The connecting rod bearing (conrod bearing) is a critical component of an engine conrod. Fitted at the mating interfaces between the conrod and either the crankshaft or the gudgeon pin, it falls under the plain journal bearing category. Based on its mounting location, conrod bearings are categorised into big-end bearings (conrod bearings) and small-end bearings (gudgeon pin bushes). The big-end bearing, commonly referred to as the conrod bearing shell, is fitted inside the conrod big-end journal bore and mates with the crankpin; the small-end bearing sits at the small-end eye of the conrod, mating with the gudgeon pin or crosshead pin.
Both bearing types serve distinct primary functions. The big-end bearing primarily reduces crankpin journal wear, minimises conrod friction and vibration, and provides mechanical support, journal alignment, and heat dissipation assistance. The small-end bearing is primarily designed to prevent gudgeon pin wear. Operating in tandem, they ensure smooth running of the crank-conrod assembly under high temperatures, extreme cylinder pressures, and alternating inertial loads.
The conrod big-end bearing is a split-type plain journal bearing. In conventional trunk-piston conrods, the upper half of the big-end bearing housing is forged integrally with the conrod shank, whilst the lower half (bearing cap) is manufactured separately and secured to the rod via conrod bolts, with pre-formed bearing shells seated within both halves. In crosshead-type engines, the big-end bearing is usually fabricated as an independent assembly, with anti-friction white metal lined directly onto its inner bore. The big-end bearing shell features precision oil holes and lubrication grooves to facilitate pressurised oil flow.
The construction of the conrod small-end bearing varies according to the engine design. Trunk-piston conrods typically utilise a cylindrical bushing pressed into the small-end eye, fabricated from phosphor bronze, leaded bronze, or steel-backed bushes lined with anti-friction material. Certain high-speed, compact diesel engines utilise needle roller bearings instead. The bush bore incorporates oil supply drillings and, in some designs, machined distribution grooves for optimal oil entry; to eliminate dry-running on cold starts, dedicated oil reservoirs are often incorporated. Conversely, crosshead-engine small-end bearings are split-type plain bearings with anti-friction lining cast directly into the bore, complete with dedicated surface oil grooves and feed drillings.
In terms of metallurgy, conrod bearings comprise replaceable bearing shells. Each shell features either a two-layer construction (steel backing and an anti-friction lining) or a three-layer tri-metal construction (steel backing, intermediate lining, and a precision overlay). The backing is pressed from 1 to 3 mm low-carbon steel plate to ensure adequate hoop strength and seating stiffness. Anti-friction linings fall into three primary categories: white metal (Babbitt alloy), offering excellent anti-scuffing properties but lower fatigue resistance; copper-lead alloys, delivering superior load-carrying capacity, fatigue endurance, and thermal resistance at the expense of conformability and corrosion resistance, widely favoured in heavy-duty commercial diesel engines; and aluminium-based alloys (such as aluminium-tin, aluminium-silicon, and high-tin alloys). High-tin aluminium alloys containing over 20% tin are widely used across petrol and diesel passenger cars due to their well-balanced load capacity, fatigue strength, and embeddability. To enhance running-in performance, an electroplated tin or lead-tin flash overlay is applied to the bearing surface.
Conrod bearings rely on forced-feed lubrication from the engine oil pump to establish a hydrodynamic fluid film across the journal surfaces. Engine oil is routed through the internal crankshaft oil galleries to the crankpin journals, discharging through the big-end bearing oil feed holes into the running clearance. This generates a high-pressure hydrodynamic wedge that lifts and cushions the journal, ensuring full fluid-film lubrication. In contrast, small-end bush lubrication operates under more demanding conditions due to high peak firing pressures combined with oscillating, non-rotational motion. Trunk-piston conrods guide lubricant into the small end via internal drillings, squirt holes, or splash grooves; crosshead conrods employ dedicated surface distribution grooves, occasionally supported by secondary high-pressure booster circuits to guarantee oil film integrity.
Common conrod bearing failures in service include adhesive wear, journal scoring, lining fatigue spalling, and bearing wiping. Among these, bearing spin or melting (severe thermal seizure and wiping of the bearing lining) represents a catastrophic failure mode. This condition is primarily triggered by oil starvation, such as insufficient engine oil pressure, sludged oil galleries, or degraded lubricant; similarly, excessively tight oil clearance restricts adequate lubricant throughput, leading to rapid frictional overheating and bearing meltdown. Sustained heavy loading also elevates sump temperatures and causes thermal breakdown of oil viscosity, drastically increasing the risk of bearing failure.
Bearing knock (conrod knock) is the definitive acoustic symptom of bearing clearance failure. It is typically characterised by a sharp, metallic "clack-clack" tapping sound that becomes pronounced under sudden throttle tip-in; the sound is audible at idle, peaks in clarity across the mid-range RPM band, and tends to blur or diminish at high revs; it shows negligible variation with operating engine temperature; performing a cylinder power balance or cylinder cut-out test on the affected cylinder results in an immediate reduction or elimination of the knocking noise. Key root causes include excessive journal-to-bearing oil clearance; thermal wiping caused by lubrication failure; stretched or loose conrod cap bolts; flaking or melted bearing alloy; and out-of-round crankpin ovality. If neglected, bearing knock will rapidly escalate into severe internal engine damage or total crankshaft seizure—where the bearing shells weld to the crankpin, locking the engine completely.
Conrod bearing servicing centres strictly on oil clearance verification and precision shell selection.
Oil clearance inspection is routinely carried out using the Plastigauge method. The service procedure is as follows: thoroughly degrease the crankpin journal and bearing shells; lay a strip of Plastigauge across the journal crown, ensuring it clears the oil feed hole; torque the conrod cap bolts down to factory specifications without rotating the conrod or crankshaft; remove the bearing cap and measure the width of the compressed gauge strip against the calibrated scale to determine the exact radial oil clearance. If the measured clearance exceeds OEM service limits, new bearing shells must be fitted; if clearances remain excessive with new standard shells, the crankpins must be ground to the next undersize or the crankshaft replaced entirely.
For selective fitting, bearing shell grades must match the crankpin journal undersize and the conrod big-end housing bore dimensions. When correctly seated in the housing, each bearing half must exhibit a specific bearing crush height, protruding approximately 0.03 to 0.06 mm beyond the mating parting faces. If crush height is insufficient, the correct grade must be sourced; under no circumstances should the conrod mating faces be ground or filed. Modern automotive engines utilise precision select-fit color-coded bearing shells, completely eliminating the need for bearing scraping or shim packing.
During final assembly, strict precautions are essential: conrod bodies and caps are fractured or machined as matched pairs—match marks must align on the identical side, and caps must never be interchanged or reversed; shells with oil drillings must accurately align with the rod's oil feed galleries; a generous coat of fresh engine oil or assembly lube must be applied to the bearing face; conrod bolts must be torqued in the prescribed cross-pattern sequence to specified torque-to-yield or angle specifications in progressive stages. Furthermore, many OEMs mandate the renewal of conrod bolts and bearings whenever disassembled. Mainstream factory workshop manuals stress that clearance checks and bearing selection must adhere strictly to OEM technical specifications to ensure long-term engine reliability.
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