Connecting Rods For Detroit Diesel 71 Series Inline Engines: Transmitting Power from Pistons to Crankshaft

October 25, 2024

Connecting Rods For Detroit Diesel 71 Series Inline Engines (271, 371, 471, 671): Transmitting Power from Pistons to Crankshaft

Connecting Rod For Detroit Diesel 71 Series Inline Engines With 1 Piece Pistons

The connecting rods play a crucial role in the operation of the Detroit Diesel Series 71 engine. They connect the pistons to the crankshaft, transmitting the force generated by the combustion process to the crankshaft, which then converts this force into rotational power. Connecting rods must withstand significant stress, heat, and pressure, so ensuring their integrity is critical to the engine’s performance and longevity.

Connecting Rod Parts Catalog For 71 Series Inline Engines

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Key Functions of Connecting Rods:

Transmitting Combustion Force:
- Connecting rods transmit the linear force of the pistons, created by the combustion process, to the crankshaft. As the pistons move up and down, the connecting rods rotate the crankshaft, enabling the engine to generate power.
Supporting Piston Movement:
- The connecting rods help guide the pistons as they move within the cylinder liner. Proper alignment of the connecting rods ensures that the pistons move smoothly and efficiently, minimizing wear on the pistons and cylinder walls.

Components of Connecting Rods:

Small End:
- The small end of the connecting rod attaches to the piston pin (also known as the wrist pin). This joint must allow for smooth movement as the piston oscillates during the engine’s cycles, while also providing a secure connection to transmit force.
Big End:
- The big end of the connecting rod attaches to the crankshaft journal via the connecting rod bearing. This end is exposed to significant rotational force and friction as it transfers power to the crankshaft.
Connecting Rod Bearings:
- Bearings are fitted into the big end of the connecting rod. These bearings provide a smooth surface for the crankshaft journal to rotate within, minimizing friction and wear. They are critical for maintaining the correct clearance between the crankshaft and the connecting rod.
Cap and Bolts:
- The big end of the connecting rod is split into two parts: the rod itself and a cap. The cap is bolted to the connecting rod, enclosing the crankshaft journal and holding the bearing in place. The connecting rod bolts must be torqued to the correct specification to ensure a secure connection without overstressing the rod or bolts.

Critical Tolerances and Measurements for Connecting Rods:

Rod Bearing Clearance:
- The clearance between the connecting rod bearing and the crankshaft journal is a critical measurement. For the Detroit Diesel Series 71 engine, the connecting rod bearing clearance typically falls within 0.001 to 0.003 inches (0.025 to 0.076 mm).
- Measurement Process: Measure the diameter of the crankshaft’s rod journals using a micrometer, then use a plastic gauge to measure the clearance between the journal and the connecting rod bearing. Excessive clearance can lead to oil starvation and increased wear, while insufficient clearance can cause excessive friction and overheating.
Small End Bushing Clearance:
- The small end of the connecting rod contains a bushing that allows the piston pin to rotate smoothly. The clearance between the piston pin and the small end bushing should generally be between 0.001 to 0.002 inches (0.025 to 0.05 mm).
- Measurement Process: Use a micrometer or bore gauge to measure the internal diameter of the small end bushing. Compare this measurement to the diameter of the piston pin. If the clearance exceeds the manufacturer’s limits, the bushing should be replaced.
Rod Length and Alignment:
- The overall length of the connecting rod and its alignment must be checked to ensure that it remains within tolerance. Misalignment can lead to uneven wear on the piston, cylinder liner, and crankshaft.
- Measurement Process: Use a rod alignment gauge or specialized equipment to measure the rod’s length and check for any bending or twisting. Rods that are bent or twisted may need to be replaced to prevent damage to other components.
Connecting Rod Bolt Torque:
- Proper torque on the connecting rod bolts is crucial for securing the cap to the connecting rod and preventing bearing misalignment or loosening during operation. For the Detroit Diesel Series 71 engine, the connecting rod bolts should typically be torqued to around 50 to 55 lb-ft (68 to 75 Nm).
- Torque Procedure: When tightening the connecting rod bolts, ensure that they are tightened evenly and in sequence to avoid putting uneven stress on the rod. Use a calibrated torque wrench to achieve the correct torque specification.

Connecting Rod Inspection and Maintenance:

Visual Inspection for Wear and Damage:
- Inspect the connecting rods for signs of wear, cracks, or bending. Pay particular attention to the small end and big end of the rod, where most of the stress occurs. Cracks can develop due to fatigue, while scoring on the rod bearings may indicate poor lubrication or foreign material in the oil.
- Check the rod bolts and cap for wear or damage. If the bolts are overstressed or show signs of stretching, they should be replaced.
Measuring Bearing Wear:
- Use a plastic gauge to measure the clearance between the connecting rod bearing and the crankshaft journal. If the clearance is too large, it may result in excessive bearing wear or oil starvation. Bearings that show signs of scoring, pitting, or discoloration should be replaced.
Checking for Rod Alignment:
- Any bending or twisting in the connecting rods can result in uneven forces on the piston and crankshaft, leading to accelerated wear. Use a rod alignment tool to check the straightness of the connecting rods. Bent rods should be replaced to ensure smooth engine operation.
Small End Bushing Inspection:
- Inspect the small end bushing for wear or signs of excessive clearance. If the bushing shows signs of wear or the clearance exceeds the manufacturer’s limits, the bushing should be replaced to ensure the piston pin fits snugly and moves freely.
Lubrication:
- Proper lubrication is critical to maintaining the health of the connecting rods. Ensure that the oil passages within the rods are clean and free of debris, allowing oil to flow freely to the rod bearings. Blocked oil passages can lead to bearing failure and increased friction, ultimately causing damage to the crankshaft and connecting rods.

Connecting Rod Removal and Installation:

Removing the Connecting Rods:
- To remove the connecting rods, the cylinder head, oil pan, and piston must first be removed. After gaining access to the crankshaft, remove the connecting rod cap bolts and separate the rod cap from the rod.
- Lifting the Piston and Rod: Carefully push the piston and connecting rod assembly out of the cylinder liner. Take care not to damage the cylinder liner or piston during the removal process.
Installing the Connecting Rods:
- Before installing the connecting rods, ensure that the crankshaft journals and rod bearings are properly lubricated. Install the connecting rod and piston assembly into the cylinder liner, and then attach the rod to the crankshaft journal.
- Torqueing the Bolts: Once the rod cap is reinstalled, torque the connecting rod bolts to the manufacturer’s specification using a torque wrench. Double-check that the rod moves freely within the cylinder liner and that the piston-to-cylinder clearance is within specification.

Common Symptoms of Connecting Rod Problems:

Knocking or Tapping Sounds: A common sign of connecting rod problems is a knocking or tapping noise during engine operation, especially under load. This noise may indicate worn rod bearings or excessive clearance between the bearing and crankshaft journal.
Vibration or Excessive Wear: Bent or misaligned connecting rods can cause vibration and uneven wear on the crankshaft and pistons. Vibration may be felt in the engine and could indicate a need for immediate inspection.
Low Oil Pressure: Excessive bearing clearance or blocked oil passages can lead to a drop in oil pressure, which may cause poor lubrication and increased wear on the connecting rod bearings.
Piston Damage: If the connecting rod is misaligned or bent, it can cause uneven force on the piston, leading to damage such as scuffing, scoring, or even cracking.

Conclusion To The Connecting Rod Section:

The connecting rods in the Detroit Diesel Series 71 engine are essential for transmitting the force from the pistons to the crankshaft, converting the energy from combustion into rotary motion. Regular inspection of the connecting rods, including checking for wear, alignment, and bearing clearance, is essential for preventing major engine failures. By maintaining proper rod bearing clearance, ensuring correct torque on the connecting rod bolts, and replacing worn components, operators can ensure that their engine runs smoothly and efficiently for years to come.