The pressures and rotational forces applied to the crankshaft by the pistons eventually cause wear and stress on the crankshaft. The lack of proper lubrication or the addition of abrasives greatly accelerates this wear.
Provided the wear is not outside specified limits, the crankshaft can be reconditioned. Before reusing the crankshaft, examine the crankshaft carefully for obvious wear and damage.
To inspect the crankshaft, remove the oil pan.
- Inspect the threads at the front of the crankshaft, the keyways, and the pilot bushing bore.
- Check the main bearing journals and connecting rod journals.
- When inspecting the journals, run your fingernail across their surfaces to feel for nicks and scratches.
- If a journal is scored, it will have to be polished before accurate micrometer readings can be obtained.
- Also see if the journal diameters show signs of heat checking or discoloration from high operating temperatures.
- Inspect the areas around fillets, thrust surfaces, oil passages, counter weights, seal surfaces, and flange.
- Are any of the sealing surfaces deeply worn, sharply ridged, or scored?
- Are there any signs of surface cracks or hardness distress?
- Check the vibration damper (harmonic balancer) and flywheel.
- Are the mounting surfaces eroded or fretted?
- Are there indications of damage from previous engine failures?
If any or all of these conditions are present, the parts need to be repaired or replaced.
Early diagnosis of bearing failure most often will spare the crankshaft and only the bearings will need to be replaced.
Compare the measurements of the crankshaft journals with the specifications. if the measurements are within specifications, the crankshaft can be reinstalled in the engine. if the measurements are not within factory specifications, the crankshaft must be machined before it is reinstalled in the engine.
The lateral movement of the crankshaft is controlled by a thrust bearing and a thrust surface on the crankshaft. If the thrust surface is worn, causing excessive end play, the crankshaft will have to be replaced, the surface built up, or an undercut radius machined into the journal.
Keep bearings in respective position order during disassembly so that the cause of unusual wear conditions can be diagnosed. a bent crank can be indicated when one bearing wears more than the others. the resulting bearing wear is usually worse at the center. if the inspection of the crankshaft indicates excessive warpage, the shaft may be straightened.
Crankshaft misalignment. Main bearing wear caused by a bent crankshaft or a misaligned crankcase. Courtesy of AE Clevite Engine Parts.
The crankshaft can be checked for obvious cracks by "ringing" the counterweights with a light tap of a hammer. A dull sound indicates the presence of a crack. An additional check is to inspect for stress cracks.
Stress cracks can develop around the fillet. If the crankshaft is going to be used under extreme conditions, or if there are any doubts concerning cracks, use the Magnetic Particle Inspection (MPI) to check the crankshaft.
Excessive torsional vibrations usually result in a crack near the number 1 piston connecting rod journal. Causes of excess vibration include:
- An unbalanced crankshaft,
- A defective vibration damper,
- A wrong flywheel,
- A damaged or improper convertor drive plate, or
- An improperly balanced torque converter.
The loads placed on the crankshaft can cause it to warp into a bow shape. If a warped crankshaft is installed with new bearings, the bearings will fail prematurely.
Always check crankshaft straightness before returning it to service. If the crankshaft is out of specifications, it should be replaced. However, some machine shops use a special press to straighten the crankshaft.
Do not store a crankshaft lying down. In this position, it may warp or become bent. Store the crankshaft in a support designed for the crankshaft.