Lower Unit gear Ratios

[198688]

I am looking for a Lower Unit for my 1966 Merc 650. I know that there are an absolute ton of lower units that will bolt up. (65 through 150 HP to be exact). The question is, Do the lower units that were built for the larger HP models have different gear ratios? And if so how can I tell what the gear ratio is on my unit and any other units that I may come across?

 

[rickdb1boat]

Re: Lower Unit gear Ratios

Yes, there are different lower uits that will fit your motor. Most times the ratio can be made up for in prop pitch. As far as determining the ratio, I believe it has to do with turning the flywheel exactly one complete turn and seeing how may turns the propshaft makes. Maybe others will know for sure..

 

[ob]

Re: Lower Unit gear Ratios

Pretty close Rick.Example: The gear ratio of my 70 hp Rude is .414 or 12/29. .414 means that for every .414 revolutions of the propshaft the crankshaft turns once.This can also be transposed from 12/29 by dividing 12 into 29 and getting 2.416. This means that for every 2.416 revolutions of the crankshaft the propshaft turns once.Divide 29 into 12 and you get 414.

 

[198688]

Re: Lower Unit gear Ratios

OK. But how did you know that the gear ratio was .414 and not .415?

 

[ob]

Re: Lower Unit gear Ratios

#1 because I have a manual for my engine.#2 divide 29 into 12 on your calculator. .414 is a rounded figure.<br /><br />I did some research but can't find what your 65 hp merc had originally for its gear ratio.

 

[198688]

Re: Lower Unit gear Ratios

I have an original parts manual for my motor and I don't see any indication of the gear ratio anywhere. I just wonder if all 4 cyl models had 1 gear ratio and 6cyl had another?? And the hp Differences within each cylinder category could be offset by the pitch of the prop???

 

[ob]

Re: Lower Unit gear Ratios

You could probably offset the engine load on a lower unit that had a closer gear ratio by decreasing prop pitch ,however you would sacrifice top end speed by running with a less steep prop to allow your engine to attain RPM.I'm sure the higher HP motors had a closer to 2 to 1 ratio than the lower HP models.I'll keep searching to see if I can come across what your design gear ratio was and post back if I do.This will give you a starting point in locating the proper lower unit.