What caused my engine bearing to fail?
This guide will outline the different types of failures that can occur with your engine's rod or main bearings.
Aluminum Bearing Fatigue
Appearance: The rod or main bearing will display cracks or will appear cratered. This is due to loss of aluminum lining material.
Why did the bearing fail?
This type of failure typically occurs in gas applications where engine tuning is critical. The failure could happen when the engine’s power surpasses the chosen bearing material. The increased power could be the result of fuel detonation that was caused by either advanced ignition or improper engine tuning. Detonation is an uncontrolled burn of fuel in the cylinder and this results in the bearing being overloaded.
How do I prevent this bearing failure from occurring again?
Be sure to select the proper bearing for the engine’s power and load. This could mean selecting a steel backed bearing rather than aluminum. Also, make sure to check your bearing clearances again. Make sure that your engine is properly tuned. This might require retarding the engine ignition or using a fuel with higher octane.
Babbitt Overlay Fatigue
Appearance: The rod or main bearing will display irregular cracks and craters in the Babbitt overlay. The lead material may display flaking and in advanced wear situations, fatigue of the copper-lead layer.
Why did the bearing fail?
Like the aluminum bearing, the failure could happen when the engine’s power surpasses the chosen bearing material. The increased power could be the result of fuel detonation that was caused by either advanced ignition or improper engine tuning.
How do I prevent this bearing failure from occurring again?
Be sure to select the proper bearing for the engine’s power and load. Be sure to recheck the bearing clearances and try using a higher-octane fuel.
Bronze Intermediate Layer Fatigue
Appearance: When the top Babbitt material is missing and the bronze intermediate layer is exposed, cracked, and cratered.
Why did the bearing fail?
This type of failure typically occurs when the engine is over-fueled, or the engine is detonated.
How do I prevent this bearing failure from occurring again?
Correct the engine tuning or use a higher-octane fuel.
Surface Wear
Appearance: The rod or main bearing will show signs of excessive wear. The bearing will be shiny, blackened and/or appear melted. The intermediate bronze layer will be exposed. In severe cases the bearing will be thinned and bearing material will be transferred to the crankshaft journals.
Why did the bearing fail?
This type of failure occurs when the bearings experience oil starvation (lack of lubrication).
How do I prevent this bearing failure from occurring again?
The oil supply was either run low, or it was contaminated. The contamination could be due to metal from another failure, or due to dirt/debris introduced into the lubrication system.
Wear Along the Rod Bearing Edges
Appearance: The edges of the bearing will show the top layer shiny or fatigued.
Why did the bearing fail?
This failure typically occurs when the rod housing bore is distorted. This can be from heat distortion from a previous failure where the connecting rod was not replaced or repaired. This can also be caused by over-torqueing or detonation.
How do I prevent this bearing failure from occurring again?
The connecting rod should either be replaced or repaired at a machine shop. A rod repair require the removal of the rod cap. The cap and rod are then machined along the parting edges. The rod is then reassembled. The housing bore is now slightly smaller and out-of-round. Using a honing mandrel, the big end is now honed back out to factory specifications. During these repairs it is recommended to replace the rod bolts or studs. It is also recommended to replace the piston pin bushing and offset boring it once installed. This will correct the center-to-center distance of the connecting rod and compensate for the material removed from the initial big end repair.
Improper Journal Geometry
Appearance: The edges of the bearing will show the top layer shiny or fatigued.
Why did the bearing fail?
This is like the failure when the connecting rod is not geometrically correct. This can also occur when the bearing journals is out-of-round.
How do I prevent this bearing failure from occurring again?
Replace or redress the crankshaft grinding wheel.
Cavitation
Appearance: The bearing overlay will appear dull in inconsistent areas.
Why did the bearing fail?
This failure occurs when there are bubbles in the oil. At high RPM’s oil pressure can rapidly fall, causing bubbles due to evaporation (boiling). As the pressure rises, cavitation bubbles contract at high velocity. This collapse results in impact pressure, that can erode the bearing material.
How do I prevent this bearing failure from occurring again?
A stronger bearing material may be required or run tighter oil clearances.
Bearing Spinning in the Housing
Appearance: The back side of the bearing will appear polished.
Why did the bearing fail?
This failure occurs when the bearing does not get proper lubrication (oil starvation) It can also occur when there is improper crush due to the housing bore being larger than the factor specification.
How do I prevent this bearing failure from occurring again?
Ensure that your engine is not run low on oil or that the oil system is not preventing the engine bearings from being properly lubricated. Check and measure your housing bores to make sure that they are within the factory specification range.
Relief Area Fatigue
Appearance: Cracks along the crush relief area.
Why did the bearing fail?
The parting line of the bearings were improperly crushed. This causes an excessive gap between the journal and bearing surfaces.
How do I prevent this bearing failure from occurring again?
Check the housing diameter and avoid over-torqueing the connecting rods. It is likely that the connecting rod housing bore is out of spec.
Damage Caused by Foreign Debris
Appearance: Scores or scratches on the bearing surface.
Why did the bearing fail?
This failure typically occurs during installation. It is critical that your crankshaft oil passaged are cleaned after machining. This can also happen when introducing foreign debris into the engine via the oil or air system.
How do I prevent this bearing failure from occurring again?
It is good practice to also clean new bearings, rod housing bores and block main saddles prior to installing the bearings. Newly rebuild engines have tight bearing clearances. Any foreign debris will prematurely wear bearings. Take caution when installing connecting rods into your cylinders. It is recommended to use bolt boots on the rod bolts to prevent any accidental contact of the rod bolt and the crankshaft rod journal. Accidental contact can go unnoticed and dig into the bearing surfaces. Also be sure to check your air and oil system for potential entry points. Replace your air and oil filters.