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OMC Cobra to OMC Cobra/Volvo SX Conversion-gctid403455

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    OMC Cobra to OMC Cobra/Volvo SX Conversion-gctid403455

    This has been a desired project for me for a long time. I finally decided to go for it in the fall of 2010 when a leak inside the boat got so that I could not leave the boat in the water unattended for any length of time. The leak turned out to be the connection piece from the outside of the transom to the inside of the transom. Since I had the outdrive off to make this repair, I decided I was not going to put the original Cobra back on. During the winter of 2010/2011 I found a 1994 OMC Cobra SX outdrive on Ebay that looked in reasonable shape for $1500 to my door. I later found complete Volvo SX Transom shield, including Trim cylinders, Gimbal Ring and Tilt housing for $700. I bought a new crankshaft mounted water pump and mounting bracket for around $200. I now had most the main components I needed. I had a very busy summer in 2011 so was unable to complete the project. That and every time I thought I'd do one phase that should only take a day, it ended up being a real challenge that took a good parts of several days. I will list the challenges in the order that I encountered them.

    1. The conversion required either Volvo SX Trim cylinders (shorter) or spacers to be installed in my OMC cylinders. Since I had the newer Volvo Cylinders, I chose to use them. Should be easy enough, just remove the 4 tubing nuts where the cylinder hoses connect to the manifold on the transom shield. Wrong! These fitting were either corroded of Loc-tited or both into the manifold. It took parts of several days to get them removed, requiring ordering 2 different tubing wrenches and a lot of frustration. Then I had to do it all over again to remove the Volvo Cylinders on the newer transom shield that I bought. I ended destroying two trim hoses at $94 a pop to replace. Removing one of the bad hoses from the Volvo cylinder pulled the threads out of the hole for the fitting in the cylinder. I ended up over drilling it and threading it to ┬╝" npt. Then I cut off the new hose fitting and installed a brake fitting and flared the tubing. Job finally done.

    2. The pivot pins were corroded and I had to drive them out with a hammer and a punch. One Sx pivot pin came with the Volvo transom assembly but I had to buy another one for the ends of the trim cylinders for $100. The Cobra/Volvo SX pivot pins are larger diameter than the Cobra pins.

    3. Next was to install the new Crank mounted water pump. I very carefully cut a 5"X10" hole in the back wall of the aft cabin to get better access to install the pump and for future maintenance. Just three bolts and pump is mounted right? Wrong! I struggled for an afternoon trying to get the three bolts started without success. Working by mostly feel alone I finally figured out that the bolts that came with the pump were too short so I got longer ones and finally got them started. For some reason, the holes in the pump flange were a larger than the bolts so there was no way to center the pump flange to the crank and after several tries to align it I gave up. I went to the hardware store and got some bushings that fit the bolts nicely. I had to turn them down on the outside to make them fit the pump flange. Now the pump doesn't wobble when the engine turns over. All I have left to do here is make the hole that I cut into a nice hatch.

    4. The next challenge was difficult at first then with some thought it was solved. I had previously pulled out the short shift cable for the original cobra and now I needed to fish in the new long shift cable to go to the helm. I tried feeding it in through both sides but it would not make the bend and it would stop. I fished some long cord through the hose and out the outdrive by using the shop vac on blower mode. Just cover  of the hole with the blower and feed the string in the gap till it exits out the outdrive. I then tied the cord around the cable with several half hitches spaced about 1 inch apart but I was still not able to pull and push the cable through. Next I tried pushing a piece of ¼ inch irrigation tubing through the hose and out the outdrive just to see if I could get something bigger through. It worked and I noticed that the rod in the cable would just barely squeeze into the tubing so I forced it in and then used the tubing and the string to pull the cable in through the outdrive and out the hose.

    5. Last to do was connect the new shift cable to the helm and adjust and I was ready for a start. There was not much of a challenge here except finding the correct ends for the cable.

    I probably have another $200 in hose for the new pump, new mounting bolts, O-rings, shims and misc. hardware and $100 for a new shift cable. I also had to buy a new Propeller and hub for $420 so this has cost me about $3408. I expect to get some return on my old Cobra drive which is in good condition. ($1000-$1500)

    Project was completed in July 2012. The first sea trial was July 5 and was to test everything and find out what propeller I needed. The starting point was the propeller from my old Cobra which was a SS 14.25X17 Rapture. It had great hole shot, low boat speed and over revved by 7-800 RPM. I knew it was not going to be the correct prop because my gear ratio changed from 1.41 to 1.51. I then tried a 14.5X21 Aluminum prop and I had a very hard time getting it up on plane and revs were too low. The next try was a 14X19 four blade SS prop and I think it is very close to where I need to be. I still had a little trouble getting it on plane but that was with a heavy load of people riding mostly in the stern. Once I redistributed the load it was fine. This boat has always been stern heavy and has never been one to jump out of the hole. Shifting is very nice, easy and smooth and that is what I was after. That and the peace of mind that comes with the SX drive verses the non SX Cobra drive.

    I give many thanks to those on this forum and another forum that have been an assistance to me.

    Good for you. You will enjoy the cone clutch gear engagement.

    So, are you calling this an OMC Cobra SX, or a Volvo Penta SX?

    A few notes for you.

    Heat is our friend here on many of the items that were stubborn.

    It's likely that had you warmed or heated some of these items up before attempting to remove them, these fittings and pins may have survived.

    Log that in your bag of tricks for next time.

    You may still have an issue with your crankshaft pump.

    The crankshaft pump flange indexes into the three bolt pattern symmetrically, but what you may have missed, is that the two raised dowels in the stamped steel crankshaft pulley actually align or phase in the pump mounting flange.

    IOW, there is only one correct indexing out of the possible three bolt hole pattern if your 5.8L crank pulley has the raised dowels stamped into it.

    If your pulley does not have the raised dowels, then you are probably OK.

    See Doug's recent thread regarding his Johnson Crankshaft pump installation.

    In post #9, I posted some images of the dowels and corresponding flange alignment bores.

    This would be for the GM pulley... not sure about the 5.8L Ford.

    If your pump has a slight wobble to it, the likely reason would be that the flange may be teedering on the two un-symetrically positioned alignment dowels.

    If so, the fix is very simple.... just remove the three bolts, re-index the flange as to align with the dowels......, and reinstall the bolts.

    You won't need the bushings any longer, either.... but again, I'm not certain about the 5.8L pulley having the dowels.

    BTW, the three mounting bolt holes in the pump mounting flange are over-sized by design, as these pumps fit the SBC, BBC and the Ford engines.

    Have fun, and I hope that it all continues to work well for you.

    Rick E. (aka RicardoMarine) Gresham, Oregon
    2850 Bounty Sedan Flybridge model
    Twin 280 HP 5.7's w/ Closed Cooling
    Volvo Penta DuoProp Drives
    Kohler 4 CZ Gen Set

    Please, no PMs. Ask your questions on forum.



      I didn't have any indexing on my Ford pully but that sure would have made it easier. The bushings I made seem to work fine and there is no wobble at idle up to 4000 RPM.

      I don't know why I didn't think of heat as I have used it many times before on my day job.

      Well, I could call it a Vobra or maybe a Colvo, I hadn't thought of it. Since it is a '94, I think both Volvo and OMC produced the drive. An advantage I just realized, instead of one source of dwendling parts, I now have two sources of parts!

      I am still not sure what to do with the shift interrupter switches and ESA. They are still mounted and wired in but nothing actuates the switches any more. Any Ideas?

      Anyway, thanks for your assistance now and in the past. If the boat was worth it, I'd consider putting a DP lower on it but I already have more in it than I'll ever get out of it. I'm done spending on this boat for now. Now time to enjoy it... until someone makes me a desent offer on it, which isn't likely. haha