Engine Building Fundamentals: Crankshaft Polishing
- The surface finish of the crankshaft journals is critical to the reliability and life expectancy of the engine bearings.
While a brand new crankshaft should offer a perfectly smooth journal finish that's ready to drop into the engine block, second-hand crankshafts may show some minor scoring or damage to the journals.
In some cases, this damage may require the crankshaft journal to be ground to a smaller size to remove this damage, which we'll discuss separately.
However, often, light scoring can be removed by polishing the journal.
Polishing is achieved while the crankshaft is installed in a crankshaft grinder so that it can be rotated about the journal axis.
A fine, abrasive belt is then run against the crankshaft journal as the crankshaft rotates.
This process polishes the journal and removes the light score marks.
Typically, the amount of material removed during the polishing process is very minimal, so it has little effect on the final bearing clearances.
However, this still needs to be considered when any polishing is performed.
Your machinist will be able to advise as to whether damage to your crankshaft will be repairable by polishing, or whether grinding will be required.
A good indicator of the severity of damage to a journal is to run your fingernail across the score marks.
You'll be surprised that sometimes, visible scoring can't actually be felt with this test.
However, if the scores are bad enough that you can feel them with your fingernail, you'll normally need to grind the journal rather than just polishing it.