The Detroit Diesel Series 71 oil pump consists of several essential components, each playing a critical role in maintaining steady oil flow and pressure throughout the engine. While the primary design of the oil pump is consistent across the Series 71, certain models—particularly those configured for marine or inclined applications—feature additional components to handle unique operational challenges. Here’s a breakdown of each component, its function, and variations across different models in the Series 71 engine family.

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1. Drive Gear

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The drive gear is the primary gear that powers the oil pump, engaging directly with the crankshaft to synchronize the pump’s operation with engine speed.

• Function: As the crankshaft rotates, the drive gear turns the driven gear, which creates the necessary pressure to move oil through the system.
• Model-Specific Variations:
  • The length of the drive gear varies slightly depending on the engine model. In three- and four-cylinder engines, the drive gear length is 6-15/16 inches, while in six-cylinder models, it is 6-15/32 inches.
  • The difference in gear length is designed to maintain proper alignment and engagement, ensuring consistent oil flow across different engine sizes.
    

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2. Driven Gear

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The driven gear works in tandem with the drive gear, rotating to create the suction and pressure needed to move oil through the pump and into the engine’s oil passages.

• Function: The driven gear is directly engaged with the drive gear, and together they form the pump’s gear set. The interaction of these gears creates the displacement necessary to generate oil pressure. The gear set’s rotation is designed to achieve consistent pressure regardless of engine speed.
• Clearance Requirements: Proper clearance between the drive and driven gears, as well as between the gears and the housing, is critical for optimal operation. The specified clearance range is 0.005 to 0.012 inches across all Series 71 models. This clearance can be adjusted by adding or removing shims under the pump mounting feet. Improper clearance can result in insufficient oil flow, excessive wear, or binding.
  

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3. Oil Pump Housing

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The oil pump housing contains the internal components of the pump, including the drive and driven gears, and provides the structure through which oil flows. The housing is a precisely machined component, ensuring a tight fit for gears and bushings.

• Function: The housing routes the oil from the sump through the pump and out to the engine’s oil passages. It also provides the mounting base for the relief valve and bushings that support the drive and driven gears.
• Model-Specific Differences:
  • In models with a scavenging feature (found mainly in marine engines or engines used in inclined applications), the housing includes additional passages and ports for the scavenging pump. This secondary pump helps move oil from the front to the rear of the oil pan, compensating for oil movement in inclined or dynamic conditions.
  • The scavenging pump configuration also requires additional screens and piping to ensure consistent oil pickup, even when the engine is tilted.
    

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4. Relief Valve and Spring Assembly

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The relief valve assembly is integral to the oil pump’s ability to regulate pressure. This valve opens when oil pressure exceeds the specified limit, diverting excess oil back to the intake side of the pump to prevent over-pressurization.

• Function: The relief valve maintains a safe and consistent oil pressure, typically set around 105 psi (724 kPa). The valve consists of a plunger and spring that allows it to open when pressure is too high, bypassing excess oil to protect engine components from potential damage.
• Inspection and Maintenance: The relief valve’s spring tension must be checked regularly to ensure it opens at the correct pressure. A weakened spring can allow pressure to drop below safe levels, reducing lubrication efficiency. Replacement of the spring and plunger may be necessary if wear or weakening is detected.
• Model Variations: While the relief valve setting (105 psi) is standard across most Series 71 models, variations in application or specific marine configurations may lead to slight modifications in the valve design to adapt to higher demands in extreme or inclined conditions.
  

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5. Bushings

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Bushings support the drive and driven gears, allowing them to rotate smoothly within the housing without metal-to-metal contact. Properly functioning bushings reduce friction and wear, ensuring long-term durability of the pump’s moving parts.

• Function: Bushings provide a low-friction interface for the rotation of the gears. They are critical for maintaining the alignment and reducing wear on the gears and housing.
• Model-Specific Variations:
  • For engines with dual oil pumps (main pump and scavenging pump), bushings are required in both sections of the pump to support additional gears.
  • Bushings in the scavenging section may experience unique wear patterns due to the angled operation and increased oil movement. Regular inspection of these bushings is especially important in marine engines, where constant inclination may cause additional stress on the scavenging pump’s components.
• Clearance Requirements: Bushings must be installed with precise clearance. If bushings are replaced, they should be reamed to fit within the specified tolerances, typically aligned with the gear clearances of 0.005 to 0.012 inches. Insufficient clearance can cause excessive friction, while too much clearance can lead to gear misalignment and pressure loss.
  

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6. Scavenging Pump Components (Marine and Inclined Applications)

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For marine applications or engines used in inclined installations, the oil pump may be equipped with a scavenging pump. This addition is essential for managing oil distribution when the engine operates at an angle, as it prevents oil from pooling at one end of the pan, which can lead to oil starvation.

• Function: The scavenging pump transfers oil from the front of the oil pan to the rear, ensuring consistent oil pickup by the main pump, regardless of engine inclination.
• Key Components:
  • Scavenging Gear Set: In addition to the primary gear set, the scavenging pump contains its own set of gears that draw oil from the front sump area and move it toward the main oil pickup point.
  • Spacer and Screens: A spacer between the main and scavenging sections, as well as specialized inlet screens, are used to prevent debris from entering the scavenging gear set. These screens require periodic cleaning to prevent clogs.
  • Additional Piping and Ports: Extra piping routes oil between the scavenging pump and the main pump, and additional ports are incorporated into the pump housing to allow seamless oil transfer.
• Inspection and Maintenance: The scavenging pump components should be inspected regularly in marine applications, where constant inclination may accelerate wear on the scavenging gear set and screens. Clogged screens or worn gears can reduce the effectiveness of the scavenging function, leading to oil starvation during inclined operation.

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Summary of Model-Specific Maintenance and Inspection Guidelines

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To ensure optimal performance of the oil pump in all Series 71 engines, model-specific variations should be noted and addressed during routine maintenance:

1. Clearance Checks: Gear-to-housing and gear-to-gear clearances should be checked and adjusted with shims to the specified range of 0.005 to 0.012 inches for all models.
2. Gear Length Compatibility: Ensure that the drive gear length matches the engine model (6-15/16 inches for three- and four-cylinder engines, 6-15/32 inches for six-cylinder models).
3. Relief Valve and Spring Inspections: Regular checks for tension and movement to maintain a consistent pressure setting around 105 psi.
4. Scavenging Pump Inspection (if applicable): In marine models, ensure all scavenging components—such as screens, spacers, and additional gears—are clean and free from wear.

By thoroughly understanding and maintaining these key components according to each model’s requirements, operators can achieve reliable oil circulation, proper pressure regulation, and effective lubrication, helping extend the life and efficiency of the Detroit Diesel Series 71 engines across diverse applications.