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Main bearings create a hydrodynamic support for the crankshaft
When the shaft rotates, it drags oil into a converging clearance and forms a pressure wedge that separates journal from bearing. The oil pump supplies flow, but journal motion generates much of the supporting film. At start-up, shutdown and very low speed, boundary lubrication and overlay material protect the surfaces.
Clearance must be large enough for oil flow yet small enough to retain pressure and control shaft position. Geometry, viscosity, temperature, load and surface finish all interact.
Every shell position has a defined job
| Feature | Purpose | Installation consequence |
|---|---|---|
| Grooved shell | Distributes pressurised oil around part of the journal. | Must occupy the specified block/cap position. |
| Plain shell | Maximises loaded bearing area. | Do not interchange just because diameter matches. |
| Oil hole | Connects block gallery to bearing groove. | Misalignment can starve the journal. |
| Locating tang | Positions the shell during assembly. | It does not prevent a loose bearing from spinning. |
| Thrust face/washer | Controls crankshaft axial movement. | Direction and location are critical. |
| Crush height | Clamps the shell firmly in its housing. | Filing edges destroys retention and heat transfer. |
Standard and repair sizes require measurement
| Marking/context | Meaning | Required evidence | Unsafe assumption |
|---|---|---|---|
| STD | For the specified standard journal range. | Measured journal and housing within limits. | Original crank automatically remains standard. |
| 0.25 mm undersize | Thicker shell for journal ground smaller by specified amount. | Machined diameter, finish and fillet verified. | Use it to cure low pressure without grinding. |
| 0.50 mm undersize | Next defined repair dimension where supported. | Engine data and machinist's measurements. | All crankshafts permit this repair. |
| Selective grade/colour | Fine production size class. | Block/crank codes and maker's selection chart. | Colour is a brand or performance grade. |
| Thrust variation | Controls end-float after machining in some engines. | Measured thrust surfaces and engine procedure. | Thicker thrust faces can hide damage safely. |
Bearing construction balances strength and conformity
A steel backing provides stiffness and crush. Intermediate copper-lead or aluminium-based layers carry load, while a thin overlay offers conformability, embedability and protection during boundary contact. Modern lead-free constructions have distinct properties and must match the engine design.
The soft surface is deliberately sacrificial. Polishing or scraping it changes clearance and removes its protective function.
Read damage as a failure record
| Appearance | Probable mechanism | Checks before rebuild | Typical root cause |
|---|---|---|---|
| Wiped/scored surface | Oil-film loss or debris. | Galleries, pickup, pump, filter and journals. | Starvation, contamination or wrong oil. |
| Fatigue flaking | Repeated load exceeds material support. | Clearance, detonation, alignment and load history. | Overload or distorted housing. |
| Bright edge wear | Journal or tunnel misalignment. | Crank bend, cap location and line bore. | Distortion or incorrect cap installation. |
| Back fretting | Shell movement in housing. | Housing bore, crush and fastener clamping. | Loose cap or damaged tunnel. |
| Thrust face worn | Excess axial load or poor lubrication. | Clutch, torque converter and crank end-float. | Constant clutch load or converter fault. |
| Spun bearing | Loss of crush and oil film, severe seizure. | Block/cap and crank require full assessment. | Not repairable by new shells alone. |
Oil pressure is a system measurement
Low pressure at hot idle can reflect excess bearing clearance, thin or fuel-diluted oil, a worn pump, leaking gallery plug, cracked pickup or relief-valve fault. Measure with a known gauge at the specified oil temperature and engine speed.
A warning lamp threshold is too coarse for rebuild decisions. Conversely, acceptable pressure does not prove every bearing is undamaged if debris or local oilway restriction exists.
Journal measurement and surface finish
Measure each main journal in several angular and axial positions with a calibrated micrometer to identify diameter, taper and ovality. Inspect oil holes, fillet radii, cracks and hardness. A journal can be shiny yet geometrically unsuitable.
Grinding requires correct indexing, radius and surface texture, followed by thorough gallery cleaning. Polishing cannot correct deep scoring, excessive taper or heat cracking.
Housing bore, cap and tunnel alignment
Main caps are location- and orientation-specific. Mixing them or installing one backwards changes the tunnel. With fasteners tightened by the prescribed method, measure housing bores and alignment. Look for fretting, pulled threads and debris under cap registers.
A distorted block may need specialist line boring or honing with compatible bearing sizing. Do not machine caps casually to “tighten” clearance.
Clearance measurement methods
Preferred dimensional calculation compares measured journal diameter with the installed bearing inside diameter under correct cap torque. A supported deformable-gauge strip can provide an assembled cross-check; keep oil off the test surface, do not rotate the crank and compare at the widest point.
One spot does not expose taper or ovality. Record every position and resolve conflicting measurements before assembly.
Thrust bearing and crankshaft end-float
Set the crank against the thrust faces as the engine procedure describes, then measure end-float with a dial indicator. Excess clearance can move timing components and alter clutch or torque-converter relationships; insufficient clearance can seize as parts heat.
Investigate continuous forward load from a clutch release system or torque converter if thrust wear is severe. Replacing the thrust bearing without fixing that load invites recurrence.
Cleanliness and oil-gallery control
Strip plugs where required and brush galleries until cleaning media remains clean. Remove abrasive from crank drilling intersections and inspect oil jets. Keep washed parts covered and use lint-free materials.
Excess liquid sealant can detach and block a gallery or pickup. Apply only the specified bead in the defined position.
Shell installation fundamentals
Dry backing, lubricated running face
The shell back and housing must be clean and dry for intimate heat transfer and correct crush. Apply the specified assembly lubricant to the journal-facing surface, not behind the shell.
Respect location and oil-hole alignment
Lay out the set in engine order and compare every groove, hole and thrust face with the removed arrangement and service data. Never cover a feed drilling.
Turn the crank as stages are completed
After each cap is tightened in sequence, confirm the crank rotates with expected smooth resistance. A sudden tight point means stop and investigate.
Fasteners, torque angle and cap loading
Renew torque-to-yield bolts where specified, lubricate threads and under-head surfaces only as instructed, and use a calibrated torque/angle method. Different lubricant changes clamp load dramatically. Side bolts and bedplate sealant can affect tunnel geometry, so follow the complete sequence.
Do not reuse a cap with stretched threads or substitute a higher-strength bolt without engineering approval.
Oil choice, priming and first start
Use the engine-approved viscosity and performance specification, accounting for rebuild and running-in instructions. Fill the filter where allowed, prime the pump and establish pressure before ignition or fuel is enabled if the procedure supports it.
On first start, watch pressure immediately and listen for knock. Do not let the engine idle indefinitely if the camshaft or other components require a defined break-in speed. Cut the filter open after the recommended interval and investigate abnormal metal.
When new bearings are not enough
A spun shell, heat-discoloured crank, damaged cap register, cracked journal or out-of-line tunnel needs specialist assessment. Installing fresh shells over damage can fail within minutes and may destroy a salvageable block.
Likewise, replacing only the visibly worst bearing leaves contaminated oilways and the original cause intact.
Practical crankshaft-main-bearing FAQs
Q: What does STD mean on a main bearing?
A: It is intended for the specified standard journal range, which still must be measured.
Q: Can 0.25 mm bearings fix low oil pressure?
A: Only after the crank is correctly machined to that repair size; they cannot be fitted to a standard journal.
Q: Are upper and lower shells interchangeable?
A: Often not; grooves, holes and load area determine their positions.
Q: Does the locating tang stop a bearing spinning?
A: No. Correct crush and oil film retain it; the tang mainly locates assembly.
Q: Can Plastigage replace micrometer measurement?
A: No. It is a useful supported cross-check but cannot fully assess taper, ovality or housing geometry.
Q: Should oil be placed behind the shell?
A: No. The back and housing should be clean and dry unless engine data explicitly says otherwise.
Q: Can scratched shells be polished?
A: No. Polishing removes designed overlay and changes clearance.
Q: What causes edge wear?
A: Crank or tunnel misalignment, cap error or housing distortion are common causes.
Q: Why does a thrust bearing wear rapidly?
A: Check end-float, lubrication and continuous axial load from clutch or torque-converter systems.
Q: Can main caps be swapped?
A: No. They are machined with the block in fixed positions and orientations.
Q: Is an oil-warning lamp enough to assess pressure?
A: No. Use a known gauge at specified oil temperature and speeds.
Q: Must oil galleries be cleaned after grinding?
A: Yes. Residual abrasive can destroy the new bearings quickly.
Q: What proves the rebuild is healthy?
A: Correct clearances/end-float, free rotation, immediate stable pressure, normal sound and clean follow-up filter inspection.
