Re: which lower unit will work for '91 force 120 hp?
Because until Mercury bought the Force line sometime around 1990, the bolt pattern and water pumps for the lower units from 1979 on was exactly the same. It stayed the same until Merc changed to a Mercury lower unit sometime around 1994.
Since you have a 1991 engine, the original lower unit had a slightly larger Merc designed water pump so earlier units will fit right in. NOW, if you wanted to put a 1990 or later lower in a pre 1989 engine leg, then a small bit of grinding might be necessary to clear the water pump casing.
After 1989 the lower units had a two piece drive shaft but for several years, the splines stayed the same as the one piece drive shaft.
The only thing you need to be careful about is the engine leg length. You can not fit a 25 inch leg lower onto a 20 inch leg and vice-versa. If the later lower has a two piece shaft and does not have the shaft and collar included, do not buy it. Two piece shafts are very expensive and not available. Used ones are very difficult to find.
And I know about performance because I have actually done it. The 2 to 1 ratio gears (from 85-90 HP engines) with a 2 inch higher pitch prop will give 3-5 MPH more top speed than the 1.72 ratio lowers (from 125 engines), even though the theoretical top speed is the same.
BTW: you can tell the difference between 20 and 25 inch midlegs by looking at the angle behind the lower rear shroud,. If the midleg angles back directly below the shroud, it is 20 inch. If it drops straight about 5 inches before angling rearward, it is 25 inch. Also look at the bottom swivel mount. If it is only about 4 inches high with 4 bolts holding it, it is 20 inch If it is about 10 inches with 8 bolts, then it is 25 inch.
