a good strong coating or you can use GelCoat or even cheap BilgeCoat.
Make sure to cut 2" holes into the stringers as to act as drains so water doesn't puddle up on the outsides of the stringers.
Don't leave any wood bare.
Make sure you have a bilge drain hole.

After the boat is all painted you are ready to cut the new hole into the transom.
You will need a Template, which is made by Mercruiser and a Drilling Fixture too.
The new motor will come with a template and drilling instructions.

Pay careful attention to getting your CENTERLINE properly marked and your "X" DIMENSION exactly as stated in the book. The "X' dimension is the height of the gimbal in relation to the very bottom of the boat.

Once you have your centerline and "X" dimensions all measure and marked you will install the Drilling Fixture (jig).
The fixture will make drilling the holes and steering pocket area easy and precise.
Then you cut out the remainder of the hole.

Once the hole is cut the bare wood should be sealed with epoxy and let dry.

Then you install the gimbal assembly, the engine and last of all the drive unit.

Coupler failure can be reduced or eliminated by checking and adjusting the engine alignment on a regular basis.
Checking the alignment requires removal of the sterndrive unit.
A special Alignment Tool is slid into the drive coupler from behind and the tool must insert and turn easily. If the tool is too tight or can not be easily fully inserted into the coupler splines, the engine mounts must be adjusted until it will.

The alignment must be checked with the engine rotated in 90 degree increments. This is to insure that the coupler is "true" and won't wobble.

It is standard policy to check the engine alignment whenever the drive is removed.
The drive should be removed at least once each season to allow for an inspection of the gimbal bearing, bellows, shift cable and alignment. It also provides an opportunity to properly grease the driveshaft yoke splines and cross bearings.

Click HERE for new Drive Couplers.

Engine Alignment

Sterndrive-Information
Change your Alpha Gear Lube
Change your Bravo Gear Lube
Why does my drive lube have white streaks?
Why does my drive lube have gray mush?
Replace Your Alpha One Water Pump
Replace your Alpha Gen II Water Pump
Rebuild your Bravo Raw Water Pump

Troubleshoot & Replace your Alpha Shift Cable
Troubleshoot & Replace your Bravo Shift Cable

Replace your Alpha One Bellows & Gimbal Bearing
Replace your Alpha Gen II Bellows & Gimbal bearing
Replace your Bravo Bellows & Gimbal Bearing
Why does my drive "Growl" when I turn?
Water is leaking into my boat from the transom

Replacing your Trim Sender Switches
My Trim doesn't work but my Trailer Switch does
My Trim Gauge is Pegged

Rebuild your Alpha One Upper Driveshaft Housing Assembly
Rebuild your Alpha One Lower Gear Housing Assembly

Check your Engine Alignment
Check for Excessive Steering Play
Troubleshoot an Overheating Problem
Should I remove my thermostat?
Why do I overheat when going fast?
Troubleshoot a No-Crank Condition
Troubleshoot a No-Start Condition
My Engine Backfires and/or Surges and has Lack of Power
What is an Anti-Syphon Valve?
My Engine blows Steam from the Exhaust
Why do I hear a "clakity" sound while at idle?
Exhaust Manifolds...IMPORTANT Information
How do I Winterize my engine?

What is Galvanic Corrosion?
I have a Bravo III Galvanic Corrosion Problem
What is a Mercathode?
All about Zinc Anodes
How do I check my carburetor?
What carburetor do I have? A Rochester or a Mercarb?
I have water in my fuel
My fuel smells like varnish
My fuel is old
My engine has been sitting for several months without being run.

I have a Mercruiser 470, 485, 488, 190, 3.7L Engine
I have coolant dripping from the front of my engine
Can I install a regular Alternator instead of the Stator-Rotor?

Converting an OMC to a Mercruiser


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