How to find the gear ratio of an alpha drive.

[viper1]

I have a sticker on my Mercruiser That says Mercruiser transom assembly # OB852835 Can this be traced to find exact out drive and gear ratio. I've googled and have yet to find anything. I'm weighing buying another upper drive unit or trying to redo this one. But need to at least know gear ratio. Anyone know how to determine this from this number. As the labels are torn off. And I haven't found a stamp on the youk either.

 

[alldodge]

I have a sticker on my Mercruiser That says Mercruiser transom assembly # OB852835 Can this be traced to find exact out drive and gear ratio. I've googled and have yet to find anything. I'm weighing buying another upper drive unit or trying to redo this one. But need to at least know gear ratio. Anyone know how to determine this from this number. As the labels are torn off. And I haven't found a stamp on the youk either.

In the adults only section you can see how to do it by verifying teeth
http://forums.iboats.com/forum/engi...formation-for-mc-1-thru-alpha-i-gen-ii-drives

Or you could put it in gear and rotate to get close

 

[thumpar]

What engine does it have?

 

[achris]

The drive serial number is usually stamped into the housing under where the port side decal was. If there is no serial number, good chance that's a replacement housing. Another method would be to let me know what the performance was and I can run the numbers through my slip calculator. I would need, WOT revs and speed (in knots) and you prop diameter and pitch (in inches) or the prop part number.

The transom number isn't much help as any drive can be put behind it. (And that number comes up as a TR/TRS :noidea Your 3 litre engine would have had a 1.98:1 drive from factory.

Chris........

 

[viper1]

The one I removed first has 1.98 R on it the second one label is gone and no stamp at all. It also has this number. 08858350. So I think so Im going to assume it takes the 1.98:1 Ok, thanks for the info.
Question... would it be a big mistake changing it out to a 1.84? Its on a 1989 24 ft Starcraft Islander 3.0L 140 181 cid
Thought it may lose some power.

 

[Bondo]

Question... would it be a big mistake changing it out to a 1.84? Its on a 1989 24 ft Starcraft Islander 3.0L 140 181 cid
Thought it may lose some power.

Ayuh,.... What's the prop pitch on it that runs well, with the 1.98:1..??

Generally speakin', our props run 'bout from 15" to 23",....
If yer startin' at the wrong end, there's No prop to make up the difference in the drive ratios,.... it's all in the math,....

A 3.0l will be a Dog, with any ratio taller than 1.98:1,....
Unless it's pushin' a bathtub,....

That's how I ended up with a 4.3l in my barge in the 1st place, all those years ago,....

 

[achris]

The one I removed first has 1.98 R on it the second one label is gone and no stamp at all. It also has this number. 08858350. So I think so Im going to assume it takes the 1.98:1 Ok, thanks for the info.
Question... would it be a big mistake changing it out to a 1.84? Its on a 1989 24 ft Starcraft Islander 3.0L 140 181 cid
Thought it may lose some power.

24 feet is a lot of boat for a V8 to be pushing. Asking a 4 cylinder to do it is a very tall ask. I suspect even with the 1.98 drive you've got a 'small' prop. Going to a 1.84 will require an even smaller one. (When I refer to small and large, I'm referring to the pitch, not the diameter.) The simply math is the new prop would have to be 1.84/1.98 (0.92) smaller than the current prop. Assuming the current is 13", that means the new would have to be 12"... Good luck finding a 15" diameter prop in 12" pitch....