Re: compatible force lower units?
As long as the 115 is not a Mercury midleg and lower unit. ANY 3 or 4 cylinder from ANY year from 1980 on with the exception of 70 horsepower will fit any 3 and 4 cylinder Force at least to 1990 except 70 HP.
Only thing to watch for is drive shaft length--some were for a 20 inch transom and some were 25.
Later models (from 1990) had a 2 piece drive shaft and the water pump was different but earlier model lowers will still just bolt right up.
Quick way to tell is Force lower units have a push-pull shift rod. Mercury lower units have a splined rotating shift rod. Force uses six bolts up through the lower plus one hidden inside the exhaust snout, Mercury uses 4 studs with nuts plus another in the tail capped with a plastic plug.
Oh, Yeah! The 85HP is most likely (but not necessarily) a 2 to 1 gear ratio and the 115 is most likely a 1.78 to 1 ratio, so check the lower unit. Spin the drive shaft or flywheel by hand while in gear. If the flywheel goes around twice and the prop spins a bit more than once, it is 1.78.
If the lower unit is 2 to 1, then you must go up 2 pitches in prop to have the same wide open throttle RPM.
