Analysis of Wear in Diesel Engine Main Bearings and Connecting Rod Bearings: A Systematic Breakdown from Failure Symptoms to Overhaul Diagnosis

During the overhaul of a diesel engine, main bearings and connecting rod bearings are among the most critical components to inspect. Although they are merely internal sliding bearings, they endure complex stresses—including combustion pressure, inertial loads, and high-speed rotation—over long periods. The wear patterns on their surfaces provide an accurate reflection of the actual operating conditions of the lubrication system, crankshaft assembly, and overall assembly precision.

To experienced mechanics, a removed bearing shell is not just a wear part slated for replacement; it is a “failure record” that documents the engine’s operational history. By analyzing wear locations, color changes, crack patterns, and surface damage characteristics, one can often quickly diagnose underlying issues such as lubrication anomalies, geometric deformation, oil contamination, or fatigue damage. Consequently, accurately interpreting bearing wear characteristics during an overhaul is often more practically valuable than simply installing a new set of bearings.

Bearing Seizure and “Burned” Bearings Caused by Lubrication Failure
During normal diesel engine operation, there is no direct metal-to-metal contact between the crankshaft journal and the bearing shell; instead, a hydrodynamic lubricating oil film—ranging from a few to several dozen micrometers in thickness—completely separates the two. This oil film not only bears the full load but also dissipates heat generated by friction, serving as the foundation for the bearing’s long-term, stable operation.

If the oil supply is insufficient or the oil film fails to form stably, the crankshaft journal will directly strike and grind against the bearing shell, causing the temperature at the contact interface to spike instantly. Under these conditions, the Babbitt metal or tin-lead anti-friction layer on the bearing surface undergoes plastic deformation first, followed by localized melting, scuffing, or even delamination of the anti-friction layer that exposes the steel backing—a phenomenon commonly known in overhauls as a “burned bearing.”

During inspection, if the bearing surface exhibits extensive mirror-like polishing, or if the anti-friction layer appears displaced or melted—accompanied by blue or black high-temperature annealing discoloration on the crankshaft journal—it can generally be concluded that the engine has suffered from severe oil starvation or extreme lubrication failure. Common causes for the breakdown of the oil film include:

Insufficient oil supply pressure due to excessive oil pump wear, an oil pickup screen clogged with sludge, or a stuck pressure-limiting valve;

Failure to change the oil for an extended period, resulting in severe degradation and a complete loss of lubricating and shear-resistance properties;

Fuel injector leakage or poor atomization, causing fuel to seep into the crankcase, dilute the oil, and compromise its viscosity;

Aggressive acceleration or high-load operation immediately after a cold start without a warm-up period, subjecting the bearings to heavy loads before the oil film has fully established;

Use of lubricating oil that fails to meet required viscosity grades or quality standards.

If an overhaul involves merely replacing the bearing shells without thoroughly identifying and resolving underlying issues in the lubrication system, the new shells will typically fail again shortly after installation.

Abrasive wear: A hallmark of contaminants entering the lubrication system
In addition to inadequate lubrication, abrasive wear is another common cause of premature bearing shell failure.

Under normal conditions, the soft alloy on the bearing shell surface possesses a degree of embeddability, allowing it to absorb minute particles and protect the crankshaft journal from scratching. However, if a large number of hard particles enter the bearing clearance along with the oil, these impurities act like grains of sand on sandpaper, continuously cutting into the metal surface during high-speed rotation.

Upon disassembly and inspection, continuous, regular circumferential scratches are typically visible on the bearing shell surface; sometimes, tiny metal fragments or sand particles can be seen embedded in the anti-friction layer with the naked eye. If the crankshaft journal also exhibits scoring grooves running in the same direction, the failure can generally be attributed to abrasive wear. These hard particles often originate from the following sources:

Incomplete flushing of oil passages in the cylinder block or crankshaft after a previous overhaul, allowing residual machining swarf to enter the bearings during circulation;

A damaged air filter or poorly sealed intake piping, allowing large amounts of silica dust to be drawn into the cylinders and subsequently scraped into the crankcase;

Metal debris flaking off due to abnormal wear of components such as pistons, timing gears, or camshafts;

A clogged oil filter causing the bypass valve to remain open, allowing unfiltered, dirty oil to flow directly into the main oil gallery.

In such cases, simply replacing the bearing shells does not solve the root problem; it is essential to thoroughly flush all oil passages, replace piping and high-precision filter elements, and trace the specific source of the contamination.

Fatigue Spalling: A Trace of Prolonged High-Load Operation
During engine operation, bearing shells must constantly withstand the intense, fluctuating loads generated by combustion in the cylinders. Over time, microscopic fatigue cracks gradually initiate within the anti-friction layer.

Early-stage cracks are extremely difficult to detect with the naked eye, but under the continuous impact of cyclic loading, they propagate both in depth and width. Eventually, the alloy layer suffers from pitting-like erosion or even large-scale detachment, resulting in typical “fatigue spalling.” Since the upper connecting rod bearing shells and lower main bearing shells endure the highest combustion pressures, this type of damage is usually most pronounced on these components.

The primary causes of premature fatigue spalling include:

Long-term engine operation under overload or over-speed conditions;

Unauthorized adjustment of fuel injection quantity or tampering with boost pressure, leading to excessive peak cylinder pressures;

Slight deformation of the crankshaft or connecting rod, causing excessive localized stress concentration on the bearing shells;

Deformation (loss of roundness) of main bearing bores or connecting rod big-end bores, resulting in uneven load distribution;

Installation of inferior aftermarket bearing shells with poor bonding strength between the anti-friction layer and the steel backing. Therefore, when fatigue spalling is observed during disassembly and inspection, do not simply attribute it to the “end of the component’s service life”; it is essential to simultaneously measure the crankshaft’s roundness, the alignment of the connecting rods, and the geometric dimensions of the bearing housing bores.

Cavitation Damage: A Special Type of Wear Caused by Fluid Cavitation
During the overhaul of certain machines, a specific type of bearing shell damage may be encountered: localized areas densely covered with pinhole-like or honeycomb-like micropits. The surface shows neither scoring marks nor high-temperature discoloration; instead, it resembles a surface uniformly blasted by a high-pressure fine-sand gun.

This is not caused by mechanical friction but is “cavitation damage” triggered by fluid cavitation.

When lubricating oil flows at high speed through bearing oil holes or encounters sudden changes in the loaded clearance, the local oil pressure may momentarily drop below the oil’s saturation vapor pressure, generating a multitude of tiny vapor bubbles. As the oil film moves into a high-pressure zone due to rotation, these bubbles collapse instantly, releasing microscopic, high-speed liquid jets. This continuous, high-frequency hydraulic impact acts upon the metal surface, eventually “gouging out” alloy grains bit by bit.

Scenarios prone to inducing cavitation include:

Aging or failure of the crankshaft torsional vibration damper, inducing severe torsional vibration in the crankshaft;

Excessive bearing clearance, leading to unstable oil pressure fluctuations;

Deterioration of the oil’s anti-foaming properties, preventing foam generated at high temperatures and speeds from dissipating quickly;

Long-term use of low-quality lubricating oil.

Cavitation is often a signal of abnormal crankshaft vibration or instability in the lubrication flow field; when encountering such issues, be sure to simultaneously inspect the operating condition of the torsional vibration damper and the dynamic balance of the crankshaft.

Uneven Wear Indicates Abnormal Geometric Precision
Ideally, wear marks on the bearing shell should be evenly distributed along both the axial and circumferential directions.

If disassembly reveals that the bearing shell is worn down to the steel backing on only one side, or exhibits a distinct diagonal cross-pattern of wear, it indicates that the crankshaft axis and the connecting rod bore axis have lost their proper coaxial alignment. This type of asymmetrical, uneven wear is primarily caused by the following critical defects:

The connecting rod was bent or twisted during a previous failure or a liquid-induced shock event (hydro-lock);

The crankshaft itself exhibits slight bending or deformation (radial runout exceeding specifications);

The cylinder block was subjected to excessive heat or severe overloading, causing the centerline of the main bearing housing bores to shift;

Imprecise positioning or misalignment of the bearing cap (or its locating register) occurred during final assembly after an overhaul.

Uneven wear not only drastically reduces the effective load-bearing area of ​​the oil film but also easily triggers localized overheating and bearing seizure (“spun bearings”). Upon encountering this type of wear, the first step must be to carefully verify mechanical geometric precision using measuring tools; under no circumstances should one simply force-install new bearings.

Chemical corrosion is easily overlooked.
Not all bearing failures stem from mechanical stress or abrasive friction.

If, upon removal, the bearing shell appears dull or dark—having lost its original metallic luster—and shows signs of powdering or flaking when lightly scraped with a fingernail, it has likely suffered from chemical corrosion.

During prolonged engine operation, if the engine oil becomes acidic for any reason, acidic substances will preferentially react chemically with the lead in the copper-lead alloy anti-friction layer. As the lead dissolves and leaches out, the alloy structure collapses, causing the bearing to lose its original load-bearing and anti-friction capabilities.

Key factors accelerating oil acidification include:

Excessively long oil change intervals, leading to the depletion of alkaline additives and the total loss of oxidation resistance;

Long-term use of low-quality, high-sulfur fuel, allowing acidic combustion gases to blow by into the crankcase;

A damaged cylinder head gasket or a leaking oil cooler, allowing coolant to mix with the oil, causing emulsification and acid formation;

Prolonged operation at low temperatures and light loads, preventing acidic condensate and fuel vapors within the crankcase from being properly vented.

Therefore, when encountering such “chemically degraded” bearings during an overhaul, one must not only inspect the bearings themselves but also investigate potential cross-contamination between oil and coolant, as well as the quality of the fuel and oil typically used by the customer.

An overhaul is not merely about replacing parts; it is about restoring the engine’s overall precision.
In actual repair workshops, one frequently encounters rework cases involving “recurring bearing seizure.” Often, the issue is not that the newly installed bearing shells are of poor quality, but rather that the root cause behind the failure of the original set remains unaddressed.

A high-quality diesel engine overhaul must be executed systematically, focusing on key stages: measurement, repair, cleaning and assembly, and verification.

Before starting work, avoid relying on guesswork or “feel.” Instead, use micrometers and bore gauges to measure the actual dimensions of crankshaft journals, main bearing bores, and connecting rod big-end bores. Accurately calculate roundness, taper, and final fit clearances to ensure they fall within the tolerances specified in the technical manual. Additionally, verify connecting rod straightness and twist using inspection equipment, and re-check crankshaft radial runout on V-blocks; components exceeding limits must undergo proper grinding repairs or be decisively replaced.

Cleaning oil passages is often the most overlooked yet critical step. It is not enough to simply wash the engine block’s exterior; one must remove the plugs from the crankshaft’s diagonal oil passages and the block’s main oil galleries. Use long-bristled brushes, high-pressure water jets, and cleaning agents to thoroughly remove accumulated sludge and metal shavings from the internal passages.

During final assembly, best practice dictates using Plastigage to measure and verify bearing clearances. Subsequently, tighten main bearing caps and connecting rod bolts in a diagonal, crisscross pattern, strictly adhering to the manufacturer’s torque specifications and angular tightening requirements.

Finally, during the initial startup and break-in period, closely monitor the oil pressure gauge, oil temperature, and listen for any unusual noises. Once the initial break-in is complete, immediately replace the engine oil and install a high-precision filter to flush out any microscopic metal particles that may have remained in the system after assembly.

Conclusion
Although main and connecting rod bearing shells are merely basic sliding components within a diesel engine, they faithfully record every change in lubrication, load shock, and instance of mechanical wear throughout the machine’s entire service life. For maintenance technicians, analyzing bearing shell wear goes far beyond simply determining whether a part remains fit for service; the true value lies in using these wear patterns to trace back to the root causes—uncovering underlying issues related to the lubrication system, crankshaft assembly, assembly precision, and operating conditions.

A truly high-standard overhaul should not stop at merely replacing damaged components; instead, it must employ systematic inspection and precision repair to restore the engine block and crankshaft to their original geometric tolerances and optimal lubrication conditions. Only by completely eliminating the underlying causes of the bearing damage can the cycle of recurring failures be broken, ensuring the repaired diesel engine is capable of long-term, stable, and reliable operation.

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