Re: Lower Unit Compatability - 1989 Force 85
The same lower unit with VERY MINOR changes, was used on all Chrysler 3 and 4 cylinder engines from late '79 --say '80 right through Force '89. Although Merc started making changes in 1990 (two piece drive shaft and slightly different water pump) That same lower unit will fit the 1990 3 and 4 cylinder Force with possibly the exception of the 3 cylinder 70 HP.
Engines: ALL 75 through 140 HP.
The only major difference was that some 85HP and 90 HP engines had 2-1 gearing instead of 1.78-1. Not sure about 75s. So if your engine has 2-1 gearing and you use a 1.78-1 lower unit, you will notice a loss in performance to the tune of 3-5 MPH.
So in summation, As long as you check the gear ratio, You can use the late Chrysler 1 piece lower unit, the early white Force 1 piece lower unit, or the Dark Force (NO, not Star Wars) up to '90 and possibly as late as '92 lower unit. Not sure the exact year Merc went to the Merc midleg and lower unit--but it was later than '90, of that I am certain. Single and dual exhaust are interchangeable; that is, the bolt pattern is the same. However, there may be an extra sleeve inside the midleg and of course, you must use a thru hub propeller.
I prefer the single though.
NOTE: It is quite acceptable and even preferable to use a 2-1 gear ratio to replace a 1.78 to 1 ratio. Just get a prop with 2 inches more pitch. For example: if you have a prop with a 17 pitch, use a prop with 19 pitch. You will notice a gain in speed.