Replace OMC Cobra with Mercruiser

Hi, all--

I'm been doing a lot of reading, searching and head-scratching and now I'm posting my questions in the hopes of getting some of the super-smart and knowledgeable people on this forum to help me out (and maybe some other folks in a similar fix).

After wrestling with a junk, aftermarket lower on a Cobra outdrive, I'm within a heartbeat of venturing down the repower path for a mercruiser. I've rebuilt the lower unit, only to have it fail about 10 hours later (similar failure) and I'm not sending any more good money after the (now) bad. My wife and I love the boat (it's in great shape and suits our needs perfectly), so I'm committed to getting the old tub back on the water. I'm simply just unwilling to invest another penny in a dead product line.

I've done the homework for repowering with a Volvo SX rig, which is the easiest (bolt-on), but Volvo is WAY too fond of their drives for this to be practical (big $$$). (If anyone is interested in hearing what I've learned, let me know. It's MUCH simpler than you might think) So, I'm looking at trading time for dollars and converting to a mercruiser alpha setup (or similar).

I've seen references to a documented repower of a Bayliner boat, but all of the links are dead. If anyone can point me to a source of info regarding this project, I'd greatly appreciate it. In fact, if you were involved in this task (or one similar) and have the documentation/photos/etc to prove it, I am MORE than happy to host said info on my website for anyone else to view. I'll keep it live or, at minimum, provide a static repository where it can be found. I CAN'T be the only person out there with a need for this info (or maybe I'm just too dumb to find it??).

Here's a couple of things that I *think* I know, with some questions sprinkled in:

--The Cobra transom cutout is identical to the Mercruiser Alphas (even the bolt holes should match up)

--A Chevy V8 is a Chevy V8, but I need to figure out what the interchangeability of the flywheels/couplers/bellhousings will be. I feel like most of the GM bellhousing bolt patterns are the same, which sort of simplifies the parts search, but I gather that the 5.7 had two common flywheels and this will affect couplers, starters, and bellhousings. True/False?

--Once I've figured out the flywheel/coupler/bellhousing combination, will the forward engine mounts need to move relative to the OMC placement? Or was OMC kind enough to copy all of the relative installation dimensions? Further, if they did, will the OMC flywheel bellhousing bolt to the Merc transom plate? Still further, can the OMC inner transom plate be retained and only the outer Merc ring, gimbal, etc be replaced?

--Can the OMC trim pump be retained for actuating the Merc trim rams or is there a basic incompatibility? (fittings, oil type, pressure requirements)

--Power steering? The pump on both of my boats look identical, but are their manufacturer spec'd differences in the two? Will it matter? It looks like they will physically interchange.

FYI - I'm NOT afraid of tackling a big project (I understand it will be) and plan to give back to the community by thoroughly documenting the project, but would like the advantage of other's experience before breaking out the sawzall and acetylene rig. I hope that these are intelligent questions, but as I'm on the upward slope of the learning curve, I don't know how basic they are. I look forward to input from everyone.

Thanks in advance,

Chris

Re: Replace OMC Cobra with Mercruiser

Ayuh,........

Rather than answering each question,.....Which I can,+ will.......
From the general read of your post,.....
You're going about this somewhat Upside Down.......
The reason is mainly,.....
Don't even Think about trying to graft a Mercruiser Outdrive to your OMC motor...........
Scrap out ALL of your OMC,........ Sell it off as Parts.........

Further, if they did, will the OMC flywheel bellhousing bolt to the Merc transom plate? Still further, can the OMC inner transom plate be retained and only the outer Merc ring, gimbal, etc be replaced?

Click to expand...

Doing This,...... Will leave you with a Frankenstien boat that'll be a Bear to Build,+ Worse to work on..........

Either go with the Volvo swap,......
Or,....
Go with the Mercruiser swap, but from the Pulleys to the Prop............

Re: Replace OMC Cobra with Mercruiser

I've thought about this too, even though my Cobra is still fine, and came to the same conclusion as Bondo did, there are too many diffrences to try to adapt power steering and trim parts from OMC to Merc. So either you get a donor boat and strip out all the Merc stuff to replace the OMC, or do the Volvo swap, depending upon what you price stuff out at, the Volvo drive can wind up costing 2K more than the Merc Alpha, but keep in mind you are not having to pull the engine, and replace the entire transom mount, trim pump & lines, & power steering and also deal with any differences in the wiring. And also some have said that the Volvo is really more comparable to a Bravo than an Alpha, which may account for some of the price difference. So when the time comes, even though the lower price of the Alpha drive is very attractive, plus parts are everywhere and so is service, if I were doing it myself I might go for the Volvo just not to have to totally strip the boat out, in the end it may not be that much more money, and certainly a heck of a lot less work.
BTW, what aftermarket lower did you have trouble with? GLM?

After pricing it out, the Volvo was coming in at about 4400 plus the pivot housing, and seawater pump. To do the Merc, you are looking at close the same #s because you have to replace the entire transom housing, trim parts and power steering. Now this is with new parts, of course if you find a donor boat with servicable Merc stuff it could well be cheaper.

Re: Replace OMC Cobra with Mercruiser

To do the Merc, you are looking at close the same #s because you have to replace the entire transom housing, trim parts and power steering.

Click to expand...

And,............

Your OMC dressed motor Still won't bolt up................

Re: Replace OMC Cobra with Mercruiser

Howdy,

I'll jump on this since i just finished my repower in AUG.

Just a couple of notes to add (re-iterate) While you could probably use the OMC trim pump.....(it may not produce enough pressure to trim the Merc actuators under full power....) It's not worth trying it. New Merc trim pumps are around $250-300 all day on ebay.

Don't even try to use the OMC bell with a Merc transom plate. you're wasting your time.

Yes the transom hole and bolt pattern is the same when going to an Alpha. (I had to drill 2 additional holes for the Bravo and yes, I had to relocate the front engine mounts going from the Ford 460 to the 454...you may have to relocate them too)


Your best bet would be to completely remove the OMC stern drive and engine if for no other reason than to clean up the entire engine compartment and repaint it for the "new" engine and drive......you will have to install the merc bell housing and coupler. You'll also probably need merc manifolds and risers to go with the merc Y-tube...(not to be confused with U-tube!!)


You're going in the right direction. I considered a Volvo too and would have used one if I could have found one for the right price. I got my 1997 454/Bravo III(125hrs since new) for about $5k and COULD NOT pass it up!!!
http://i14.photobucket.com/albums/a317/nixypixy07/dsc_0028.jpg

Cheers,


Rick

Re: Replace OMC Cobra with Mercruiser

I'm going 1 step farther; replacing an '84 OMC stringer with alpha gen 2.

http://forums.iboats.com/showthread.php?t=225270

I found an almost new leg on eBay for $1100, then got all the other parts (except engine & exhaust manifolds, will use my 5.7 and ss headers) for $900. Including trim pump, trim cylinders, gimbal housing, inner transom plate, PS cylinder, bell housing, coupler, Y pipe, motor mounts, all fasteners.
I will have to modify my headers a bit to meet the Y pipe.

Then sold the OMC leg, the trim hydraulics, and the tilt parts for $2250. Took the OMC bell and mid drive to the scrap for $22 more.

Yes, all SBC bell housing bolt patterns are the same but for a minor exception, the dowel pins are different sizes. I needed to make a pair of bushings.

My steering cable slipped right into the Merc parts, the power steering hose fittings even match up.

Re: Replace OMC Cobra with Mercruiser

Man! You must some kind of Electrical Engineer!

Anyone that can make a Merc go where a Stringer came out is some kind of craftsman!

Re: Replace OMC Cobra with Mercruiser

LOL. Thanks, HT.
I am considered very "handy".
I hold an electrician's and gas fitter's license, also do central and car A/C, carpentry, plumbing, welding, mechanical... have even had a couple of watches apart, AND managed to get them back together and working. Need strong glasses for that now, though.

Re: Replace OMC Cobra with Mercruiser

Thanks to everyone who posted replies-- I didn't hear what I was hoping I would hear, but ya'll are honest and that's why I like this board.

I'd really prefer the Volvo option, but although the SX is technically a superior option (and way easier to install), I can't bring myself to pony up to the sticker.

On the other hand, it looks like the solution is clear, buy the entire donor boat and swap the guts over. Looks like I need to pull all the OMC tin off the engine and replace with the Merc stuff and then it should be *relatively* straight forward.

Still, I'd really like to gaze upon any photos or documentation from a Cobra to Alpha conversion, so if you have it in your heart.... DonS was it you??

And anyone who has the gumption to convert a stringer to anything other than a flower pot has my absolute and eternal respect...

BTW, "Miss Danelle" was intended as a one-shot donor. I bought her with a cracked block, but my uncle insisted we try to resurrect her as she has very low hours and was always garaged. Of course, she floats, she runs, and now she's family...

I guess I'm off to find another "junker" that I can't justify repairing....

Fair winds,

Chris

Re: Replace OMC Cobra with Mercruiser

Volvo used to make an adapter gimbal housing to mate the SX to the OMC cobra transome shield. thereby saveing you any transom or engine removal/modifications.

Re: Replace OMC Cobra with Mercruiser

rodbolt said:

Volvo used to make an adapter gimbal housing to mate the SX to the OMC cobra transome shield. thereby saveing you any transom or engine removal/modifications.

Click to expand...

This is true - I have one. Any Volvo SX housing will bolt into the Cobra transom shield. With a new cable (maybe, I didn't get that far) and an engine mounted water pump, the conversion is complete.

Again, my hangup was the pricing - $4500+ for a new drive, $3000 for "rebuilt", and upwards of $2000 for a used one. Alpha-powered boats are out there all day long for less than $2500 for the whole thing. Shoot, a new water pump is $200+, for a brass pump that in slightly different configuration costs about $50.

I think that the kicker is that SEI now has their fault-free 3 year warranty which (in exchange for a bunch of labor) makes the Volvo conversion moot for my needs (less than 300 HP, fresh-water use only, etc). All for less than $1300 (or about what the local Volvo dealerships wanted for the gimble housing alone).

I'm no fan of the Merc and, believe it or not, I'm not super cheap, but the economic differences are just too great.

PS - Lou - I think that the garbage lower IS a GLM, but can't confirm it because there are no stampings that I can find on the housing. Either they aren't there, or they are ground out/filled. The first failure was caused by the driveshaft shearing at the pinion gear. The second looks like a new bearing that failed and dumped chunks into the oil, destroying the lower gearset (and polluting the entire drive with junk). I'm still working on a warranty replacement for the failure, but not optimistic about it...

Chris

Re: Replace OMC Cobra with Mercruiser

well to convert it you will need the shield and drive assy for the alph, with minor mods to the engine,including manifolds and risers, the OMC engine will work.
the shift interrupt will most likly need to be rewired bypassing the ESA and wiring it like a merc.
bell housing is different the shaft coupler may be able to be reused if its the correct length and will clear the bell housing opening.
you may find its cheaper to buy a boat or do a repower complete.
the stringer width should be ok but the mounts will have to be relocated on the stringer.
and you will find the list goes on.

Complete Repower ?

Complete Repower ?

Hi- We are very attached to our boat and I'm contemplating a complete repower. The SEI Alpha Gen II drive seems to be a no brainer, but any recommendations on where to get a complete engine and all the rest of it? I want to stay with a 4.3L... All new parts would be great wouldn't it?

Re: Complete Repower ?

Re: Complete Repower ?

Best to start a new thread with your question 208, lest your question gets lost after a couple of replies from those that never read anything but the original post.