Is Motorcycle Piston Slap Caused by the Crankshaft?

[JuliaDella]

Motorcycle piston slap is not caused by the crankshaft. Piston slap refers to the lateral knocking of the piston against the cylinder wall, which is equivalent to a very high-frequency side-to-side oscillation. Piston Slap: Since the new piston and the old cylinder are not originally matched, even if they appear to be in good condition, the excessive clearance between them causes the piston to oscillate within the cylinder under the pressure of high-pressure gases during the engine's power stroke, resulting in knocking against the cylinder wall and producing noise. Piston Pin Slap The main cause of piston pin knocking noise is the excessive clearance between the piston pin and the connecting rod bushing (or the connecting rod bearing itself) or the piston pin bore. During engine operation, the piston pin moves up and down, striking the connecting rod bushing (bearing) or the piston pin bore, generating knocking noises.

[DeRemington]

Knocking in motorcycles is often not directly caused by the crankshaft. I've been riding motorcycles for decades and encountered many abnormal noise issues. The knocking sound is more like noise from improper engine combustion, such as ignition timing that's too early or using gasoline with a low octane rating, causing abnormal combustion of the air-fuel mixture that impacts the cylinder walls. If the crankshaft has problems, it's usually a metallic tapping sound, located closer to the bottom of the engine, whereas knocking is a high-frequency metallic banging sound, generally near the cylinder head. Once, my bike had a knocking sound, and switching to higher-octane gasoline fixed it; when the crankshaft failed, it was due to bearing wear, and the sound was completely different. I suggest starting with the basics: check the spark plugs or fuel, and perform regular maintenance to avoid carbon buildup worsening the problem. Don't underestimate these small things—they can prevent major repairs. Safe riding is crucial, so address any issues promptly when they arise.

[StCollin]

It's not quite accurate to say that piston slap in motorcycles is caused by the crankshaft. I often help friends with motorcycle repairs, and when diagnosing such abnormal noises, it's usually necessary to identify the sound's location. Piston slap mostly stems from detonation due to faults in the ignition system or fuel mismatch. If the crankshaft has issues, it might cause connecting rod noise or bearing noise, but that's not typical piston slap. During actual inspections, use a stethoscope to differentiate: if the sound is from the upper part of the cylinder, it's piston slap; if it's from the bottom, it might be crankshaft-related. Excessive carbon buildup can also cause similar problems. For prevention, keep the engine clean, use the correct gasoline, and don't delay oil changes. The risks are significant; long-term neglect could lead to piston damage. It's safest to get it checked at a reputable shop.

[Charlotte]

Knocking in motorcycles is generally not caused by the crankshaft. As an ordinary rider, I've had a similar experience - when I first started riding, I mistakenly thought the crankshaft was broken and was quite worried. Later, a knowledgeable friend told me that knocking is mostly caused by combustion issues, such as using low-quality gasoline or aging spark plugs. If the crankshaft fails, it produces a different knocking sound, often accompanied by vibrations. After I replaced the spark plugs, the knocking sound disappeared, and the crankshaft was still fine. I recommend using high-quality fuel and paying attention to sound changes during cold starts. Small issues are easy to overlook, but early detection can save money and hassle. Regular maintenance is key - don't let small problems escalate. Safety first - if you notice any issues, stop and inspect immediately.