What Are the Phenomena and Causes of Piston Slap in Motorcycles?

[DiHarper]

Piston Slap: New pistons and old cylinders are not originally matched. Although they may appear to be in good condition, excessive clearance can cause the piston to swing within the cylinder under high-pressure gas during engine operation, resulting in knocking against the cylinder wall. The characteristics include: a clear knocking sound during engine idle; more pronounced knocking when the engine is cold, which diminishes or disappears as the temperature rises; in multi-cylinder engines, the knocking sound weakens or disappears when the affected cylinder is deactivated; injecting a small amount (about 3mL) of engine lubricating oil into the cylinder through the spark plug hole and slowly turning the crankshaft via the kickstarter for about a minute allows the oil to flow between the piston and cylinder, temporarily reducing or eliminating the knocking sound during engine operation. Additionally, piston reverse ellipticity, bent or twisted connecting rods, or excessive tightness in the piston pin and connecting rod bushings (bearings) can also cause piston slap. Piston Pin Knock: The main cause of piston pin knocking is excessive clearance between the piston pin and the connecting rod bushing (or the piston pin bore), leading to the piston pin striking the bushing or bore during engine operation. Characteristics include: a distinct knocking sound during idle and medium-speed operation; the knocking frequency increases with engine speed; the sound does not diminish with rising temperature and may become more pronounced (a key difference from piston slap); the knocking sound weakens or disappears when the cylinder is deactivated, but a noticeable knock or two occurs upon reactivation; the knocking is louder when a stethoscope is placed on the upper half of the cylinder and quieter on the lower half; advancing the ignition timing intensifies the knocking. Piston Ring Slap: This occurs when worn or broken piston rings move loosely within their grooves, striking the cylinder wall. Severe cylinder wear creating a ridge can also cause knocking when new rings hit the ridge. In two-stroke engines, wear of the piston ring locating pin can displace the ring gap into the cylinder port, leading to knocking. Characteristics include: a duller knocking sound; the volume and frequency increase with engine speed, becoming more chaotic; deactivation reduces but does not eliminate the sound, and a 'swishing' noise may occur upon reactivation if the ring is broken.