1978 Johnson 4HP outboards Service Manual, Page 71Get this manual
eObserve location of keyway in impeller and key in driveshaftSlide water pump assembly down driveshaft and engage keyImpeller housing cover will contact spacer in water pump cavity before cover flange contacts face of gearcase leaving space of about 0.010" (0.25 mm) See Figure 6-9Slight finger pressure will seat cover on face of gearcaseThis fit is necessary for proper operation of the water pumpAttach pump to gearcase with screws dipped in OMC Gasket Sealing CompoundTorque to specifications
1Water tube grommet 2Screws 3Impeller housing cover 4Impeller plate 5Impeller 6Impeller drive key 7Impeller housing cup 8Spacer 9Washer
Figure 6-7Water Pump Components
dPlace washer in gearcasePlace spacer with two small projections located in forward hole in water pump cavitySee Figure 6-8Place impeller plate in cup and assemble cup to impeller housing coverSee Figure 6-10Oil and insert new water tube grommet in cover1About 0.010" (0.25 mm) press to gearcase face