Different gear ratios on lower unit?

[ThomW]

Hey all. A buddy of mine wore out the upper gears in his 4.3 Cobra lower unit. We are weighing the option of swapping out the gears versus replacing the upper half of lower with a used unit. Since we do not have a machine press, switching gears is near impossible. My question is this: I know that the upper half of a 3.0 cobra would fit on this lower unit, but I believe they have different gear ratios. What effect, if anything would that have on the unit? Would that work, or would other gears have to be swapped out to make the ratios match? Thanks!!!

 

[Bondo]

but I believe they have different gear ratios. What effect, if anything would that have on the unit?

Ayuh,...... The motor will be screamin' a million rpms, 'n the boat will be goin' nowhere,......

Ya Need the right gear ratio,.....

 

[ThomW]

Thanks Bondo. I thought so, but another "mechanic" told me that the Cobra 3.0 and 4.3 lower units were close enough in ration that it would not be a huge impact. Slightly more torque on take off, and a little less top end speed. I wasn't convinced so I wanted to check with the vast knowledge on this site. I am correct that I need a machine press to replace the gears in upper half of the lower, right?

 

[harringtondav]

I've only been in a Cobra once to do a seal job. The drive gear is locked onto the shaft with a threaded ring that requires a special spanner, heat, and a BFH to loosen it. Then the gear slips off the splined shaft.

That is the easy part. Any time you replace an upper gear set it is absolutely critical that you follow the procedures for shimming the gear match, and setting backlash. If you don't you'll be replacing the gear set soon.

Boatinfo has the Cobra manual and the procedure.

 

[ThomW]

Ayuh,...... The motor will be screamin' a million rpms, 'n the boat will be goin' nowhere,......

Ya Need the right gear ratio,.....

So I just wanna make sure I am sure on this. From what i have seen, the gear ratio on the 4.3 drive is 21:21 and the 3.0 is 21:19. If I am keeping the lower unit and lower gears from the 4.3, and only want to put the upper gear box and gears of a 3.0 drive (with a 21:19 ratio), that will cause serious performance issues to the boat? Thanks for bearing with me.

 

[ThomW]

Sorry. The 4.3 is a 21:19 ratio and the 3.0 is a 21:18 ratio. Unless I am mistaken.

 

[Bondo]

So I just wanna make sure I am sure on this. From what i have seen, the gear ratio on the 4.3 drive is 21:21 and the 3.0 is 21:19. If I am keeping the lower unit and lower gears from the 4.3, and only want to put the upper gear box and gears of a 3.0 drive (with a 21:19 ratio), that will cause serious performance issues to the boat? Thanks for bearing with me.

Ayuh,..... The Upper used, determines the gear ratio,......