OMC Cobra vs Mercruser keyhole

[chconger]

Hi All

Is there any difference? I am in the middle of a restore (see my thread below) and just wondering: if i wanted to say farewell to the OMC, do I have to muck with the keyhole?

Already cut for the cobra;

Thanks
Chris

 

[matt167]

Re: OMC Cobra vs Mercruser keyhole

Keyhole is the same, a couple bolt holes at the top are different. You can go Merc, Volvo or OMC Cobra with the same keyhole.. I would absolutely ditch the OMC at this point.

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Thanks! That's good to know. Cant afford the switch now, (trying to reload the money gun just for foam) but good to know its fairly straight forward.

Cheers
Chris

 

[Idlespeedonly]

Re: OMC Cobra vs Mercruser keyhole

I would like to take that little boat in the middle of your pictures for a spin someday.

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

I would like to take that little boat in the middle of your pictures for a spin someday.

OK then; 24 seconds in, and you are on the mahogany;

Cheers
Chris

 

[Tnstratofam]

Re: OMC Cobra vs Mercruser keyhole

That has got to bee one of the coolest videos I've seen in a while. That's also one of the coolest boats I've seen. Absolutely beautiful!

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Hey Thanks!

It was fun to build too.

Cheers
Chris

 

[HT32BSX115]

Re: OMC Cobra vs Mercruser keyhole

Hi All

Is there any difference? I am in the middle of a restore (see my thread below) and just wondering: if i wanted to say farewell to the OMC, do I have to muck with the keyhole?

Already cut for the cobra;


Thanks
Chris

Howdy Chris,

Only slight differences. I replaced a 1987 460 King Kobra with a 1997 Mercruiser 454 Bravo III

I had to drill 2 additional holes for the 2 extra studs in the Bravo.

If you install an Alpha Gen II, you'll need to drill those additional (top) holes too.

Other than that they're the same. Same X-dimension too. The distance between the drive shaft and propshaft are the same.


Comparison:

Cheers,


Rick

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Howdy Chris,

Only slight differences. I replaced a 1987 460 King Kobra with a 1997 Mercruiser 454 Bravo III

I had to drill 2 additional holes for the 2 extra studs in the Bravo.

If you install an Alpha Gen II, you'll need to drill those additional (top) holes too.

Other than that they're the same. Same X-dimension too. The distance between the drive shaft and propshaft are the same.

Cheers,


Rick

That's astonishing. Thanks! I never expected this to be so easy. So its "just" money now...:facepalm:


Cheers & Thanks All

Chris

 

[HT32BSX115]

Re: OMC Cobra vs Mercruser keyhole

That's astonishing. Thanks! I never expected this to be so easy. So its "just" money now...:facepalm:


Cheers & Thanks All

Chris

I also had to relocate the engine mounts and I used the complete Mercury wiring harness so I had to rewire the instrument panel but that wasn't very hard at all. I used my old steering wheel and cable (the cables are universal) and did replace the OMC throttle/shift quadrant (and throttle/shift cables) but it was pretty old and I wanted a much more modern one.

I spent the most time on repairing the transom, and bilge. there was some fiberglass work needed after I moved the engine mounts.

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

I also had to relocate the engine mounts and I used the complete Mercury wiring harness so I had to rewire the instrument panel but that wasn't very hard at all. I used my old steering wheel and cable (the cables are universal) and did replace the OMC throttle/shift quadrant (and throttle/shift cables) but it was pretty old and I wanted a much more modern one.

I spent the most time on repairing the transom, and bilge. there was some fiberglass work needed after I moved the engine mounts.

Thanks. Engine mounts would be the largest thing there, and I just built / installed mine, so I wont be too timid to cut them out and make new ones wherever required.

Is there any kind of table or drawing set on line showing the different mount requirements?

 

[HT32BSX115]

Re: OMC Cobra vs Mercruser keyhole

Thanks. Engine mounts would be the largest thing there, and I just built / installed mine, so I wont be too timid to cut them out and make new ones wherever required.

Is there any kind of table or drawing set on line showing the different mount requirements?

Not really.

What you do is, install the gimbal mount and inner transom plate(which is the rear eng mounts) ........... and put the engine in with a lift and bolt the flywheel housing mounts to the transom plate mounts.

Then, get the engine in approx alignment.

Left/right and up/down(rear) alignment is basically taken care of with the rear mounts.

Up/down (in the front) is where the front mounts come in.

Holding the engine with the rear mounts and the lift, you locate where the front mounting pads will need to be (with the front mounts approx in the middle of their adjustment)


Once you have the pads/stringer/blocks etc all glassed in the right place and the right height, you can rest the engine (front mounts) on the pads to locate where you will need to drill the holes for the lag-bolts. Everything is based on where the engine will sit once aligned.

Then when you get it all bolted down and tight, you'll do another final alignment (using the alignment tool)

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Not really.

What you do is, install the gimbal mount and inner transom plate(which is the rear eng mounts) ........... and put the engine in with a lift and bolt the flywheel housing mounts to the transom plate mounts.

Then, get the engine in approx alignment.

Left/right and up/down(rear) alignment is basically taken care of with the rear mounts.

Up/down (in the front) is where the front mounts come in.

Holding the engine with the rear mounts and the lift, you locate where the front mounting pads will need to be (with the front mounts approx in the middle of their adjustment)


Once you have the pads/stringer/blocks etc all glassed in the right place and the right height, you can rest the engine (front mounts) on the pads to locate where you will need to drill the holes for the lag-bolts. Everything is based on where the engine will sit once aligned.

Then when you get it all bolted down and tight, you'll do another final alignment (using the alignment tool)

Thanks!

I already have my forward mounts glassed in to the same XYZ of the old mounts.

So my plan was to mount the engine as you describe above (EDIT: with the transom plate in) to locate the holes for the mounts, then mark, pull up the engine again, drill holes, epoxy seal the wood, remount engine, and final adjust.

Never done it before so grateful for any insight.

Thanks
Chris

 

[HT32BSX115]

Re: OMC Cobra vs Mercruser keyhole

Thanks!

I already have my forward mounts glassed in to the same XYZ of the old mounts.



So my plan was to mount the engine as you describe above (EDIT: with the transom plate in) to locate the holes for the mounts, then mark, pull up the engine again, drill holes, epoxy seal the wood, remount engine, and final adjust.

Never done it before so grateful for any insight.

Thanks
Chris

Nice job!

I don't think those (3/4" plywood?) boxed mounts would be heavy enough for a 454............ I would probably make then a LOT bigger, heavier and SS lag screwed into the main stringers before glassing it all in...............

With them open like that, you could use bolts instead of lag bolts. but that would place all the torque from the engine on the glued/glassed in corner joints.

You may want to consider adding some additional layers of plywood to make it thicker to distribute the loads and use longer SS bolts. Once the bolts are in, you could box it in and fill with epoxy/glass powder slurry.

Or even leave it open enough at the bottom so you could get a wrench in there. I do like the idea of using bolts with a nut on the bottom instead of lag screws/bolts.

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Nice job!

I don't think those (3/4" plywood?) boxed mounts would be heavy enough for a 454............ I would probably make then a LOT bigger, heavier and SS lag screwed into the main stringers before glassing it all in...............

With them open like that, you could use bolts instead of lag bolts. but that would place all the torque from the engine on the glued/glassed in corner joints.

You may want to consider adding some additional layers of plywood to make it thicker to distribute the loads and use longer SS bolts. Once the bolts are in, you could box it in and fill with epoxy/glass powder slurry.

Or even leave it open enough at the bottom so you could get a wrench in there. I do like the idea of using bolts with a nut on the bottom instead of lag screws/bolts.

Actually they are 1.5" thick, not 3/4, fortunately.

Unfortunately, they are to hold a 302, not a 454.

Here is an image of one on the bench with the old one in the background.

Box joint, total 1.5 inch;

Compare original and new:

Appreciate any advice.

Cheers
Chris

 

[HT32BSX115]

Re: OMC Cobra vs Mercruser keyhole

Pretty nice! You are one serious woodworker!

I might have also used SS screws and hardwood gussets while waiting for the epoxy to harden.

I like overkill, and it looks you are all over it!!

 

[chconger]

Re: OMC Cobra vs Mercruser keyhole

Pretty nice! You are one serious woodworker!

I might have also used SS screws and hardwood gussets while waiting for the epoxy to harden.

I like overkill, and it looks you are all over it!!

Thanks! Ill sleep better. Perhaps another round of glass for fun....

Cheers
Chris