Main and connecting-rod bearings support the crankshaft and rods on a thin pressurized oil film. Bearing selection is a precision measurement task: journal diameter, housing-bore size, bearing thickness, crush, oil clearance, crankshaft finish and intended use all matter. A package labeled “standard†is not proof that it will produce the correct clearance in a particular block and crankshaft.
Key takeaways
- Measure the crank, block and rods with calibrated tools before selecting final bearing size.
- Standard, undersize and extra-clearance descriptions must be interpreted using manufacturer terminology.
- Clearance is calculated or measured with bearings installed in correctly torqued housings.
- Oil viscosity, temperature, pump system and engine use must agree with the builder's clearance plan.
Main bearings vs. rod bearings
Main bearings sit in the block and main caps, supporting the crankshaft. Rod bearings sit in the connecting-rod big ends around crankpins. Thrust control may be integrated into one main bearing or use separate washers, depending on engine design.
Shells can include oil holes, grooves, tangs and upper/lower-specific construction. Their positions are not interchangeable unless the technical documentation says so.
How hydrodynamic lubrication works
As the journal rotates, it draws oil into a converging space and generates a supporting pressure wedge. The journal runs on this oil film rather than directly on the bearing surface during normal operation. Oil supply, viscosity, clearance, journal geometry, load and speed influence film stability.
Loss of oil pressure, contamination, detonation, distorted housings and incorrect clearance can collapse the film and damage the bearing.
Bearing construction and materials
Modern bearings commonly use layered construction with a steel backing and engineered intermediate and surface layers. Material systems vary in fatigue strength, conformability, embedability and compatibility with crankshaft finishes. Performance bearings may use different overlays, eccentricity or crush characteristics.
Choose the material recommended for the shaft, power level and duty cycle. A harder performance bearing is not automatically better for every street rebuild.
| Measurement | What it reveals | Typical tool |
|---|---|---|
| Journal diameter | Size, taper and out-of-round | Outside micrometer |
| Housing bore | Block or rod geometry under fastener load | Dial bore gauge referenced to micrometer |
| Installed bearing bore | Finished inside diameter | Dial bore gauge |
| Oil clearance | Difference between installed bore and journal | Calculated from measured diameters |
| Crank endplay | Axial thrust clearance | Dial indicator or specified method |
What standard and undersize mean
A standard bearing is intended around an unground standard-size journal, but actual clearance still varies with crank and housing dimensions. When a crankshaft journal is ground smaller during reconditioning, a bearing with additional wall thickness is used to match that reduced journal. Catalog language may call this an undersize bearing based on journal diameter.
Do not confuse bearing undersize, housing oversize and extra-clearance variants. Read the manufacturer's dimensional definition and part number.
Measuring journals
Clean the crankshaft and measure each main and rod journal at several positions and directions to identify taper and out-of-round. Compare with the current crankshaft and bearing specifications. A polished surface can still be dimensionally wrong or have unsuitable roughness.
Oil-hole edges, fillet radius and journal width must match the selected bearings and rods. Grinding should be completed by a qualified crankshaft machinist.
Housing bore and bearing crush
Main caps and rod caps must be assembled in their original orientation with specified fasteners, lubrication and procedure before housing measurement. Distortion, fretting, incorrect caps or a fastener change can alter the bore.
Bearing shells are manufactured with interference so their ends extend slightly beyond the housing parting line. Correct clamp load creates crush, retaining the shell and transferring heat. Filing shell ends or mixing caps is unsafe.
Oil-clearance planning
Required clearance varies by journal size, engine architecture, material, oil, temperature, speed, load and intended use. Too little can reduce oil flow and risk contact as parts expand; too much can reduce pressure and film control. Generic rules are not substitutes for engine, bearing and builder specifications.
Measure installed bearing bores and subtract measured journal diameters. A crushable checking strip can provide a secondary assembly check when used correctly, but it does not replace full geometric measurement.
Oil holes, grooves and orientation
Align required oil holes and install grooved and plain shells in their specified locations. Some performance modifications alter groove or feed arrangements, but changes require engineering knowledge because they can affect load area and oil distribution.
How to choose the right bearings
- Confirm exact engine, block, crankshaft and connecting rods.
- Inspect and measure journals for size, taper, out-of-round and finish.
- Assemble and measure main and rod housing bores with intended fasteners.
- Choose bearing size and material with the machinist and manufacturer.
- Install clean dry-backed shells in clean housings.
- Torque using the exact procedure, then measure installed bearing bores.
- Verify oil clearance and crank endplay against the build plan.
- Check crank rotation and final cleanliness throughout assembly.
Common mistakes
- Ordering standard bearings without measurement.
- Mixing caps or reversing rod and main shells.
- Measuring housings without final fasteners and torque procedure.
- Using abrasive material on bearing backs or housing saddles.
- Assuming a different oil viscosity corrects bad clearance.
- Ignoring crank fillet and bearing edge compatibility.
- Installing over debris or applying lubricant behind the shell.
- Failing to verify thrust clearance.
Browse FSP engine bearings and related engine components. Confirm exact dimensions and manufacturer part numbers with the machine shop before ordering.
Frequently asked questions
Can bearing clearance be checked with only a crushable strip?
It can provide a useful verification when used correctly, but precision builds should measure journals and installed bearing bores with calibrated instruments.
Can worn journals be fixed with thicker bearings?
Crankshaft condition and geometry require machinist evaluation. Bearings are not a substitute for correcting taper, damage or poor finish.
Are performance bearings always looser?
No. Material and design do not determine finished clearance by themselves; measured part combinations do.
Can main and rod bearings use the same clearance?
Specifications are engine- and build-specific. Measure and follow the respective requirements.
Engine-building note: Bearing failure can destroy an engine rapidly. Use calibrated measurement, current specifications, controlled cleanliness and a qualified machinist.
