Crankshafts For Detroit Diesel 71 Series Inline Engines (271, 371, 471, 671): Converting Linear Motion into Rotational Power

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The crankshaft is one of the most critical components of the Detroit Diesel Series 71 engine. It converts the up-and-down (linear) motion of the pistons into rotational motion that powers the engine’s drivetrain. The crankshaft operates under intense pressure and stress, and its ability to function properly is essential for overall engine performance. Proper maintenance and inspection are key to ensuring the crankshaft remains in good condition.

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Key Functions of the Crankshaft:

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1. Converting Linear to Rotary Motion:
   • As the pistons move up and down inside the cylinders, the crankshaft converts this reciprocating motion into rotary motion that powers the machinery or vehicle.
2. Bearing Support:
   • The crankshaft is supported by several main bearings that provide a smooth surface for rotation. These bearings also help reduce friction and wear as the crankshaft spins at high speeds.
3. Connecting Rod Journals:
   • The crankshaft contains connecting rod journals, which are the surfaces that the connecting rods attach to. These journals must be precisely machined and lubricated to reduce wear and friction as the crankshaft rotates.
4. Counterweights:
   • Counterweights are built into the crankshaft to balance the engine and reduce vibration. These counterweights ensure smooth operation, especially at higher RPMs, and help prevent damage to the crankshaft and other internal components.
     

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Crankshaft Components:

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1. Main Journals:
   • These journals are the portions of the crankshaft that rest in the engine block’s main bearings. They must be smooth and properly sized to fit within the bearings, allowing the crankshaft to rotate with minimal friction.
2. Connecting Rod Journals (Crank Pins):
   • The connecting rod journals (also called crank pins) are offset from the center of the crankshaft and connect to the connecting rods. These journals convert the reciprocating motion of the pistons into rotary motion by transmitting the force from the connecting rods.
3. Thrust Bearings:
   • Thrust bearings prevent the crankshaft from moving axially (side-to-side). Proper thrust bearing clearance is essential to maintaining alignment between the crankshaft and other engine components, preventing excessive wear.
4. Oil Passages:
   • The crankshaft is drilled with oil passages that allow pressurized oil to flow from the engine’s oil pump into the main and rod bearings. This ensures that all moving parts are properly lubricated to reduce friction and wear.
     

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Critical Tolerances and Measurements for the Crankshaft:

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1. Main Bearing Journal Clearance:
   • The clearance between the crankshaft’s main bearing journals and the main bearings is a critical measurement for ensuring proper lubrication and preventing excessive wear. For the Detroit Diesel Series 71 engine, this clearance should typically be between 0.0015 to 0.0035 inches (0.038 to 0.089 mm).
   • Measurement Process: Use a micrometer to measure the diameter of the main bearing journals. Then, use a plastic gauge or micrometer to measure the clearance between the journals and the bearings. If the clearance exceeds the specified range, it may be necessary to replace the bearings or machine the crankshaft.
2. Connecting Rod Journal Clearance:
   • The clearance between the crankshaft’s connecting rod journals (crank pins) and the connecting rod bearings should generally be between 0.001 to 0.003 inches (0.025 to 0.076 mm).
   • Measurement Process: Similar to the main bearings, use a micrometer to measure the diameter of the connecting rod journals, and then use a plastic gauge to check the clearance. If the clearance is too large, it could result in poor lubrication, leading to increased wear and potential engine damage.
3. Crankshaft End Play (Axial Clearance):
   • The end play of the crankshaft (the axial movement) should be checked to ensure that the crankshaft isn’t moving excessively from front to back. The acceptable end play for the Detroit Diesel Series 71 engine is typically between 0.004 to 0.012 inches (0.10 to 0.30 mm).
   • Measurement Process: Use a dial indicator to measure the crankshaft’s axial movement. Excessive end play can cause issues with timing, vibration, and improper engagement of the engine’s components.
4. Crankshaft Runout:
   • Runout refers to how much the crankshaft deviates from a straight line while rotating. Excessive runout can cause vibration and uneven wear. For the Detroit Diesel Series 71, crankshaft runout should not exceed 0.002 inches (0.05 mm).
   • Measurement Process: Mount the crankshaft in V-blocks and use a dial indicator to measure the runout at several points along the length of the crankshaft. If the runout exceeds the allowable limit, the crankshaft may need to be re-machined or replaced.
     

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Crankshaft Inspection and Maintenance:

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1. Visual Inspection for Cracks and Wear:
   • During engine overhauls or routine maintenance, inspect the crankshaft for cracks, scoring, or discoloration. Cracks can form due to excessive stress or heat, and scoring may indicate poor lubrication or foreign material in the oil.
   • Crankshaft Cracks: Pay special attention to high-stress areas such as the fillets between the crankshaft journals and counterweights. Any visible cracks may require replacement or re-machining of the crankshaft.
2. Measuring Journal Wear:
   • Use a micrometer to measure the diameter of the crankshaft’s main and rod journals. Compare the measured values to the manufacturer’s specifications. If the journals are worn beyond tolerance, they may need to be machined to an undersized specification, with corresponding undersized bearings installed.
   • Out-of-Round Journals: Journals can wear unevenly, resulting in an out-of-round condition. This can cause vibration and poor bearing contact. Measure at multiple points around the journal’s circumference to check for out-of-roundness.
3. Polishing or Machining the Crankshaft:
   • If the crankshaft’s journals are lightly worn but still within specification, they can be polished to remove minor imperfections. This process helps restore a smooth surface for the bearings.
   • For journals that are worn beyond limits, machining the crankshaft to an undersize can restore proper clearance. After machining, undersized bearings must be installed to compensate for the material removed.
4. Inspecting the Crankshaft Oil Passages:
   • The crankshaft contains internal oil passages that deliver lubrication to the journals and bearings. During maintenance, it’s essential to ensure that these passages are clean and free of debris.
   • Cleaning the Passages: Use compressed air and a solvent to flush out any debris or sludge that may have accumulated in the oil passages. Blocked passages can lead to bearing failure and excessive wear.
5. Replacing Bearings:
   • Bearings are wear items that should be replaced if they show signs of scoring, pitting, or uneven wear. When replacing bearings, it’s crucial to use the correct size (standard or undersized) based on the condition of the crankshaft journals.
6. Checking for Proper Thrust Bearing Clearance:
   • The thrust bearing helps control the crankshaft’s axial movement. Check the thrust bearing clearanceduring engine assembly to ensure it falls within the specified range (typically 0.004 to 0.012 inches). Excessive clearance can lead to poor alignment and increased wear.
     

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Crankshaft Removal and Installation:

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1. Removing the Crankshaft:
   • Before removing the crankshaft, drain the engine oil and remove the oil pan, connecting rods, and main bearing caps. Ensure all fasteners are removed in the correct sequence to avoid stressing the crankshaft or block.
   • Lifting the Crankshaft: The crankshaft is heavy and may require specialized lifting equipment for safe removal. Be careful to avoid scratching or damaging the journals during the removal process.
2. Installing the Crankshaft:
   • Before installing the crankshaft, ensure that all journals and bearings are properly lubricated. Install the main bearing shells and set the crankshaft in place, ensuring it is aligned correctly.
   • Torque Specifications: Use a torque wrench to tighten the main bearing cap bolts to the manufacturer’s specified torque values. Improperly torqued bolts can lead to bearing misalignment and crankshaft damage.
   • Check for proper end play and ensure that the thrust bearing is seated correctly to prevent excessive axial movement.
     

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Common Symptoms of Crankshaft Problems:

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1. Vibration: Excessive runout, worn journals, or imbalanced counterweights can cause engine vibration, which may be felt throughout the vehicle or machinery.
2. Knocking or Tapping Sounds: If the crankshaft journals are excessively worn or the bearings are failing, the engine may produce knocking or tapping sounds, indicating a need for immediate inspection.
3. Low Oil Pressure: Blocked oil passages or excessive bearing clearance can lead to low oil pressure, which may result in poor lubrication and increased wear on the crankshaft and bearings.
4. Excessive End Play: If the crankshaft moves too much along its axis, it can cause timing issues, improper engagement of the engine’s components, and potential damage to the thrust bearings.

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Conclusion About The Crankshaft

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The crankshaft is the heart of the Detroit Diesel Series 71 engine’s power delivery system. Regular inspection and maintenance of critical tolerances like journal clearances, runout, and end play are essential for maintaining engine performance and preventing catastrophic failures. Ensuring that the crankshaft and its supporting components (bearings, oil passages) are in good condition will help the engine operate smoothly and reliably for many years.

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