OMC Cobra Lower unit

[CaptCrunch205]

Hello I have a question about the lower unit of the OMC Cobra. A friend of mine blew out his lower unit this weekend. It's definitely the gears in the lower unit. His upper gears are pristine and still look new. They don't look a bit worn. I have a lower unit for a 4.3 that is the same year as his. I bought it when I had my open bow Four Winns with a 4.3 and never used it. I didn't sell with the boat because I thought if I ever needed it with my 5.0 it might be good for a back up. Can we just swap it out with his bad one? What do we have to look for to make sure that nothing in his upper is bad? I am assuming that the driveshaft could possibly be messed up anything else we need to look for? I know the guy really well that I bought the lower unit from to begin with and he guaranteed it to be in good working condition. It has been stored inside with oil in it. Let me know what else we might need to look for or need to do before we bolt them up. Thanks

 

[matt167]

If he has anything but a 4.3L the drive ratio won't be right and it might work fine, but will likely need a weird prop and never be 100%.. For sure would get him back in the water if it is a good drive.

I think the best repair for a blown OMC lower would be an SEI drive conversion kit. Could use it with an SEI or Mercruiser leg

 

[CaptCrunch205]

The outdrive is the exact same year and motor. I was just wondering what would be needed to swap out the lower units. Yes a new one would be better, but it's not in his budget right now and I am trying to help him out. Obviously I will go through the shift adjustments, and whatever else while we have his outdrive off. Definitely going to replace the bellows and gimbal bearing while it's out. Just wondering what look for to make sure his upper hasn't been compromised. We drained it down and obviously the we're metal shavings on the oil. I opened the top and his upper gears literally look brand new. No wear, chips, scratches or anything that sticks out and says Ooo..... that doesn't look good. Just want to get him back on the water and he can look at getting his lower rebuilt or get a new one at the end of the season. Thanks.

 

[HT32BSX115]

Just wondering what look for to make sure his upper hasn't been compromised. We drained it down and obviously the we're metal shavings on the oil. I opened the top and his upper gears literally look brand new. No wear, chips, scratches or anything that sticks out and says Ooo..... that doesn't look good.

Howdy,

What ever you do, if you plan to use that upper (from the drive where the lower was damaged) you should disassemble it and COMPLETELY flush, clean, and carefully inspect the gears, bearings, and reseal/re-shim before bolting on a "new" lower.

The oil is shared between both and it's imperative that both be "rebuilt" and/or taken apart when it appears that only one was damaged (upper or lower)

Regards,

Rick

 

[CaptCrunch205]

Thanks Rick. Are you talking a complete teardown? Shafts, gears, etc? I have never tackled that big of a job and don't know if I have all the tools to complete the task. The lower was completely gone through by the mechanic I previously took my boat to. I was hoping to separate the bad lower unit and flush and clean the upper carefully then mate it up with the lower I have and do a pressure test, replace the bellows, gimbal bearing and adjust the shift linkage and see how it performs.

 

[HT32BSX115]

Thanks Rick. Are you talking a complete teardown? Shafts, gears, etc?

I am.

Since most modern sterndrives share oil top to bottom, metal-contaminated oil will have been circulated throughout as the "lower" slowly self-destructed.

If you were to install a different lower unit on that drive without a disassembly/full cleaning it wouldn't last very long. There's no easy way to ensure that you completely flush all the metal particles out of a drive without taking it apart.

If you don't want to do that, it would be better to just replace or rebuild the drive altogether.

If you're going to do that, a better proposition might be to get an SEI adapter kit + new SEI drive with a warranty etc.

 

[CaptCrunch205]

What do you use to flush the outdrive? Would you use diesel fuel or some kind of motor flush? We are going to tackle it with some help from the guy that used to work on my boat. He can’t do any physical work anymore, but he said he would help us with the harder parts of the process. Just wondering what to use to start flushing the outdrive.

 

[kpg7121]

What do you use to flush the outdrive? Would you use diesel fuel or some kind of motor flush? We are going to tackle it with some help from the guy that used to work on my boat. He can’t do any physical work anymore, but he said he would help us with the harder parts of the process. Just wondering what to use to start flushing the outdrive.

It might be a good idea to pick up or download an OMC shop manuel. Forget Seloc Or Clymer except as knee pads. Take pics & keep track of all shims & where they go.

I agree with previous poster on the SEI conversion but it's your buddies call & the conversion last time I checked I think it was was about $2k. Best of luck.

 

[CaptCrunch205]

I already have an OMC service manual. I was planning on pics and keeping track of all the details. We are gonna start on it this weekend.

 

[HT32BSX115]

What do you use to flush the outdrive? Would you use diesel fuel or some kind of motor flush?

Diesel fuel, Kerosene, "Motor Flush" are just light distillates.

Any of them would "flush"....... The problem is that you cannot guarantee you'll get it all. Then at some point later, those particles of metal that didn't wash away start moving around until they either get into a roller bearing, bushing, dog-clutch or gear-face.

Because of the extreme pressures in the drive gears, minor damage turns into catastrophic failure fairly quickly. (same reason a Cobra drive "upper" will destroy itself in mere minutes if the upper gears become uncovered from a bubble floating to the top in a drive that was filled wrong)

You could probably ask some of the other people here that have actually pulled a drive apart that had an upper or lower "blow-up" and ask them how much damage or contamination they had in the other side of the drive.

It's been talked about many times here in this and other forums. If you have a catastrophic failure in a lower or upper in any drive, the consensus is to disassemble the seemingly undamaged part for inspection........NOT to just "flush" it and call it good.......... OR, you'll just be pulling it all apart in the future to rebuild/replace both.


Cheers,


Rick

 

[CaptCrunch205]

Thanks Rick. Actually we are going to tackle the tear down starting tomorrow. Kpg7121 said to take pics and keep track of the shim placement. That’s what we plan on doing. Plus I can get verbal help from the mechanic that used to work on my boats. He can’t do any physical work, but his brain is still sharp. I just wanted to know what I could use to flush and clean things as I go. I am just worried about messing up the upper unit, but my friend says let’s do it. So tomorrow we start. Thanks for all the insight on things.

 

[HT32BSX115]

Thanks Rick. Actually we are going to tackle the tear down starting tomorrow. Kpg7121 said to take pics and keep track of the shim placement. That’s what we plan on doing.

I think that'll be the right plan.

I like to use JET-A1 (or kerosene) for cleaning solvent because it's cheaper than any "regular" cleaning solvent you can usually buy.....and it's not nearly as oily as #2 Diesel (although diesel will work) .

You can get Jet-A at many small airports (the bigger ones will refuse to sell it to you). Most farm supply co-ops will have K1 (kerosene...essentially the same stuff, slightly more expensive) and some will even have it "at the pump" (like you can buy gasoline or diesel)

I wouldn't use gasoline though.

An important key to rebuilding any high torque gearbox/differential/outdrive etc.... is keeping everything pristine clean" .....And, you may want to replace all the seals when going back together.

Regards,


Rick