Swap lower units, Force 125 into a 85 hp power head

[ima.riverrat]

I have a quick question I hope someone can answer. I have a Force '89 85 hp and a Force '86 125 hp. the power head on the 125 is bad and the lower unit on the 85 is bad.

Now my question is will the gear difference be ok for this swap, to put the 125 lower end onto the 85 power head? they have the same shaft length so it would be a direct swap.

Also, The same size prop is on both motors now... should I change?



Thanks for any help



Rich.

Livin' in a Van down by the River

 

[tater76]

Re: Swap lower units, Force 125 into a 85 hp power head

Is the tooth count on the end of the driveshaft the same on each unit?

 

[pnwboat]

Re: Swap lower units, Force 125 into a 85 hp power head

The 125 HP motor comes from the factory with a 1.73:1 gear ratio. The 85 HP comes from the factory with a 2:1 gear ratio. Putting the 1.73:1 ratio on the the 85 HP motor will decrease the performance. It will work but don't expect the 85 HP motor to perform as well with the 1.73:1 lower unit.

The real test is to try it out and see if it meets your needs. You may have to decrease the propeller pitch to get decent hole shot to get up onto plane quickly.

 

[ima.riverrat]

Re: Swap lower units, Force 125 into a 85 hp power head

Is the tooth count on the end of the driveshaft the same on each unit?

yes it is

 

[ima.riverrat]

Re: Swap lower units, Force 125 into a 85 hp power head

The 125 HP motor comes from the factory with a 1.73:1 gear ratio. The 85 HP comes from the factory with a 2:1 gear ratio. Putting the 1.73:1 ratio on the the 85 HP motor will decrease the performance. It will work but don't expect the 85 HP motor to perform as well with the 1.73:1 lower unit.

The real test is to try it out and see if it meets your needs. You may have to decrease the propeller pitch to get decent hole shot to get up onto plane quickly.

Ok, That's what I was thinking... It is my bass boat, but we do have family time tubing/skiing, so this would be a bad thing to get out of the hole quick... I do have another 85 hp lower unit to use but it has a damaged skeg...

 

[pnwboat]

Re: Swap lower units, Force 125 into a 85 hp power head

A damaged skeg doesn't hurt anything unless it just bent in a way that affects steering. You could also bolt on a skeg guard if there's enough of the old skeg left.

I put a 1988 85HP leg on my 1988 125HP motor. The difference was pretty significant (much better performance, hole shot etc). If you're running the 85HP motor, I would think that you would see a drop in performance with the 125HP leg because of the gear ratio change. The 3 and 4 cylinder Force motors really like the higher 2:1 gear ratio. Kind of depends on the hull type, weight, prop pitch, etc too.

 

[SkiDad]

Re: Swap lower units, Force 125 into a 85 hp power head

it will probably be OK if you run a 15 pitch prop - a 17 is going to prob not work for skiing - guessing you probably had a 17 pitch up to now.

 

[ima.riverrat]

Re: Swap lower units, Force 125 into a 85 hp power head

A damaged skeg doesn't hurt anything unless it just bent in a way that affects steering. You could also bolt on a skeg guard if there's enough of the old skeg left.

I put a 1988 85HP leg on my 1988 125HP motor. The difference was pretty significant (much better performance, hole shot etc). If you're running the 85HP motor, I would think that you would see a drop in performance with the 125HP leg because of the gear ratio change. The 3 and 4 cylinder Force motors really like the higher 2:1 gear ratio. Kind of depends on the hull type, weight, prop pitch, etc too.

Nice... Yea I know the condition of the 125hp leg, owned since new, the other 85hp leg I picked up and seamed to shift ok and stay in gear, that head and leg combo had a Bad cylinder. So I guess it's put the new water pump on the 85hp leg instead of my old 125...Thanks for the advise!

 

[ima.riverrat]

Re: Swap lower units, Force 125 into a 85 hp power head

it will probably be OK if you run a 15 pitch prop - a 17 is going to prob not work for skiing - guessing you probably had a 17 pitch up to now.

Actually they both had 15's.