- In order to inspect the pistons, the engine usually must be disassembled. perform the cylinder leakage test and compression test before disassembling the engine to locate the source of engine problems.
- Inspect the pistons for obvious wear and breakage.
- Also inspect piston rings, connecting rods and rod bearings.
- Closely examine all piston skirts and bearings for unusual wear patterns that may indicate a twisted rod.
A twisted connecting rod caused this wear pattern on the piston skirt. Courtesy of Dana Corp., Perfect Circle Division.
There are several different types of piston noise resulting from cracked pistons, piston slap, excess piston pin clearance, and other causes.
- Retarding the spark timing will generally reduce the intensity of piston noise.
- Grounding out the plug during the power balance test can increase a piston noise. This is the opposite result from the way a bad connecting rod reacts to ignition grounding.
- The term piston slap is used to describe the noise made by the piston when it contacts the cylinder wall.
Piston Slap. Courtesy of Ford Motor Company.
- This noise is usually heard only in older, high-mileage engines that have worn pistons or cylinder walls.
- Sometimes the noise gets louder during acceleration, often clearing up when the engine warms up.
- Piston slap at top dead center (TDC) and bottom dead center (BDC) causes oval wear as the piston rings scrub the sides of the cylinder.
- Look for worn or collapsed piston skirts when excess oval cylinder wall wear is found.
- The noise is most noticeable when the engine is cold or under a load.
- Cracked pistons are often the source of noise.
A cracked piston.
- The noise is sometimes higher pitched than a crank related noise and could be confused with a valve train noise.
- Cracked pistons are often the result of a broken timing chain or improper valve timing, which can allow a valve to strike a piston on some engines.
Piston Pin Noise
- Noise from excessive piston pin clearance makes a "double click" sound at idle or fast idle.
- Pin noise often becomes more intense after the installation of new piston rings. The noise will gradually become less as the rings wear.
- With the engine running at the speed where the most noise occurs, grounding the plug wire will sometimes increase the noise even more. The noise level might not increase, but it will not become less. Piston inertia causes the noise, which is why the noise does not go away when the the spark plug is shorted out.
- The noise usually becomes less or goes away when the engine warms up.
Diagnosing Piston Wear
Scuffing results from excessive heat. It occurs when the cylinder wall and piston momentarily weld to each other as the piston stops at TDC. The welds are constantly made and broken.
- Scuffing on both skirts is a problem usually caused by insufficient clearance between new pistons and cylinder walls.
- Scuffing on only one skirt can be caused by excessive idling at too low an rpm or by lugging the engine.
- In either case, there is not enough oil thrown from the rods to provide adequate cylinder wall lubrication .
A scuffed piston skirt. Courtesy of Federal-Mogul Corporation.
- Scuffing can also be caused by cylinder wall hot spots that are the result of poor cooling system maintenance.
- Tight wrist pins cause what is commonly called four corners scuffing. Both skirts are scuffed on the edges next to the piston pin.
Scuffing near the wrist pin. Courtesy of Federal-Mogul Corporation.
- This is usually a result of an external cause such as too lean an air/fuel mixture, which causes the top of the piston to run too hot and expand against the cylinder bore.
- A piston that overheats because of cooling system problems or abnormal combustion will expand excessively near the piston pin. This can cause scuffing of the piston skirt near the pin.