Swapping Short and Long Shafts between outboards

[collespaccato]

I would like to know if it is possible to swap the lower units between a Mariner 30hp short shaft (serial#0G193173) with a Mariner Magnum 40hp (0G137689) both seem to be in the same time bracket. I understand that the mounting bracket is different since the 40hp has tilt trim.
Thanks

 

[merc850]

I'm thinking that you just have to changed the driveshafts as long as the splines are the same.

 

[collespaccato]

I checked and compared the parts list, gear housings looks different in diagrams and driveshafts are different part numbers. Do you still think as long as the splines are the same it would work?

 

[Texasmark]

A series of engines a lot of times share components and only make slight changes to fuel/combustion parts to vary the HP rating. The question is where is that break point and why would the spline count be different on a SS drive shaft when you only have a low percentage of power changing.

If you are running the same part numbers then you ought to be good to go. Since you said you are changing HP, pull up the short shaft parts list for the 40 hp and compare to 30 hp. If those two match then you are good to go.

Shift shaft has to be longer too and check the exhaust manifold directly below the engine inside the mid section housing. One mechanic on here mentioned on another post that the mid section length change in question had a different length exhaust horn......I didn't get into it but these were loop charged engines and that type of fuel scavenging uses reflected exhaust pulses to make the Loop work with the flat topped piston. Changing the length to me would change the timing for the same hp engine with the same reflection coefficient......may have been blowing smoke.....but I know not about the smoke or the different part!

 

[Sea Rider]

Why would you want to swap lower legs for, is the Magnum 40 HP lower leg also a S size ?

Happy Boating​

 

[Chris1956]

Normally the midsections will be different lengths, as will the shift shafts, water pipes and driveshafts. You might measure them. You should see about a 5" difference.

You may be able to swap powerheads. Does that work?

 

[merc850]

Yes they will be different lengths but the waterpipe, exhaust tube and shift shaft will all end on the same plane (ie. distance from the casting bottom) whether it's a long or short shaft, only the driveshaft will be longer or shorter. This is only speculation - I'm thinking about the cheapest way to make 2 different motors from one.

 

[Chris1956]

Correct, however, I was providing general info about the two shaft length motors. Some Mercs had 5" extensions on the midsection, driveshaft, water tube, shift shaft and studs. They were easier to convert.

On a related subject, would you rather swap a powerhead or a driveshaft? Powerhead wins every time.....

 

[merc850]

Powerhead for sure, buy the gasket first.

 

[Texasmark]

Yes they will be different lengths but the waterpipe, exhaust tube and shift shaft will all end on the same plane (ie. distance from the casting bottom) whether it's a long or short shaft, only the driveshaft will be longer or shorter. This is only speculation - I'm thinking about the cheapest way to make 2 different motors from one.
View attachment 352893

Off topic but what you see in your picture is why Mercury Marine, through their extensive testing in the late 1950's testing at Lake X, Florida, added shock absorbers to their engines and engine kill switches later so that when the engine kicked up and the RPMs were significant, preventing damage to the transom when the prop returned to the water and the step function thrust was applied to the top of the transom.