Upper Unit Gear Ratio Question

[ljsweeney]

I'm the idiot who cooked his OMC Cobra upper unit (mated to a 5.0L engine) after a botched gear lube fill.

I've got a line on a replacement unit with a gear ratio of 21:18. The manual for my boat states that the upper unit drive ratio is 21:17. Of course, the guy selling the unit says it will not be an issue, performance will not suffer, I won't have to reprop, blah, blah, blah.

Is he right? Can I run a 21:18 upper unit without issue? Should the splines be the same? Is he just trying to unload a unit I cannot use on me?

Thanks!

 

[HT32BSX115]

Re: Upper Unit Gear Ratio Question

I've got a line on a replacement unit with a gear ratio of 21:18. The manual for my boat states that the upper unit drive ratio is 21:17. ................. Can I run a 21:18 upper unit without issue? Should the splines be the same? Is he just trying to unload a unit I cannot use on me?

Thanks!

Howdy,


Well, it depends on the over all drive ratio. The over all ratio is a combination of the upper and the lower in a Cobra. The service manual does indicate the total ratio someplace I think. You could use a 1.43,1.41 or 1.5 drive and it will work ok....

OMC used a couple of different ratios for the SB V-8's

Any of them are probably close enough to work satisfactorily.


If you want to know your ratio (and the drive is off the back,) just have someone turn the input shaft (in gear) exactly 20 revs. and count prop turns. If it's 20 to 14 you have a 1.43 drive......you can use any combination of turns you want with a simple calculator

Just pick a combination of turns that gives you a result that matches the available drive ratios without having to measure a fractional turn (otherwise you have to use a protractor!!)

 

[ljsweeney]

Re: Upper Unit Gear Ratio Question

If you want to know your ratio (and the drive is off the back,) just have someone turn the input shaft (in gear) exactly 20 revs. and count prop turns. If it's 20 to 14 you have a 1.43 drive......you can use any combination of turns you want with a simple calculator

Just pick a combination of turns that gives you a result that matches the available drive ratios without having to measure a fractional turn (otherwise you have to use a protractor!!)

If I hadn't seized the upper unit bearings, I'm sure that would tell me what I need to know. <sigh>

Thanks for your excellent reply - I just wish I could use your advice.

I'm going with the 21:18 upper. Wish me luck.

 

[Joshua Nichols]

Re: Upper Unit Gear Ratio Question

You toasted the upper... You realize all those metal shavings fall down into the lower too.. You can do what you wish, but I think you should get a new drive.. Why risk it? Bearings and shims with a lot of metal in them.. That just sounds like future disaster...

 

[a70eliminator]

Re: Upper Unit Gear Ratio Question

The 21:18 is the correct one for your 5.0 engine. No such thing as a 21:17, better double check you manual.

 

[Howard Sterndrive]

Re: Upper Unit Gear Ratio Question

my manual shows a 21:17 for the 5.0
I think 21:18 will be fine, but I think you'll be changing the prop

 

[a70eliminator]

Re: Upper Unit Gear Ratio Question

Really? This is from my factory 1989 Cobra service manual:
2.3L 21:21
3.0L 21:21
4.3L 21:19
5.0L 21:18
5.7L 21:16
5.8L 21:16

 

[Joshua Nichols]

Re: Upper Unit Gear Ratio Question

my manual shows a 21:17 for the 5.0

mine too... guess it is going to depend on year

 

[Howard Sterndrive]

Re: Upper Unit Gear Ratio Question

weird - my manual is MED/MEF (1989 model year),
it was published in 88 and it lists 21:17 for the 5.0 in two places.

 

[Fordiesel69]

Re: Upper Unit Gear Ratio Question

Pay to get your drive washed internally.

 

[a70eliminator]

Re: Upper Unit Gear Ratio Question

weird - my manual is MED/MEF (1989 model year),
it was published in 88 and it lists 21:17 for the 5.0 in two places.

That is weird oh well, I think he'll be just fine with the 21:18

 

[HT32BSX115]

Re: Upper Unit Gear Ratio Question

You toasted the upper... You realize all those metal shavings fall down into the lower too.. You can do what you wish, but I think you should get a new drive.. Why risk it? Bearings and shims with a lot of metal in them.. That just sounds like future disaster...​

You need to heed this advice.....

If you're infact bolting the "new" upper to the lower that had the catasprophic failure, the whole mess will not last long.
​

 

[a70eliminator]

Re: Upper Unit Gear Ratio Question

did it go crunch crunch, or just get hot and seize up? The condition of the lube in the lower would tell the story, there is only a small passageway through the intermediate (hollow) driveshaft for oil equalization between upper and lower, my thinking is that the lower gears are somewhat isolated from the upper, I would definately inspect it close.

 

[Joshua Nichols]

Re: Upper Unit Gear Ratio Question

Pay to get your drive washed internally.

What do they do? Pull out the bearing carrier and pump safety kleen though it? Never heard of that service

 

[ljsweeney]

Re: Upper Unit Gear Ratio Question

Just to answer a few questions:

The manual does indeed state 21:17 for the upper unit gear ratio. The unit in question is an '88. I spoke to some knowledgeable folks while sourcing the part and they stated that 21:17 was used for the 5.0 only briefly until they standardized on 21:18.

The upper did seize up, but it did not grenade. The gears did not lose any teeth, and they were only slightly bluish. It would appear that the bearings seized.

I closely inspected the fluid that drained from the lower unit prior to disassembly. It did not smell burnt, had a good color, and most importantly, I did not find any metal shavings.

I know I'm taking a chance here, and the really, really safe thing to do would be to either replace both upper and lower units, or have the lower rebuilt/resealed while replacing the upper. However, budget is a concern here - I was able to source a tested, torn-down and reassembled upper with a small warranty for only a grand. If this turns out to be a bad move, then I'll still have to replace the lower. If this works, I'll be saving a ton. Plus, I have used this opportunity to become better acquainted with the inner workings of my boat. I just got it last fall, so this is my first full boating season with it.

I mated the new upper with the existing lower yesterday, and to avoid drive unit installation issues, replaced the u-joint bellows. I'll be installing the 'frankendrive' later today, properly filling it with lube (this time), and testing the hell out of it before dropping the boat in the river again. I'll let you know how it works out.

Finally, I'd like to say that this forum is a wealth of knowledge! I simply could not have even tried to fix this mess without everyone here and all the information contained in the old threads. Thanks so much.

 

[ljsweeney]

Re: Upper Unit Gear Ratio Question

Good news - Frankendrive lives!

After mounting up the drive unit (it slipped right in - thanks to all who labored with a tight u-joint/bellows clearance before me), and filling her (properly this time) with gear lube, she runs like a champ. After the initial muffs-on run, I found a little bit of metal on the magnetic drain plug (no more than I saw last weekend when I started this mess), and nothing after the second, much longer run. She shifts good and doesn't make any unusual noises.

The real test will be tomorrow when I get her wet for the first time since breaking/fixing her, but I am optimistic.

Thanks again to all forum members, past and present, who have been through what I have been through and were gracious enough to post their travails on this site. I am indebted to each and every one of you. I hope i can someday return the favor.

 

[a70eliminator]

Re: Upper Unit Gear Ratio Question

So many times a person uses the forum to get input and then they're gone, good to post conclusions, glad it's working out for you may have dodged a bullit, hope your sea trial goes well I think it will.

 

[ljsweeney]

Re: Upper Unit Gear Ratio Question

Thanks, man.

The initial sea trial was excellent, albeit rain shortened. I plan on extensively shaking her down tomorrow - it's going to be in the 90s.

It's nice to have her back.

 

[ljsweeney]

Re: Upper Unit Gear Ratio Question

Extended sea trial completed yesterday. She's running strong and not doing anything odd or worrisome. I could not be happier.

The first official "Summer of the Boat" has begun.

 

[dutchmafia]

Re: Upper Unit Gear Ratio Question

Did the same thing this weekend. Found a replacement drive. 21:18 ratio. My question is whether it matters whether the replacement drive goes into a Ford or GM. The replacement drive is a Ford. Engine is a GM. Anyone know whether that matters?