OMC Stringer conversion

27 Jun 99

Author: William B. Arnold


Changing from the stringer to another type now days, sure sounds better and better, considering cost, heck it cost to repair must all of them. Before considering changing the drive, think if you really want to get rid of the boat and buy another that has another type drive. It isn't that you should sell the boat, but the work can be a bit daunting and taxing. Figure whether or not you want to buy a brand new drive I.E, bravo II or a Volvo-Penta outright. Think of how much money you wish to spend in the boat. Prepare yourself for all the little decisions that need to be made. Also consider if you will want to change the motor to something else. One thing leads to another. To do this little project without making anything, you will have to get the complete setup that bolted to the same engine 302/351W, or be prepared to change the motor too. Forethought goes along way.


That big hole in the back of the boat has to filled. Some background on this subject. On the OMC type such as all the pre-Cobra models, the outdrive pushes against the motor mounts and utilizes a rubber boot. On most any other type the outdrive push against the transom, thus placing all the strain here. The later types do not utilize a rubber, but mount directly to the transom. You must check the thickness and condition of the transom wood and the thickness of the fiberglass. The glass should be around 1/4 thick, the wood itself would be either 3/4 or two sheets of 3/4 plywood. This is an area of concern, since all the future forces to propel the boat will be in this area.

This is the: "I want to do this, but really haven't made up my mind, and I'm not sure what I'm getting into" step.

#2. My scenario:

I could have obtained a 888 Mercruiser complete from prop to bell housing for around $2200. Originally, out of a 23 cruiser with a 351W Ford motor. The 888 has the adapter plate for the 302/351w. A complete setup makes things easy. Now if you want a new Mercruiser outdrive, I.E., a Bravo II or III then you must find out if the 888 bell housing will bolt to the gimble housing. People will tell you it cannot be done, since this drive and motor combo never came standard as factory standard. The only important items are; find a Ford bell housing plate, then check to see if this plate will bolt up to the inside transom plate for the Mercruiser. If it will you are in luck. Then you need to find a Mercruiser type coupling which mounts on the crankshaft flywheel bolts. The coupling cost around 150 new. Do yourself a favor, NEVER use a used coupling! When you do, you will need the mounting studs and nuts that mount the adapter to the flywheel. These can be had at NAPA, Mercruiser, boatyards and such. Once you have the outdrive package, you should have the gimble bearing housing, if used, inspect it CAREFULLY!! The gimble bearing housing has been known to crack, normally due to corrosion, and it is around $500-$600!

Next is to buy a new gimble bearing, depending on the type gimble housing determines the bearing, i.e. 42 or 75 dollars. While you are doing this, it is a good idea if you can rent, beg borrow, steal, plead with someone for their engine alignment tool. This very important tool will be needed latter on. So, we should have these items on hand by this time; gimble bearing housing, inside transom plate assembly bracket, and the bell housing for the 302/351W or what ever motor you decided upon.

Great, hope you had fun looking and finding all the differences between all the different drives and connecting components, you may need to tap that info a bit later too.

This is the "I gotta find all these parts for the best price and condition phase, I know I have a long list, and hope I don't forget anything really expensive" step.

#3. Cutting the starting holes:

You should know the dimensions of the original transom hole and the transom mount for your new transom plate assembly. If you have located the items above and have not bought them yet, you must do one thing! Find the bottle or a six pack, sit down and party, why, now is when you must either commit or not, much like standing in front of the alter; you remember that? You need to really know to expect, but you have no idea what you are getting yourself into. Once you recover and have truly decided to do it, time has come to rip the motor out. Time also to get out a jigsaw and cut the transom so the hole is symmetrical about the center of the boat. Yes, that hole is gonna get a heck of a lot bigger than right now! Wait, I missed a step, BEFORE you start cutting, remember this rule. The original hole will have to extent across the transom symmetrically, i.e., from the vertical centerline to the left and right of it. Great time to mark the back of the boat up with a fine pencil line. Remember these lines will be used to ensure exact alignment of the new package also. Once marked, lay cellophane tape over them so you can't accidentally erase them. Now draw the hole pattern symmetrically. This will be the INSIDE hole, and if impossible to cut out from the inside, well I think you better drink a bit more. If the inside hole has to be "under" the floor, you will have to cut the floor, I am saying this as worst case scenario, since I don't know the boat type we are dealing with. Here's a bit more theory, the new hole will be the inside hole, there will be a bigger hole by two inches greater on the outside, since most of the other drive will be HIGHER in statue than the OMC hole, you will need the bigger hole to exceed the NEW drive height by 2 inches. A little hint here, I made a routing jig for the inside and outside holes and along with the Volvo-Penta mounting hole. I could part with it, i.e. cut you another one but we'd have to talk about that for awhile. This step took me around 6 days between staring at the original hole and staring at a finished transom ready for me to cut the new outdrive hole.

Also remember, that was 6 full working days, it didn't count for work stoppages like sleep, work, rain etc.. Anyway, the idea is to make a jig so you can use a router and cut the final holes. This way, making the essential perfect fit will be so much easier. It took around twenty minutes to final fit the plug to the hole after using great care in wielding the router. The router method is extremely close tolerance, the only way to do it, useless of course you happen to have access to a carbon dioxide cutting laser. 

This is "I'm gonna do this even if it kills me!"


If all went well, you should be ready to cut the hole plugs. Once finished and perfectly fitted. Do the fitting while perfectly sober! No mistakes here! All gluing will be done with epoxy fiberglass resin, and the total project should take around $350 dollars for fiberglass and all related materials. I haven't checked my receipts, still scared to do so. Quantity of epoxy resin was two gallons with some left over. You can use a polyester type resin, but I'm running away fast! I haven't engineered the forces applied to the transom during operation using the Volvo-Penta or Mercruiser drives, so I'll do the best with the best. Hell if it ever sinks, it wasn't from my workmanship. Once the plugs are fashioned and ready for installation, they have yet to undergo a few more changes. Remember that these things must be glassed together and bolted, screwed together. I used 69 bolts and screws in mime. May have been a bit of over kill, but I tell you, you can beat that transom with a sledge hammer all you want. I'm quite happy. Both plugs must have their edges chamfered. Reason: we DO NOT want any 90 angle edges, as it would create a shear point, and this is BAD. I don't like swimming in cold water, and I don't like to swim when I HAVE TO. What do you think, I want to be fish bait? Next, must draw the mounting holes on both plugs. This step will help you visualize the placement of the hardware and their relationship to the plug edges, overlaps, and the sterndrive. We don't want to have to drill through a stainless bolt or screw later. Let's do all the thinking well before we cut or drill. A mistake can create an awful lot of work later during the process. Next mark the location of all drill points for the bolts and screws. The bolt locations should be right at 2inches from each other. See, there is that two inches again. One inch from each plug edge, one inch from all outdrive mounting bolts, and one inch from the outdrive cutout. TWO inches from all others. All others not on an edge, will be in a staggered pattern. All closest to an edge will be inline and will be two inches from the next edge bolt/screw. All holes will be predrilled, however, barely sink the holes for the outdrive mounting bolts. These will be used later it ensure your handiwork is good to go, or it is time to drink heavily and sob uncontrollably, or time to drink and party since you are just too damn good, even if you are the only one who knows it!!

This is "GOD, I can't believe I just messed up a perfectly good boat."

There is no looking back, , take a lot of pictures; when you get old and gray, you will reflect on those younger days and say, "Was I crazy or What???"

#5. Bolting the mess together.

You will need someone else for this one. Once you start, you CANNOT stop until finished. It is absolutely essential you have planned this phase and the fore phase without question. There can be NO mistakes, NONE. Make sure you cut a smaller outdrive hole in both pieces, around two inches smaller. You will epoxy the inside plug, then backstop that plug on the side of the transom, to ensure a perfectly square fit with the original sheet of plywood, if you don't, man, you will be creating a lot more work for yourself later. Once completed, assemble the outside plug to the inside plug. Make sure you clamp together the inside and outside plugs at this point. Using your buddy from earlier, screw the most center BOLTS first, the outer BOLTS next. Work clockwise or counter-clockwise, don't skip any, it is most important we have NO air pockets between the sheets of plywood. Your buddy will torque the nuts down on the inside. Once the bolts have been completed, start on the screws, you should have found much epoxy oozing out of the screw holes, if not, maybe crying might be the best thing here. Anyway, once the screws have been completed, time to glass the chamfered outside edges, might as well do the inside edges next. This whole step should have taken around an hour, lots of hollering, bitching, and moaning, a lot of groaning too. This is the step your friend will totally think you are CRAZY!

This is "I must be insane!

6. Time to glass the transom.

Might as well finish the inside of the boat. Grind and finish if you didn't in the previous step. Glass the transom damn close to the original surface. I know we haven't fully covered the glassing area preparation phase, that's a whole other book. Gelcoat to match the original surface, match the color if you can. If you did it right and had great luck, the gelcoat will perfectly match. As long as you have sanded it perfectly, you will not be able to tell where the work was performed, you shoot for this point, however meeting it, you will not! Dont' be disappointed, you still have done a damn fine job. If the gelcoat doesn't match, might as well spray paint the transom for a even color.

This is "God, I must take a break, I'm getting too old for this stuff, heck, I even know I'm crazy for sure now!

7. Hole preparation

Prior to painting, route the new transom hole through the glass and all layers of plywood. Use an expensive CARBIDE router bit. Please go slow, and DO NOT make a mistake. Paint the transom, I strongly suggest spraying epoxy paint or alkyd enamel paint or polyurethane. I used Alkyd Enamel, my boat is trailered and never left in the water. After a bit, you should be looking at a great ass end, I mean a fine one!, even if it is on a boat, man doesn't she look great!

This is "I know I'm good!"


8. Mount the gimble housing, and inside transom plate assemblies

Time to swing the motor, oh, did I forget to tell you , you will need something like the Mercruiser motor mounts. This is some of those thousands of those "LITTLE" decisions you made earlier, even if we didn't cover them. Get the motor mounted. Don't get too happy, time to roundup that alignment tool you were trying to beg, borrow or steal. The alignment tool is not needed for a Volvo-Penta installation. Use the new sealing orings for the alignment tool, the motor will self align. Get the motor aligned, shove the outdrive on. Hold on tight, this moment will make it all seem worth it, well almost, we haven't got to the boating shakedown cruise yet.

Heck, the only water she has seen was when you were doing the wet sanding, oh, that's covered in another book too, sigh...

This is "We're almost there!"

9. Time to forget about the outdrive.

Now is the time when you will find out if you have all the inside parts. If not, you will soon discover the shortage. Exhaust, new steering cables, the stringer type are now wall decorations. Others may look at them and go, ah, just another set of cables, but you will see them as something much more, so much more. Leave no stone unturned at this point, make sure everything is good to go.

This is "I can't wait to get in the water!"

10. Time for a shakedown cruise!

Give her a though testing, especially well before you have the whole family and friends pile on for a great day on the water, because it will not be error free outing! You WILL have forgotten something; CRAP, this thing is sinking, where's that bung plug anyway???

11. Once you are happy, your friends and family will think you are the greatest craftsman alive, they will be amazed at your handy work, yes you too can pat yourself on the back, shoot, I'll shake your hand too! Now that everything is just peachy, this is the time to invite all those folks for some fun times on the water. I wasn't planning on typing all this stuff, but got a bit carried away. It should serve as a tool on making your decisions. Hopefully it will help you envision some of the "unseen" work that must be accomplished for a truly successful endeavor. It can be done, and done right. You do it haphazardly, she will bite you squarely in the ass, a big bite at that! Also, you will become the newest expert on the above techniques. Thinking upon the past, I should have just bought a brand new Volvo-Penta outdrive direct from the factory for my boat. It would have cost around the same for the drive system as my 280 did after corrosion control and tearing it apart and overhauling it. Sigh, there goes that experience thing again. If you happen to be anywhere around me, come over and take a look at my boat. I will gladly fill in all the blanks, be prepared to spend a day or so, as there are many topics to completely cover to ensure successful implementation. I have no idea of your mechanical, planning and execution skills, and the availability of the tools I used successfully during my venture. Yes, you will make a mess. Guys do that anyway, it is in our genes, each and everyone of us. Drives women crazy. Bottom line: for me it was worth it, I put a lot of hours on my rig, I expect no problems stemming from my workmanship, I skimped nowhere. I like my Volvo-Penta 280 and my setup. A much more reliable package, plus I don't have to carry as a whole mess of spare parts anymore. The older Volvo-Penta's do not have a large selection of props to choose from, sigh, makes no real difference in the long run. I learned quite a bit more than I knew before, and definitely proved to myself once again, proper planning and execution is paramount for an enjoyable experience. You have no limits, except what we place upon ourselves. The project was more work than I had originally planned, just helps me fine tune my planning techniques again. Remember step three, especially.

Other thoughts:

A few good words. Do it inside under controlled conditions, such as low humidity, warm 80-90 degrees and a place easy to wash down the fiberglass fibers. Fiber glass get EVERYWHERE during grinding operations. It would be great to pull the boat outdoors for the grinding phases. Stay organized.

Some tools needed:

Router, jigsaw or reciprocating saw. I use a Dewalt 12v drill, drill bit set, a large drill with a 1/2" bit, LOTS of latex gloves, a bunch of beer cans for mixing epoxy (at least 6 cases worth), utility knife, sticky back sandpaper in 80, 120 grit (open coat) buy it on a roll. A sanding backing pad for the 12v drill. 4 /12" grinder with 40 and 80 grit disks (DO NOT use those disks with holes in them, good way to kill yourself!), a batten around 6 feet long for fairing the transom, almost a two person sanding device. Use sticky back sandpaper here

Some of the materials required:

lots of 1 and 2 and 3 inch brushes

5 yards of woven cloth

5 yards of Comatt cloth

2 gallons of epoxy (could be less),

one sheet of the best 3/4 or 25/32 plywood you can find,

20 5/16 or 1/4 carriage bolts with twice the amount of fender washers

50 number 10 wood screws

2 quarts gel coat, you can get away with 1 if you are really careful, I gelcoated the hole area but couldn't match the gelcoat, so I painted with alkyd enamal over the gelcoat just for color. Nice ass end!

Hint: you can vacuum bag the transom, really wish I did, it would have made the job a lot easier and would have taken quite a bit less time. A bit costly but would be well worth it in time not spent layering. The total fiberglass layers came out to nine layers woven, 7 layers Comat, I.E. 1/2 thick. There's another story behind this too. It is extremely important leave no air gap between the existing fiberglass and the transom plywood period!

I'm quite sure I left out a few items for sure. Before I started my project, I decided to go with a Volvo-Penta 280 outdrive. Bought a used boat, motor and trailer with a 280 outdrive attached to a 4 cylinder motor. This rig was around 1K by the time I got it home. It still required a reseal kit, a propeller ($100), v-8 gear set($375), and bearings($200), raw water pump($200), a little adapting of the exhaust system, a used exhaust "Y" pipe ($200), and a bell housing($600). I always replace all bearings in a used outdrive. I hate to swim to shore from 60 miles offshore just because of a bearing failure. I also do all my own work. 

I can't rightly tell you the true total money involved. I really don't want to tally up the receipts, it'd be a bit scary. I believe if I ever do it again, and plan on keeping the boat for over 8-10 years, I will buy a new outdrive assembly complete!


I do a lot of shifting, so I went with the Volvo-Penta, probably should have gone with a Volvo SX or DP, or for less money a new Mercruiser Bravo II/III drives.

 The VP has a wider turning radius over the OMC, God I miss that, and I really miss my electric shift too. Trimming I have never had and don't miss it, my boat is trimmed by the outdrive position and I have never change it. It is good as it is gonna get. Beside, she likes rough water.  

Let me see, where was I? You need to make a couple templates out of 1/4 inch birch or any other material, Lexan would be really nice, the smoother edges you get the better! Watch out for Plexiglas, you snag it with a router, and it is gonna hurt, and if I was watching you, I'd stand back at least 20 feet from you! I used a fine 1/4" Birch plywood, worked quite well. 

Cut all fiberglass layers prior to glassing if you can, use lots of time, thought and care. Lay it up at one time, use West System SLOW curing epoxy and vacuum bag it if possible. Wish I could have stood the boat on her nose and have the transom horizontal, not my luck though, :-(. 

All hardware should be Monel or stainless, I used stainless.

Hope this leads you into the direction you are thinking. Maybe if you could catch a cheap flight out my way, I'd give you invaluable inside into the job. 200-300 dollars would be well worth it. Sure wish I had the same opportunity.

I know I have probably omitted quite a few things. Each area seems to grow into it's own chapter!