lower unit swap question 120 hp to a 90 hp lower unit

[rocketman87]

I have a 1997 force by mercury 120 hp lower unit that needs replacing...I have found a 1995 90 hp force by mercury lower unit and would like to know if they are interchangable...I have read a little and searched some ...but am in need of expert assistance if any can help...I just bought this boat and had the lower unit go out on the first trip and was ready to cry....thanks all

 

[bentle]

Re: lower unit swap question 120 hp to a 90 hp lower unit

The 1995 90hp lower unit will work. you will most likely want to use
Your drive shaft and coupler from the 120 providing they are in good shape.
I am wondering what or how the lower reacted before it stoped working.
Also the 90 lower will have a different gear ratio than the 120, so watchthe your tach and double check that the shift rod will match up with the 120.
Should have splined end but double check anyways.

 

[rocketman87]

Re: lower unit swap question 120 hp to a 90 hp lower unit

I know it has the splined end...Is it true to change it out you just need to shift it into forward gear and unbolt and take off and put the new one in and bolt it up....sounds too easy...but the mechanic quoted me a 75 dollar price to change them out so I know it cant be too complicated or they would charge more....

 

[bentle]

Re: lower unit swap question 120 hp to a 90 hp lower unit

From the book shifting into forward gear is not needed.
However when installing the lower there is a sequence to go through,
and rather than explaining it I posted a pic of the proper sequence.
Ok its not letting me upload so here goes.
Push shifted into forward gear, next you will shift the shift rod on the lower into forward gear
While rotating prop clockwise until you hear the clutch dogs make a ratchet sound.
Then install lower making sure the water tube lines up with pump housing.
with a helper you have to rotate the flywheel clockwise a bit to get the drive shaft splines to line up with the crank shaft splines.
not a really hard job as long as you have help.
Doing it alone is a pain but possible.

 

[rocketman87]

Re: lower unit swap question 120 hp to a 90 hp lower unit

Do I need to lube/grease the upper shaft when installing ...like the same stuff that goes on the prop... and what is that stuff... I am also changing installing a water pump impeller kit...any tips or other helpful items would be appreciated.thanks

 

[rocketman87]

Re: lower unit swap question 120 hp to a 90 hp lower unit

Do I need to lube/grease the upper shaft when installing ...like the same stuff that goes on the prop... and what is that stuff... I am also changing installing a water pump impeller kit...any tips or other helpful items would be appreciated.thanks

anyone ? Frank ?

 

[bentle]

Re: lower unit swap question 120 hp to a 90 hp lower unit

Grease splines except the end that fits up into the crank splines.
impeller is pretty easy to do just note the position the old impeller is in before removing.
you will want to install the new one in the same direction, and a little dab of grease on the impeller key way will help to hold it on the drive shaft when you reinstall the pump housing.
make sure to note the gaskets positions so you can get them back on correctly also.
A marine grade grease is what I use to grease up all splines excluding the end that mates up with the crank shaft splines. Something about a pre-load there and can cause crank shaft failure.

 

[rocketman87]

Re: lower unit swap question 120 hp to a 90 hp lower unit

It is on....It works thanks to all who helped...

I will have to admit I had it together and had the gears all messed up....Nuetral was forward and I had to take it to someone to drop the lowwer again and reset the shift lever I guess...anyway that was a steal at 75 dollars...


The rpm seems higher with this unit and I thought i read it would be lower....I have yet to gps it but Im thinking it is running about 5-7 mph slower with this unit...I need a different prop...

I have a Laser SS 48-16992A45 22p

What is a better prop for better takeoff and lower top end rpm's

 

[jerryjerry05]

Re: lower unit swap question 120 hp to a 90 hp lower unit

The grease should be used everywhere.
No pre-load on the Forces(that I know of).
The way to figure out the prop is with the tach.
You should be running about 52-5500rpm.at WOT.
Not sure what prop your looking for but if your way above 5500 you need a bigger prop.From a 19 to a 21
Or way under then you need from a 19 to a 17p.
Post the rpms at wot???
Then try the prop forum to get the other questions answered.(What is a better prop for better takeoff and lower top end rpm's)

 

[pnwboat]

Re: lower unit swap question 120 hp to a 90 hp lower unit

The gear ratio on the 1997 120HP lower unit is 2.07:1. The gear ratio on the 1995 90HP lower unit is 2.3:1. The engine RPM's with the 90HP lower unit will be higher to go the same speed as with the 120HP lower unit. If you're running a Laser SS 22P prop, may have to go to a 23P if you want to get more top end, however, usually you lose some low end hole shot when you increase the pitch. Like Jerry says, need to know what RPM's you are turning at WOT.

 

[rocketman87]

Re: lower unit swap question 120 hp to a 90 hp lower unit

somewhere around 6000 I only went there for a sec because I didnt want to run it above the 5250 reccommended top rpm

Do I need a bigger prop or an increase or decrease in pitch ?