From 1972 to 1989, the gear driven oil pump has went through several changes with the main body and design staying the same as in the ball check valve assembly and seals: ¹⁾

• 1972 - Feed pump speed was 1/4 of the engine speed and the return side operated at a 2:1 ratio of the feed side. The breathing system was used to return oil from the crankcase into the cam/ gear cover where the oil drained directly onto the return gears of the oil pump.²⁾
• 1975-1980 - The engine had a separate oil sump bolted to the crankcase. Return gears scavenged the crankcase oil through an external oil line.³⁾
• 1989-2003 - Using the same design of the '75-'80 pump, but with a more efficient drive gear design and improved oil routing, the oil pump speeds were doubled to 1/2 of engine speed for the feed pump and a 4:1 ratio on the return side. The oil pump drive gear was re-activated as a breather only. ⁴⁾

• The separate oil feed and scavenger (oil return) pumps are gear-type pumps and are incorporated in the same pump housing with a check valve on the oil feed side. The feed section is gravity fed from the oil tank and forced fed from the pump to the engine and back to the scavenger side of the pump which returns oil back to the oil tank. ⁵⁾
• In a gear type oil pump, the oil is transferred from the inlet to the outlet side of the pump when it is trapped between the rotating gear teeth and the gear housing.
• The oil pump seldom needs servicing. Therefore, before dis-assembling it for repairs because of no oil pressure, be absolutely certain that all possible related malfunctions have been eliminated. ⁶⁾
  • If oil in oil tank is diluted, pressure will be affected.
  • In freezing weather, the oil feed line may clog up with ice or sludge, preventing circulation.
  • Inspect oil pump check valve as it prevents the gravity flow of oil into the crankcase when the engine is not running and provides the correct oil pressure for the operation of the oil signal light switch. If the check valve is not seating properly, oil will bypass the valve and drain oil from the oil tank into the crankcase and upon starting the engine, a considerable amount of oil will be blown out the crankcase breather pipe.
  • Check for a grounded oil pressure switch wire or a faulty switch if oil indicator light fails to go out with the engine running.
  • If no oil pressure or return oil is not indicated at the oil tank, when engine is running, or an excess amount of oil is blown from the breather pipe, dis-assemble the oil pump for repair. Damage can occur by way of a foreign object lodged in the oil pump gearing. Check your FSM for dis-assembly and repair. ⁷⁾

• The oil pump is fitted with gerotors instead of gears consisting of two gerotor pumps in the same pump housing. The pump is gravity fed from the oil tank. The feed pump forces oil to the engine while the scavenger pump collects and returns oil back to the oil tank.
• A gerotor type pump has 2 elements, an inner (which always has one less tooth than the outer) and an outer gerotor. Both elements are mounted on fixed centers but are eccentric to each other. ⁸⁾
• In a gerotor type pump, oil is transferred form the inlet to the outlet as it is trapped between the rotating inner and outer gerotors. ⁹⁾
• During the first 180 degrees of rotation, the cavity between the inner and outer elements gradually increases in size until it reaches it's maximum size which is equal to the full volume of the missing tooth. The gradually enlarging cavity creates a vacuum into which the oil flows out from the inlet. During the next 180 degrees of rotation, the size of the cavity decreases forcing oil into the outlet.¹⁰⁾
• Oil is forced into a valve with a one way spring loaded cup set at 1 1/2 psi. From that valve, oil exits the pump and enters the crankcase. Oil pressure is indicated by the oil pressure switch. ¹¹⁾