OMC Cobra + GM 5.7L Gear ratio question

Keaton

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Feb 17, 2015
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41
My current stern driver is shot, the oil had tons of metal in it and was almost in a paste form. I found a guy who will sell me a whole rebuild drive(upper & lower) for cheap. cheaper then i can order parts for.
he as 2 drives available. 21:16 & 21:17.

looking at my drive I don't see it marked like the manual says it should be. I looked through the service manual and I don't see where it says which is which.

my guess is the 21:17 goes to the Ford 5.0L & the 21:16 goes to the GM 5.7L. the GM is more powerful so it can have taller gearing.

can anyone confirm this?

this is all I could find in the stern drive section but the math doesn't work out 21/17 = 1.23 & 21/16 = 1.3125
its a 1986 with a GM 5.7L
 

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Bondo

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my guess is the 21:17 goes to the Ford 5.0L & the 21:16 goes to the GM 5.7L. the GM is more powerful so it can have taller gearing.

Ayuh,.... I donno how the tooth count works with OMCs, But,....

One tooth difference can be made up for, by prop pitch change,....

Depends on the prop pitch yer runnin' now, to see how much room ya got to work with,....
If yer runnin' a 15" pitch, go with the lower gear,....
If yer runnin' a 23" pitch, go Taller,....
 

Keaton

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Feb 17, 2015
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41
I was running a 14.5" 14" pitch 4 bladed prop until i found a unmarked rock. now I'm at 14"x14" 3 blade. i want to go back to the 4 blade prop. the boat is faster and gets on plane quicker at lower speeds.

its a small light weight 18Ft boat. fresh lake water only.

so your saying
15" pitch = 21:17
&
23" pitch =21:16?
 

Bondo

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I was running a 14.5" 14" pitch 4 bladed prop until i found a unmarked rock. now I'm at 14"x14" 3 blade. i want to go back to the 4 blade prop. the boat is faster and gets on plane quicker at lower speeds.

its a small light weight 18Ft boat. fresh lake water only.

so your saying
15" pitch = 21:17
&
23" pitch =21:16?

Ayuh,.... What was the Rpms at Wot with the 14" prop,..??..??

That ain't much prop for a 350 in a tiny boat,....

Have ya done a compression test on the motor,..??
 

Keaton

Seaman Apprentice
Joined
Feb 17, 2015
Messages
41
Ayuh,.... What was the Rpms at Wot with the 14" prop,..??..??

That ain't much prop for a 350 in a tiny boat,....

Have ya done a compression test on the motor,..??

I have not done a compression test on the motor. it fires up, runs great, doesn't smoke or eat oil so there was no reason to suspect anything.

the motor will spin to 5.5K no problem, unless the tach is reading off...


what prop would you recommend for a 350 in a tiny boat like mine?
 

Bondo

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Messages
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I have not done a compression test on the motor. it fires up, runs great, doesn't smoke or eat oil so there was no reason to suspect anything.

the motor will spin to 5.5K no problem, unless the tach is reading off...


what prop would you recommend for a 350 in a tiny boat like mine?

Ayuh,..... Proper prop pitch, is the prop that allows the motor to run the Rpms at Wot that the motor is rated at,....

In yer case, 4600/ 4800 rpms, at Wot, light average load, light seas, 1/2 tank fuel, trimmed to run,...

My guess is, yer severely under-propped, or yer motor is really sick, 'n down on available Horsepower,.....

Ya need a reliable tach, so test yer's against a Shop Tach with known values,....
 
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