OMC Hydraulic Assist Gear Ratio Chart 1978-1981

Lower gear case with hydraulic assist cylinder parts such as: forward gear, pinion, shift rod, shift cable, push rod, shift plunger, detent spring, propeller shaft, and detent ball bearing will not interchange with 1982 to 1986 OMC stern drive parts.
If the 1982 complete outdrive with shift cable is used with the original 1978 thru 1981 throttle the boat put in forward position will make boat go in reverse.
Note on 1978 thru 1981 stern drives the engine should be running before shifting outdrive, or the power assist servo assembly may be damaged.
Boat engine with heavy loads or lower compression due to motor wear ( 300 plus hrs. ) may want to use a lower gear ratio.
Example : 230 hp. engine with gear ratio of 21 / 16 but low compression may want to change to a gear ratio of 21 / 17 if repairing broken gears to aid in getting the boat up on plain faster.
Exploded View OMC drawings

OMC Stern Drive Year HP upper and lower gear case 1978 to 1981.

21 / 18 Upper 13 / 26 Lower 1978 140 hp.
21 / 18 Upper 13 / 26 Lower 1978 185 hp.
21 / 17 Upper 14 / 26 Lower 1978 225 hp.
21 / 16 Upper 14 / 26 Lower 1978 240 hp.
21 / 16 Upper 14 / 26 Lower 1979 120 hp.
21 / 18 Upper 13 / 26 Lower 1979 140 hp.
21 / 18 Upper 13 / 26 Lower 1979 185 hp.
21 / 17 Upper 15 / 23 Lower 1979 200 hp.
21 / 17 Upper 15 / 23 Lower 1979 230 hp.
21 / 16 Upper 15 / 23 Lower 1979 250 hp.
21 / 16 Upper 15 / 23 Lower 1979 260 hp.
21 / 16 Upper 15 / 23 Lower 1980 120 hp.
21 / 18 Upper 13 / 26 Lower 1980 140 hp.
21 / 18 Upper 13 / 26 Lower 1980 185 hp.
21 / 17 Upper 15 / 23 Lower 1980 200 hp.
21 / 17 Upper 15 / 23 Lower 1980 230 hp.
21 / 17 Upper 15 / 23 Lower 1980 260 hp.
21 / 16 Upper 15 / 23 Lower 1981 120 hp.
21 / 18 Upper 13 / 26 Lower 1981 140 hp.
21 / 18 Upper 13 / 26 Lower 1981 185 hp.
21 / 17 Upper 15 / 23 Lower 1981 200 hp.
21 / 17 Upper 15 / 23 Lower 1981 230 hp.
21 / 17 Upper 15 / 23 Lower 1981 260 hp.
21 / 16 Upper 15 / 23 Lower