What are the phenomena and causes of piston slap in motorcycles?
2 Answers
Piston slap in motorcycles refers to a crisp, sharp "clank, clank" or "click, click" sound produced in the cylinder when the engine starts or idles, caused by the piston swinging inside the cylinder and its head or skirt colliding with the cylinder wall. It resembles a hammer striking the cylinder, hence the term piston slap. Piston slap is not a piston damage fault but rather a noise nuisance. Below are the reasons for the occurrence of piston slap in motorcycles: Around the top dead center of the compression stroke, due to the tilt of the piston and the change in the direction of lateral pressure, the contact surface of the piston with the cylinder wall shifts from the rear thrust face to the main thrust face, resulting in a lateral swing. This leads to the periodic knocking sound of the piston striking the cylinder, known as piston slap.
When a motorcycle experiences engine knocking, the engine makes a metallic tapping sound like a small hammer constantly hitting inside, especially noticeable when climbing hills or accelerating hard. I feel the bike's power weakens with some vibration. This issue is usually caused by poor fuel quality, such as using cheap gasoline with low octane ratings leading to unstable combustion; or old, carbon-fouled spark plugs causing inaccurate ignition; sometimes incorrect ignition timing can also trigger it. I've encountered this on several bikes I've ridden, and after inspection, switching to high-quality fuel and cleaning the spark plugs solved it. Ignoring this can cause severe internal engine wear, leading to piston ring damage or cylinder wall scoring, which is expensive to repair and disrupts riding. So when detected, it's best to stop using the bike immediately and have it checked at a shop. For prevention, regular maintenance combined with using good fuel can avoid knocking in most cases.
