Connecting rod bearings — also called big-end bearings — are precision shell inserts that sit between the connecting rod's lower eye and the crankshaft journal, allowing the rod to rotate freely on a film of pressurised engine oil. Manufactured to tolerances measured in thousandths of a millimetre, they are one of the most demanding components in the lubrication system: each bearing must withstand the full combustion load on every firing stroke while relying entirely on the oil wedge to prevent metal-to-metal contact. When that oil film breaks down — through oil starvation, prolonged oil change intervals, or contamination — the tell-tale symptom is a rhythmic knocking sound from the lower engine that increases with rpm and worsens when the accelerator is held at a steady load. Left unaddressed, bearing failure progresses to journal scoring, connecting rod failure, and in many cases catastrophic engine damage. MAHLE and CLEVITE are internationally recognised OE suppliers of precision engine bearings, with their shells produced to the same dimensional tolerances as the factory-fit parts. Bearing selection must match the crankshaft journal diameter and any undersize regrind marking stamped on the crankshaft, as well as the connecting rod big-end bore to ensure correct crush and oil clearance.
Connecting rod bearings are specified by the crankshaft journal diameter (standard or undersize if the crankshaft has been reground), the connecting rod big-end bore, and the bearing width. Many manufacturers stamp a colour code or letter code on the crankshaft web and on the connecting rod to indicate the factory-selected clearance class, and the replacement bearings must match both codes to achieve the correct running clearance. Using your engine code here, alongside any crankshaft regrind specification, will filter to the shells that match your exact assembly.
OEM engine bearings, supplied by companies such as MAHLE or KOLBENSCHMIDT, are produced to dimensional tolerances of less than a micron and use carefully selected alloy compositions — typically a tri-metal overlay on a steel backing — that provide the correct combination of hardness, fatigue resistance, and conformability. Reputable aftermarket suppliers like CLEVITE and GLYCO produce bearings to equivalent standards. Very cheap bearings of unknown origin can have inaccurate dimensional tolerances that result in insufficient or excessive oil clearance, both of which cause premature failure.
Yes. If one big-end bearing is worn or damaged, the others are of similar age and operating history and should be replaced as a complete set for all cylinders. Replacing only the failed shells leaves the remaining bearings at risk of failure within a short period, requiring the engine to be partially disassembled again. Main bearings, which sit between the crankshaft and the cylinder block, are a separate set and are commonly replaced at the same time as big-end bearings during an engine rebuild.
The distinctive symptom is a deep, rhythmic knock from the lower engine — sometimes described as a rod knock — that accelerates exactly in proportion to engine rpm and is most pronounced under a steady part-throttle load. Unlike valve train noise, which is typically a lighter tapping from the top of the engine, rod knock carries a heavier, lower-frequency quality and is audible even over road noise at moderate speed. An oil pressure warning light illuminating alongside the knock, or a sudden worsening of the sound, indicates that bearing clearance has increased to the point where oil film integrity is compromised and engine damage is imminent.
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