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Main bearings locate the engine's rotating foundation
The crankshaft changes piston force into torque while carrying combustion impulses, rotating inertia and loads from the flywheel and front drive. Its main journals run in housings distributed along the cylinder block. Replaceable bearing shells provide the accurately controlled surface between each journal and its housing.
These shells do not operate as dry bushes. Rotation draws oil into a narrowing gap, creating a hydrodynamic wedge capable of supporting the crank without continuous metal contact. The dimensions, alignment and lubricant supply of the complete assembly determine whether that film remains stable.
A main-bearing set can contain several distinct pieces
| Element | Role | Identification point |
|---|---|---|
| Upper shell | Sits in the block or bedplate and often receives pressurised oil. | May carry an oil groove and feed hole not present in the lower shell. |
| Lower shell | Sits in the removable cap and carries the opposing load area. | Do not interchange with a grooved upper unless expressly designed alike. |
| Flanged thrust bearing | Combines radial support with axial control faces. | Flange width, surface direction and housing position are critical. |
| Separate thrust washers | Limit crankshaft fore-and-aft movement. | Oil grooves normally face the moving crank surface; follow engine data. |
| Selective-grade shells | Fine-tune clearance within production tolerances. | Letter, number or colour systems are unique to the engine maker. |
The oil wedge depends on controlled eccentric position
Gravity and operating load move the journal slightly away from the centre of the bearing clearance. As it turns, the journal drags viscous oil into the converging space. Pressure generated within this wedge separates the surfaces. Pump pressure delivers oil to the bearing, while journal speed, clearance and viscosity establish much of the carrying capacity.
Very low speed with high cylinder load is demanding because film-generating speed is small. Cold oil may struggle to flow through restrictions; excessively hot or fuel-diluted oil may be too thin. Both conditions show why the specified oil grade and healthy galleries matter.
Bearing layers balance strength with compatibility
Steel backing
The backing gives the shell stiffness and creates an interference fit within its housing. It must transfer heat into the cap and block. Oil or debris behind it changes geometry, reduces heat transfer and can create a concentrated pressure mark.
Intermediate and running layers
Aluminium alloys, copper-based layers, overlays and engineered polymer coatings are used in different combinations. A running surface must resist fatigue while remaining sufficiently compatible with a precision steel journal. Material is an engine-design decision, not an appearance-based upgrade.
Grooves, holes and edge profiles
Oil features deliver lubricant to the bearing and sometimes to cross-drilled crank passages feeding connecting rods. Chamfers provide clearance around journal fillets. The wrong hole, width or edge shape can interrupt supply or create destructive contact even where nominal diameter appears correct.
Choose from engine identity and measured geometry
| Selection check | Information required | Consequence of a mismatch |
|---|---|---|
| Engine variant | VIN, code, build split and crankshaft type. | Wrong housing width, layer material or oil layout. |
| Journal size | Measured diameter plus standard or reground status. | Clearance can be excessive or the shaft may lock. |
| Grade system | Block, crank and shell markings interpreted by the manual. | Random colour matching gives uncontrolled clearance. |
| Housing construction | Individual caps, ladder frame or one-piece bedplate. | Shell positions and tightening procedure can differ. |
| Thrust arrangement | Flanged shell or separate washers and specified location. | Excessive or insufficient end float. |
| Oil feed features | Upper/lower position and gallery alignment. | Blocked supply to a main or connecting-rod journal. |
Standard, undersize and oversize language needs care
A crank journal that has been professionally ground to a smaller diameter needs a bearing with greater wall thickness to restore the required running gap. Catalogues may call this an undersize bearing by the journal reduction, while other descriptions focus on the thicker shell. Always verify the dimensional convention rather than relying on one word.
Block-side repair can involve different terminology. A line-bored housing or externally oversize shell is a specialist machining solution and must not be confused with a crank-journal undersize. Record actual measurements and machinist instructions on the build sheet.
Preserve cap identity before measurement
Main caps are machined in position with the block and cannot be casually swapped or reversed. Mark their number and orientation before removal if factory markings are unclear. Cross-bolts, side bolts and bedplate fasteners can impose a defined clamping sequence that affects final bore shape.
Never file mating faces to reduce clearance. Do not remove metal from a cap to “improve crush” outside an approved machining process. Dowel damage, fretting or a cap that no longer seats correctly calls for expert assessment of the tunnel.
Measure the crankshaft beyond one diameter
Use a calibrated micrometer on each clean main journal at several axial positions and orientations. This establishes diameter, taper and ovality. Check the crankshaft for bend using the specified support and indicator procedure, and inspect fillets, oil-hole edges and thrust faces.
Scoring, heat discolouration, cracks or transferred bearing material require a proper reconditioning decision. Polishing can improve an acceptable surface finish, but it cannot make an undersized or tapered journal dimensionally correct. Grinding must preserve fillet geometry and surface direction.
Housing alignment determines how the shaft is supported
With bare caps or the bedplate installed and tightened in the checking sequence, measure each housing and the aligned tunnel using suitable bore equipment. Distortion may result from overheating, cap movement, incorrect fastener torque or previous machining. A correct journal and new shell will still bind in a misaligned tunnel.
Where the engine uses a structural bedplate, approved sealant thickness at other assembly joints can affect alignment. Follow the exact measurement state in the workshop data rather than mixing assembled and disassembled dimensions.
Oil clearance must be verified for every main position
Calculated dimensional clearance
Measure the bore created by the installed shells and subtract the journal diameter, maintaining calibrated tools and stable temperature. This method can show variation around and along the bearing, provided the fasteners and housing are assembled as specified.
Crushable gauge checking
A purpose-made calibrated strip can check clearance at a defined point. The journal must not rotate while the cap is tightened. Oil beneath the strip, incorrect tightening or crank movement distorts the result. The method does not reveal tunnel misalignment, journal bend or the complete clearance profile.
Compare readings with the exact engine limit. A generic clearance-per-diameter rule is not a replacement for data covering that bearing material, oil system and duty.
Shell crush retains the bearing and conducts heat
When seated in its housing, each shell projects fractionally at the parting line. Tightening the cap develops circumferential interference known as crush. This holds the backing against the housing and enables heat transfer. The locating tab mainly establishes position during assembly; it cannot restrain a loose shell under running load.
Bright rubbing on a shell back, fretting near a parting line or a flattened locating feature may indicate lost crush or housing movement. Simply installing another bearing without checking the bore can allow it to spin again.
Thrust control needs its own measurement
Crankshaft end float permits thermal movement while controlling axial position. Excessive float can alter crank-sensor relationships, damage thrust faces and create movement at the pulley or flywheel. Too little can wipe the thrust surface as the assembly heats.
Seat thrust faces in the sequence specified, then measure axial movement with a securely mounted dial indicator. On manual vehicles, inspect for causes of sustained forward crank load such as clutch release trouble or resting a foot on the pedal. Automatic-transmission converter faults or incorrect installation can also overload thrust faces.
Wear evidence should guide root-cause investigation
| Bearing evidence | Possible mechanism | Areas to investigate |
|---|---|---|
| Wiping or smeared surface | Oil-film collapse, tight clearance or overheating. | Oil supply, measured gap, cooling and journal finish. |
| Long directional scoring | Hard debris passed through the oil film. | Gallery cleaning, filter bypass, cooler and failure residue. |
| Local mark on the backing | Particle trapped between shell and housing. | Cleaning practice and housing surface. |
| Edge-heavy wear | Crank bend, tunnel misalignment or fillet contact. | Shaft run-out, housing line and shell chamfer. |
| Fatigue cracks or flaking | Repeated overload, detonation or inadequate film. | Combustion, tune, oil viscosity and duty. |
| Thrust face worn on one side | Persistent axial load or incorrect end float. | Clutch, converter, crank finish and installation. |
| Blackened or rotated shell | Severe heating after lubrication or retention failure. | Block housing, crank and complete oil circuit. |
Low oil pressure is a system warning
Excessive bearing clearance can increase internal leakage and contribute to low pressure, but a worn pump, leaking pickup, blocked strainer, stuck relief valve, aerated oil or sensor fault can create similar evidence. Test actual pressure with suitable equipment at the specified oil temperature and speeds.
If the warning illuminates while driving, stop safely and switch off. Running the engine “for one more check” can turn repairable surface distress into a seized crankshaft or broken block.
Contamination after failure travels widely
Bearing fragments can enter drillings in the crankshaft, oil galleries, piston cooling jets, variable timing controls, turbocharger feeds and the oil cooler. Strip and clean the circuit by the engine procedure. A cooler that cannot be verified clean may have to be renewed.
Investigate why the first bearing failed: oil starvation, coolant or fuel dilution, unsuitable oil, debris, overheating, detonation and assembly error all remain capable of damaging a new set within minutes.
Assembly order protects the new surfaces
- Clean and dry the block saddles, caps, crank oilways and tools.
- Confirm each shell's number, location, oil features and thrust orientation.
- Seat shell backs dry and fully home without touching the running faces unnecessarily.
- Apply the specified clean assembly lubricant to journal-facing surfaces.
- Lower the crankshaft without dragging it across shell edges.
- Install original caps or bedplate in position and follow every tightening stage.
- Check crank rotation after each stage and investigate any newly tight position.
- Measure end float and record final oil clearances before closing the engine.
Fastener procedure affects the bearing tunnel
Main bolts can be torque-to-yield and may require renewal. Threads, washers and under-head faces must have the lubrication state stated in the procedure because friction changes achieved clamp. Side bolts and bedplate fasteners may be tightened only after primary vertical bolts reach a defined stage.
Using a higher fastener torque does not strengthen the assembly. It can distort the tunnel, overload threads and reduce clearance. If studs or an alternative fastener system are specified for a performance build, the block may need line-honing under the new clamp condition.
Oil choice and first start are controlled operations
Use the engine builder's compatible assembly lubricant and the required oil viscosity and approvals. Prime the pump and galleries by the approved method, fill the filter where permitted and verify that plugs and gallery closures are present. Avoid prolonged dry cranking.
At first start, establish oil pressure within the stated time without unnecessary revving. Stop for a warning lamp, heavy knock, leakage or abnormal crank resistance. Follow a run-in plan compatible with the bearings, rings, camshaft and oil; conflicting generic advice should not override component instructions.
Operating safety and UK road use
A main-bearing failure can cause abrupt engine seizure, loss of power or oil discharge. Do not continue driving a vehicle with confirmed low pressure, substantial crank knock or visible metallic debris. Arrange recovery and measurement.
The MOT does not measure crankshaft oil clearance or certify an engine rebuild. A valid certificate therefore gives no assurance that internal knocking or oil-pressure loss is safe to ignore.
Practical crankshaft-bearing FAQs
Q: Are crankshaft bearings the same as main bearings?
A: In this context, yes; they support the crankshaft within the block rather than joining it to the connecting rods.
Q: Does a main-bearing set always include thrust washers?
A: No. Some engines use a flanged main shell, separate washers or separately supplied thrust parts.
Q: Can standard bearings fit a reground crankshaft?
A: Only if measured dimensions meet the standard specification; a reduced journal normally needs the matching thicker shell.
Q: May upper and lower shells be swapped?
A: Not unless the engine data identifies them as identical; oil holes and grooves often differ.
Q: Can I reuse main-bearing caps in different positions?
A: No. They belong to their original block locations and directions.
Q: Will new bearings cure low oil pressure?
A: Not automatically; the pump, pickup, relief valve, galleries, oil condition and all clearances require diagnosis.
Q: Is a locating tab what stops a shell spinning?
A: No. Correct bearing crush and housing clamp provide primary retention.
Q: Can a scratched journal just be polished?
A: Only if professional measurement shows size, shape and surface remain within limits.
Q: Why measure every journal?
A: Diameter, taper and ovality can vary, so one acceptable position does not validate the crankshaft.
Q: What controls crankshaft end float?
A: Dedicated thrust faces or washers at the specified main-bearing position.
Q: Should oil be placed behind the shells?
A: Normally no; seat the backs clean and dry and lubricate the running faces as instructed.
Q: Must main bolts be replaced?
A: Replace every fastener identified as single-use and follow its exact tightening procedure.
Q: What should happen before first start?
A: Prime the complete oil circuit, confirm free rotation and account for every gallery plug and tool.
Q: Is it safe to drive with a deep crankshaft knock?
A: No. Stop the engine and arrange diagnosis before a bearing spins or the shaft seizes.
