Recognising a Connecting Rod Knock
A dull, rhythmic knock from the bottom of the engine that gets louder under load (on hills, during acceleration) and follows engine speed is the classic sign of worn connecting rod or crankshaft bearings — what mechanics call a spun bearing. It often appears after a loss of oil or oil pressure.
A loud, sudden knock: stop driving immediately. A connecting rod with too much play can break and punch through the engine block.Don't Confuse It With
| Noise | Likely cause | Severity |
|---|---|---|
| Dull knock from the bottom of the engine, louder under load | Connecting rod or crankshaft bearings | Very serious |
| Light ticking from the top of the engine | Hydraulic tappets, valve clearance | Minor |
| Rattling for a few seconds on start-up | Slack timing chain | Needs prompt attention |
| Knocking on acceleration in a petrol engine | Combustion knock (fuel, ignition) | Worth monitoring |
Why It Happened
Bearings run on a film of oil. A low oil level, a worn oil pump, infrequent oil changes or the wrong grade of oil can break that film: metal rubs against metal, wears down and develops play.
Repair or Replace the Engine?
Replacing the bearings means stripping the bottom end of the engine, often regrinding the crankshaft, and metal debris will typically have damaged other components. In the vast majority of cases, a reconditioned engine on an exchange basis is the safest solution. Supplied as a bare engine, it accepts your ancillaries and comes with a warranty of up to 24 months. Remember to have the oil cooler and turbo cleaned or replaced if they have been contaminated with metal swarf.
