What Is a Crankshaft Bearing?

A crankshaft bearing is a precision, oil-lubricated journal bearing that supports and locates the crankshaft in an internal combustion engine, keeping a pressurized film of oil between moving surfaces to prevent metal-to-metal contact. Found in the engine block (main bearings) and at each connecting rod (rod bearings), it allows the crankshaft to spin smoothly under heavy loads while maintaining alignment, reducing friction, and managing heat.

Where It Sits and What It Does

Crankshaft bearings are split, semicircular shells that cradle the crankshaft’s journals. Main bearings sit in the engine block (and sometimes in caps that bolt to the block) to support the crankshaft along its length; rod bearings sit in the big end of each connecting rod, coupling piston motion to the crank. Both rely on steady oil supply and tight tolerances to function correctly.

The key roles of crankshaft bearings in engine operation include the following functions.

• Support and alignment: Hold the crankshaft on-center to maintain proper clearances and timing.
• Friction reduction: Create a hydrodynamic oil film that separates surfaces at operating speed.
• Load handling: Distribute combustion and inertial forces across the bearing surface.
• Heat management: Transfer heat from the crankshaft to the block via the bearing shells and oil.
• Noise and vibration control: Dampen knock and whir through controlled clearances and material properties.

Together, these functions ensure the crankshaft can convert reciprocating piston motion into clean, continuous rotation with minimal wear and energy loss.

Types and Materials

Main vs. Rod Bearings

Main bearings support the crankshaft within the block, while rod bearings ride on each crank pin to connect rods to the crank. Engines also may use thrust bearings or thrust faces on specific main bearings to control axial movement of the crankshaft.

Here’s how common bearing types compare and where they’re used.

• Main bearings: Support radial loads and often incorporate thrust faces to limit crankshaft end play.
• Rod bearings: Handle high cyclic loads from combustion and piston acceleration on each crank throw.
• Thrust bearings/faces: Control axial (fore-aft) movement, crucial for transmission load and clutch operation.

While all are journal bearings, their geometry, oil-feed features, and load profiles differ to match their location and duty.

Common Constructions

Modern crankshaft bearings are typically “plain” bearings built as layered shells with specific surfaces designed for strength, embedability, and wear resistance.

Typical constructions include these material stacks and overlays.

• Bimetal (aluminum-tin on steel back): Common in modern passenger cars; good fatigue strength, lead-free, compatible with low-viscosity oils.
• Trimetal (bronze/copper intermediate layer with soft overlay on steel back): Traditional performance choice, now largely lead-free; excellent conformability and seizure resistance.
• Sputter/polymer overlays: Advanced coatings for high-load or start-stop/turbo applications; improve scuff resistance and dry-start protection.

The specific choice balances durability, oil compatibility, and the engine’s expected load and temperature cycles.

How It Works: The Hydrodynamic Oil Film

At speed, a wedge of pressurized oil forms between the spinning journal and the bearing shell, lifting the crank off the bearing surface. This “hydrodynamic lubrication” regime is what actually carries the load during normal operation.

The lubrication process can be understood in these stages.

1. Startup: Mixed/boundary lubrication until oil pressure builds; overlays protect against scuffing.
2. Operating speed: A stable oil wedge forms, fully separating metal surfaces and minimizing wear.
3. Shutdown/transients: Oil pressure drops; coatings and additive chemistry mitigate brief contact.

Maintaining correct oil viscosity, clean oil flow, and designed clearances is critical to keeping the engine in hydrodynamic mode most of the time.

Clearance, Oil Supply, and Tolerances

Bearings are manufactured in precise sizes to achieve a target clearance—typically on the order of thousandths of an inch (hundredths of a millimeter). Too tight raises seizure risk; too loose drops oil pressure and accelerates wear. Oil reaches the bearings through drilled galleries; crank journals and bearings often have grooves and holes to meter and distribute the flow. High-load engines may feature cross-drilled cranks, chamfered oil holes, and grooved main shells for improved lubrication.

Common Failure Modes and Symptoms

When bearings are starved of oil, overloaded, contaminated, or misinstalled, they can fail quickly. Early detection can prevent catastrophic damage.

Typical failure modes and what you might notice include the following.

• Oil starvation/scuffing: Blueing and scoring on shells and journals; low oil pressure warnings; sudden knocking.
• Fatigue spalling: Flaking of bearing surface under cyclic loads; metallic glitter in oil; growing noise over time.
• Debris embedment: Scratches and ridges; higher wear rate; possibly localized overheating.
• Spun bearing: Bearing shell rotates in its housing, wiping out the locating tang and blocking oil holes; loud rod knock and rapid engine failure.
• Corrosion/chemical attack: Pitting from moisture, fuel dilution, or improper oil additives; reduced load capacity.

If these symptoms appear—especially rod knock or loss of oil pressure—shut down the engine promptly to limit secondary damage to the crank and block.

Causes and Prevention

Most bearing problems trace back to lubrication faults, contamination, or incorrect clearances. Good maintenance practices greatly extend bearing life.

Key causes and the preventive steps are listed here.

• Low or wrong-viscosity oil: Follow the manufacturer’s grade and change intervals; consider duty cycle and climate.
• Oil starvation: Address clogged pickup screens, failing pumps, cracked pickup tubes, and severe oil slosh; ensure proper sump baffling for track use.
• Contamination: Replace filters on schedule; fix coolant and fuel leaks; keep assembly areas clean during rebuilds.
• Improper machining/assembly: Verify journal roundness and surface finish; measure clearances with micrometers and Plastigage; torque fasteners in sequence.
• Overheating and detonation: Maintain cooling system; tune fuel/ignition; detonation dramatically spikes bearing loads.

Addressing these factors protects the hydrodynamic oil film that bearings rely on, preventing wear and catastrophic failure.

Inspection, Service, and Replacement

Routine engines seldom need bearing service before overhaul, but high-mileage, heavily loaded, or performance engines may require earlier attention. Accurate measurement and cleanliness are essential when replacing bearings.

Typical service steps and considerations include the following.

• Measurement: Mic the crank journals; check out-of-round and taper; confirm housing bore alignment and size.
• Clearance setting: Use the correct shell grade/undersize; verify with Plastigage; target spec per service manual.
• Surface prep and cleanliness: Deburr oil holes; clean galleries; use assembly lube; align shell tangs and oil holes.
• Crank reconditioning: Polish or grind journals as needed; match undersize bearings to ground journals.
• Thrust clearance: Measure end play; install thrust bearings correctly with oil grooves oriented per spec.
• Torque and angle: Follow specified torque/angle for caps and rod bolts; replace torque-to-yield fasteners.

Because a failed bearing can damage the crankshaft and block, many repairs escalate to a partial or full rebuild that restores proper geometry and lubrication paths.

Costs and Practical Considerations

Costs vary widely by engine design and damage severity. Bearing sets often run roughly tens to a couple hundred dollars; crank polishing or grinding and undersize bearings add cost; labor dominates. A light in-car rod bearing refresh on accessible engines can be in the hundreds to low thousands of dollars, while repairs after a spun bearing or severe journal damage often require an engine-out rebuild that can run into several thousands. Preventive maintenance—correct oil, timely changes, and addressing oil-pressure warnings—remains the cheapest “insurance.”

Related Context

While internal combustion engines rely on crankshaft bearings, electric vehicles do not have crankshafts. They use different bearing types (typically rolling-element bearings) in electric motors and reduction gears. That distinction underscores how crankshaft bearings are specific to piston engines and their lubrication systems.

Summary

A crankshaft bearing is a split, plain journal bearing that supports the crankshaft in an internal combustion engine, riding on a pressurized oil film to minimize friction and wear. Main and rod bearings work together to carry heavy loads, manage heat, and maintain precise alignment. Correct oil, clean lubrication, and accurate clearances are essential for longevity; neglect can lead to scuffing, spun bearings, and costly rebuilds.

What is the life expectancy of a crankshaft bearing?

Crankshaft Bearings Service Life
Generally speaking, reasonable use and regular maintenance can prolong the service life of crankshaft bearings, which can usually reach 100,000 to 200,000 kilometers under normal working conditions.

What does a crankshaft bearing sound like?

And then after the crankshaft. It will go to the valve. Train. Now when the clearance between the crankshaft. And the bearing is excessive. That will cause excessive oil leakage.

What happens when a crankshaft bearing fails?

The crankshaft bearings are generally made to be durable but normal wear and tear can happen and it could happen faster if the recommended car maintenance and service schedule are not followed. If your crankshaft bearings fail, your vehicle will most likely have a catastrophic engine failure.

Can crankshaft bearings be replaced?

If you are smart (as I wasn’t), you’ll heed the warning signs that the crankshaft bearings are in the beginning stages of failure and replace them before they damage other, more expensive engine parts.