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OMC Hydraulic Assist Gear Ratio Chart 1978-1981

OMC stern drive gear ratios years 1978 to 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