A crankshaft is ground to clean up scored journals. The shop takes it to the next standard undersize, and the matching bearings are ordered. On assembly, the mechanic checks oil clearance with a plastic gauge. Two journals read perfectly, one is too tight, and one is too loose. The bearings are all from the same box. The difference is in the grinding: the journals were cleaned up, but not all brought to exactly the same finished size. Matching the crankshaft undersize bearing to the journal is a measurement job, not a catalog lookup.
What “Undersize” Really Means
When a crankshaft journal is worn or scored, it is ground smaller to restore a round, smooth surface. The new, smaller journal needs a thicker bearing shell to keep the same oil clearance. These thicker shells are sold as undersize bearings, named by how much smaller the journal has become.
The name refers to the journal, not the bearing thickness. A 0.25 mm undersize bearing fits a journal ground 0.25 mm smaller in diameter than standard. Each shell is about 0.125 mm thicker, because there are two shells around the journal.
Common Undersize Steps
| Undersize | Journal diameter reduction | Typical situation |
|---|---|---|
| STD | None | Original or polished journals within limits |
| 0.25 mm (0.010 in) | 0.25 mm | Light wear or scoring |
| 0.50 mm (0.020 in) | 0.50 mm | Moderate damage or a second regrind |
| 0.75 mm (0.030 in) | 0.75 mm | Heavier damage; not available for every engine |
| 1.00 mm (0.040 in) | 1.00 mm | Last step on some heavy diesel cranks; check manufacturer limits |
Not every engine offers every step, and some crankshafts have a hardened layer that limits how far they can be ground. Grinding beyond that depth leaves the journal soft, and it will wear quickly no matter which bearing is fitted.
Oil Clearance Is the Real Target
The undersize choice gets you close. The oil clearance (the gap between the journal and the bearing, filled with oil under pressure) decides whether the engine lives. Too tight, and the oil film breaks down under heat and load. Too loose, and oil pressure drops, the bearing hammers, and fatigue cracks start in the bearing lining.
That is why good crankshaft grinding targets a specific finished diameter within a tight band, not just “cleaned up.” Every main journal should land in the same band, and every rod journal should too.
Do Not Forget the Bores
Oil clearance depends on two diameters: the journal and the housing bore. If the main bearing bores in the block are out of line or out of round, or a connecting rod big end has stretched, a perfectly ground journal still ends up with the wrong clearance. A proper job measures the bores as well as the crankshaft.
Mixing Sizes
Some engines allow mains and rods to be at different undersizes, and occasionally individual journals need different sizes. This is acceptable when the manufacturer permits it, but it must be recorded clearly, ideally stamped on the crankshaft, so the next person who opens the engine does not guess.
How Merdeka Precision Handles Crankshaft Grinding
Crankshaft grinding is a core part of our crankshaft services, using specialized grinding, balancing, and measuring equipment. Journals are ground to a defined finished size for the chosen undersize, then measured and recorded during QC inspection, in line with our ISO 9001:2015 quality system.
Because clearance depends on the block as much as the crank, main bore alignment can be checked and corrected through line boring as part of our cylinder block services. Diesel engines from trucks, commercial vehicles, and other equipment in the transportation sector make up much of this work.
Ask for the Numbers
When a crankshaft comes back from grinding, you should know the undersize, the finished diameter of each journal, and ideally the expected oil clearance with the specified bearings. If those numbers are not available, the clearance on assembly is a gamble.
Does your last ground crankshaft come with a measurement sheet, or only a size stamped on a counterweight? Contact us to see what our crankshaft report includes.