What Is Rod Knock?
Rod knock is the noise produced by a worn or failed connecting rod (big-end) bearing. The connecting rod links the piston to the crankshaft. At each rotation, the big-end of the rod must pivot smoothly around the crankshaft journal. This pivot is protected by a thin shell bearing — a replaceable precision-machined insert with a layer of soft bearing material on its inner face.
When this bearing wears or fails, the clearance between the rod and the crankshaft journal increases. At each combustion stroke, the rod hammers against the journal rather than pivoting smoothly, producing the characteristic knock.
What Rod Knock Sounds Like
- A deep, hollow knock or rap — like knocking on a thick wooden door
- The sound follows engine RPM precisely — you can track it speeding up and slowing down with the throttle
- It becomes significantly louder under load (acceleration, going uphill)
- On a multi-cylinder engine, you can sometimes isolate the affected cylinder by shorting out (disconnecting) plug leads or injectors one at a time — when the knocking reduces, you've found the faulty rod
- It may be faint when cold and worst when the engine is fully warm (thinner hot oil provides less cushion)
How It Differs From Other Knock
- Main bearing knock is lower and more constant (less affected by RPM)
- Piston slap is muffled and reduces with warmup
- Detonation/spark knock is a sharp ping, worst under load, and changes with throttle immediately
Rod knock is characteristically deep, rhythmic, and worsens under load.
What Causes Rod Knock
- Oil starvation — the most common cause. Driving with the oil pressure warning light illuminated, even briefly, can fatally damage rod bearings within minutes.
- Infrequent oil changes — degraded oil loses its film strength; the bearing material wears through.
- Low oil level over a long period — even if the light doesn't come on, chronically low oil stresses the bearings.
- Very high mileage without rebuild — bearings are wear items; on an engine that's done 300,000+ km without a rebuild, wear is expected.
What Happens If You Keep Driving
Rod knock does not stabilise. The bearing material continues to wear away, clearance increases, and the knocking becomes more violent. Eventually:
- The bearing shell disintegrates completely
- The connecting rod has no support and flexes catastrophically
- The rod breaks or bends, potentially driving through the engine block (a "thrown rod")
- The engine is destroyed and may be non-rebuildable
Repair Options
- Early stage (knock just beginning, journals not scored): Rod bearing replacement as part of an engine rebuild. $1,500–$3,500 NZD.
- Scored or worn crankshaft journals: Crankshaft regrind + undersized bearings, or crankshaft replacement. Adds $500–$1,500 NZD.
- Replacement engine: Often the most cost-effective path on older vehicles. $2,500–$5,500 NZD fitted for a good second-hand unit.