Main and connecting rod bearings support the crankshaft on a thin film of pressurized oil. The space between the bearing and the crankshaft journal, called oil clearance, must fall within the engine manufacturer’s specification for that film to form correctly. Choosing the right bearing size is how a rebuilder controls that clearance.
Standard Bearings
Standard-size bearings are made for crankshaft journals at their original manufactured diameter. They are used when the crankshaft is in good condition and the journals measure within the original specification for size, taper, and out-of-round.
Undersize Bearings
When a crankshaft is reground to remove wear or scoring, its journals become smaller. Undersize bearings are thicker to make up the difference and restore the correct oil clearance. They are identified by the amount the journal was reduced, such as .010″, .020″, or .030″ undersize. The label refers to the crankshaft journal size the bearing is designed for, not to the bearing being smaller.
How to Select the Right Size
- Complete crankshaft machining first, or confirm the journals are within specification if no machining is needed.
- Measure every main and rod journal with a calibrated micrometer and record the results.
- Select bearings that match the final journal size, as specified by the machine shop.
- Verify oil clearance after installation using the method and specification in the engine’s service information.
Common Mistakes to Avoid
Do not assume a crankshaft is standard size without measuring it; a previously rebuilt engine may already have a reground crank. Keep bearing halves paired and installed in the correct positions, make sure locating tabs seat in their notches, and keep bearing backs and housing bores clean and dry so the bearing seats fully.
Find the Right Bearings
Evergreen main and rod bearings are identified by application and size. Use the Evergreen eCatalog to find bearings for your engine, and confirm the size you need before ordering.
