Outdrive Swap

[Roadblock007]

My Alpha 1 Gen 2 upper drive gear broke a few teeth. The gear ratio is 1.98:1 and it is attached to a 3.0L. If I replace the upper unit with a different one instead of doing the gears does it matter what ratio I get? If I just replace the whole drive does it matter? The ones I am looking at are from 3.0L engines but not all units are 1.98:1.

 

[greno]

I would stay with 1.98:1. You might get away with 1.84 with a different prop but you won't be able to run a V8 gear set like 1.47/1.50.

 

[Roadblock007]

I don't understand your response greno, I'm sorry. What I am thinking on doing is just replacing the top end, but wanted to know if I needed to stick with the 1.98:1 ratio. The units I am looking at are from 3.0L engines, just like what I have.

 

[greno]

Yes, you should stick with the 1.98:1 ratio you currently have.

 

[Scott Danforth]

if your getting a drive off a 3.0, it will be the 1.98:1 ratio

 

[achris]

My Alpha 1 Gen 2 upper drive gear broke a few teeth. ...

Metal from those 'broken teeth' will have made its way into the lower housing and it too will fail fairly soon.

I would replace the whole drive, and as already stated, stay with the 1.98:1 ratio. Any drive you see that is not 1.98 (or 1.94) will not be from a 3 litre engine. Merc only ever supplied 1.98 (or 1.94, see 'history of Alpha drives' in the stickies) with the 2.5 and 3 litre engines.

Chris.....

 

[Roadblock007]

I read a post that the 1.98:1 and 2:1 drives were interchangeable. Is that incorrect?

 

[achris]

I read a post that the 1.98:1 and 2:1 drives were interchangeable. Is that incorrect?

yes. But there is a 'gotcha'....

The current drive you have, 1.98, has an upper tooth count of 20/24 and lower tooth count of 17/28.

The 2.0:1 has an upper tooth count of 24/24 and a lower tooth count of 14/28...

Just beware that should you need to replace an upper or a lower in the future, you have to be very watchful of tooth count, not what the ratio that particular housing was 'designed' for...

Chris........

 

[Roadblock007]

So you are saying that if I wanted to use just the upper unit of the 2:1 outdrive it wouldn't fit with my current lower unit? I just redid the water pump and replaced all of the zincs so I was planning on flushing and using my lower unit with the upper from the new one.

 

[greno]

It's going to fit but you'll have a wrong gear ratio. You'll need to change to a 20/24 upper gear set in the 2.0 upper. Right now it should have a 24/24 gear set in it.

 

[Roadblock007]

I am very new to this, but have done several years of auto mechanics. Is it ok to put my lower end on this upper or not? What would the risks be exactly?

 

[greno]

Well, you're going to end up with a 1.65:1 ratio which is no good on a 3.0L. The prop is going to try to spin faster and you won't have the torque to do that. You about 20% difference. You may be able to buy a new low pitch prop that may help but ultimately you're going to have a less than optimal assembly from what Merc engineers designed.

 

[achris]

So you are saying that if I wanted to use just the upper unit of the 2:1 outdrive it wouldn't fit with my current lower unit? I just redid the water pump and replaced all of the zincs so I was planning on flushing and using my lower unit with the upper from the new one.

Physically, it will fit, but the drive overall ratio will not be 1.98:1. Using a 24/24 upper with your 17/28 lower will give an overall ratio of 1.65:1....

What size propeller are you currently running? (pitch)

Chris........

 

[Roadblock007]

Physically, it will fit, but the drive overall ratio will not be 1.98:1. Using a 24/24 upper with your 17/28 lower will give an overall ratio of 1.65:1....

What size propeller are you currently running? (pitch)

Chris........

I'm not sure. Where on the prop do I find that information?

 

[Rick Stephens]

I would cut to the chase here and tell you flat out that prop pitch doesn't matter - get the right 1.98:1 upper. There is no reason at all to try and jury rig the incorrect upper and lower together and hope you can prop it into some semblance of usable.

 

[achris]

Should be on the outer hub, between the blades. You'll have a part number, starts with 48-, and the last will be the pitch, like 17P... That's the pitch, in inches.

Chris.......

 

[Roadblock007]

Pitch is 19.

 

[achris]

If you wanted to put a 1.65:1 drive on, you'd have to change prop to a 15" or 16". 17" might be a bit steep, depending on what your current WOT RPM is....

Chris..........

 

[Roadblock007]

Thanks guys. I appreciate all of the knowledgeable opinions and help.