Crankshaft bearings, commonly known as main bearings (or "main shells"), are plain friction bearings fitted between the engine block’s main bearing saddles and the crankshaft main journals. Ranking among the most critical and heavily loaded plain bearings in an internal combustion engine, they shoulder the vital task of supporting the entire crankshaft assembly. They ensure the crankshaft maintains precise radial alignment during high-speed rotation whilst transferring combustion forces and rotational inertia directly to the cylinder block. Operating primarily under hydrodynamic lubrication, the spinning crankshaft journals draw engine oil into the wedge-shaped clearance between the journal and bearing shell during normal running. This creates an uninterrupted, pressurised hydrodynamic oil film that effectively "floats" the journal, ensuring non-contact metal-to-metal operation, drastically cutting friction and wear, and dissipating frictional heat into the engine block.
Crankshaft bearings feature a split plain design, where each main journal journal is cradled by a matched pair of semicircular halves: an upper bearing shell and a lower bearing shell. The upper shell sits in the cylinder block main bearing bore, whilst the lower shell resides in the main bearing cap. To prevent the shells from spinning circumferentially or shifting axially inside their housing, each shell features a stamped locating lug (commonly known as a "bearing tang" or notch) on its back that locks into a corresponding locating slot in the housing bore.
Structurally, crankshaft bearings generally employ a bi-metal or tri-metal composite layout consisting of a steel backing + anti-friction alloy lining. The steel backing, pressed from low-carbon steel plate (typically 1.5 to 3 mm thick), provides structural rigidity and deformation resistance; an anti-friction alloy is cast or sintered onto its inner contact surface. Based on alloy composition, these bearings fall into three main categories: Babbitt metal (white metal) offers exceptional conformability, embeddability, and seizure resistance, but has lower fatigue strength, making it suitable for low-to-medium-load engines; copper-based alloys (copper-lead or leaded bronze) deliver high load-bearing capacity, fatigue resistance, and superior thermal conductivity, making them a mainstay in heavy-duty diesels and high-performance turbocharged petrol engines; aluminium-based alloys (such as high-tin aluminium) strike an ideal balance between load capacity, fatigue strength, and corrosion resistance, finding widespread use in modern passenger car petrol engines. To further enhance initial running-in, most shells feature a flash-plated tin or lead-tin overlay measuring just 0.01 to 0.02 mm thick. Notably, the upper shell typically incorporates an annular or semi-annular oil groove and feed holes to align with the engine block’s main oil gallery and channel pressurised oil into the crankshaft’s internal oil passages; lower shells are generally smooth without oil grooves to maximise the effective load-bearing surface.
Typical crankshaft bearing failures include wear, fatigue spalling/flaking, bearing burn-out (seizure/spun bearing), and knocking noises.
Normal wear is inevitable over extended mileages. As bearing clearances open up, oil pressure drops, compromising the bearing’s ability to properly support the crankshaft. Fatigue spalling, often triggered by prolonged engine lugging, over-revving, or improper oil clearances, manifests as pitting or flaking of the anti-friction overlay; in extreme cases, the steel backing contacts the journal directly, accelerating catastrophic wear.
Bearing burn-out or seizure represents the worst-case scenario. When starved of oil due to low oil pressure, clogged galleries, degraded lubricant, or overly tight assembly clearances, the protective oil film collapses. This leads to severe boundary or dry metal-to-metal friction, generating extreme heat that rapidly melts the anti-friction layer. The molten metal welds itself to the crankshaft journal, locking up the engine entirely (commonly termed "engine seizure" or a "spun bearing"). Once this occurs, the crankshaft must usually be reground or replaced alongside a complete set of new bearings.
Main bearing knock is the definitive acoustic symptom of excessive main bearing clearance. It presents as a heavy, dull, metallic "clanking" or thumping sound originating from the bottom end of the block near the crankcase at idle and low-to-mid revs, deeper and heavier in pitch than a connecting rod big-end knock. The knock becomes noticeably louder under abrupt acceleration or heavy engine load, whereas engine operating temperature has minimal effect on the noise. Unlike a big-end knock, cutting ignition to a single cylinder rarely quiets a main bearing knock; however, cutting ignition to two adjacent cylinders (which share a common main bearing) will significantly soften or eliminate the noise.
Servicing crankshaft bearings centres on precise clearance measurement and matched-grade selection.
Clearance measurement is universally carried out using Plastigauge: thoroughly clean and dry the crankshaft journals and bearing faces; lay a strip of Plastigauge along the journal axis (avoiding oil holes); fit the main bearing cap and torque the bolts down to factory specifications without rotating the crankshaft; remove the cap and measure the width of the compressed strip against the calibrated scale to determine the exact radial oil clearance. Main bearing radial clearances on modern passenger cars typically range between 0.02 and 0.06 mm (refer to the vehicle’s specific workshop manual for exact tolerances); bearings must be replaced once service wear limits are exceeded.
When selecting replacements, if the crankshaft journals have been machined or reground due to wear, appropriately undersized bearings must be fitted. Standard undersize steps include 0.25 mm, 0.50 mm, 0.75 mm, and 1.00 mm (often marked as -0.010", -0.020", etc.). Technicians must ensure that the size code stamped on the back of the shell matches the journal's reground dimensions. In modern engine rebuilding, manual bearing scraping or adding shims to adjust clearance is strictly prohibited; proper clearances must only be achieved by fitting the correct grade of bearing shells, as scraping ruins the bearing's geometric roundness and compromises hydrodynamic oil film formation.
During installation, several best practices apply: upper and lower shells must never be mixed up or inverted, and locating tangs must seat positively in their housing slots; the upper shell’s oil holes must align perfectly with the cylinder block main oil gallery; apply clean engine assembly lube or fresh engine oil to all bearing surfaces; reinstall main bearing caps in their original locations and orientations without interchanging them; torque down main bearing bolts in progressive stages working outward from the centre cap towards both ends. Furthermore, crankshaft end float (axial clearance) is governed by thrust bearings (thrust washers) on the designated main journal, typically specified between 0.07 and 0.25 mm, which must be measured and verified concurrently with radial clearances. As of July 2026, leading automotive workshop standards require uncompromising compliance with OEM factory specifications throughout inspection, measurement, and assembly to guarantee optimal reliability and engine longevity following a bottom-end overhaul.
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