force 120 lower unit swap - 1991 on to a 1990

[cuddydogs]

I picked up a cheap lower unit to swap out mine. I have a 1990 120hp motor, and I found a cheap lower unit of a 1991 120hp. everything will match up (splines, shaft length, etc.) except for a couple things. There is a difference in the numbers stamped on the inside of the units. on my 1990 it is stamped Driveshaft 5 and forward gear 6 but on the 1991 unit it says driveshaft 7 and forward gear 8. what does that mean? is it ok to swap these? Also, there is some black tubing with a plug in it right inside the front cavity of the lower unit housing. The only other thing is a rubber boot type thing mounted in front of the water pump housing-which I dont think makes any difference. Thank you in advance for any info, it is greatly appreciated!

 

[cuddydogs]

Re: force 120 lower unit swap - 1991 on to a 1990

forgot to mention I did do the gear ratio test, and they are the same - 1.78

 

[Frank Acampora]

Re: force 120 lower unit swap - 1991 on to a 1990

Those numbers have to do with mating parts in a given gearcase and shimming. There is a chart which shows various combinations of gears, gearcases and shims. Some numbers also describe what engine the lower was destined for.

If everything matches and you are swapping a complete lower unit assembly, then you have no concerns.

 

[cuddydogs]

Re: force 120 lower unit swap - 1991 on to a 1990

do you have any clue to what the black tubing goes to? maybe a breather or something?