What Causes Piston Slap?

[DePenelope]

The causes of piston slap are: 1. Excessive wear of piston and cylinder leading to excessive clearance; 2. Poor lubrication of cylinder walls; 3. Incorrect piston installation direction or piston deformation; 4. Bent or twisted connecting rod; 5. Severe carbon buildup in the combustion chamber; 6. Excessive fuel injection advance angle; 7. Blocked lubrication oil passages; 8. Overly thin cylinder gasket or excessive wear of cylinder block top surface, resulting in increased compression ratio of diesel engines; 9. Overly rapid combustion speed of combustible mixture, causing excessive cylinder pressure. The effects of piston slap are: 1. Loud noise; 2. Piston cracking and cylinder head deformation; 3. Increased engine compression ratio, prone to knocking during combustion; 4. Damage to cylinder head; 5. Bending deformation of connecting rod.

[VanDaisy]

I've encountered piston knocking quite a few times during my over 20 years of driving. The most common cause is excessive clearance between the piston and cylinder wall, which could result from long-term wear or improper maintenance. Insufficient lubrication or poor oil quality can worsen the situation. Especially during cold starts, when the engine temperature is low and metal contracts, the increased clearance often leads to that 'tapping' sound. Carbon buildup can also squeeze the piston's movement and amplify the noise. If left unchecked, it might develop into cylinder scoring or piston ring damage, which would mean much higher repair costs. From my personal experience, regular oil changes and wear inspections can effectively prevent this. Changing the oil filter each time also helps clean carbon deposits. If the sound persists, it's safer to have a professional mechanic check it out.

[AidenAnn]

My new car has been driven for less than three years, and recently I kept hearing a metallic knocking sound from the engine, which made me quite worried. After checking, I found out it was piston slap. The usual causes include excessive piston clearance, such as insufficient engine oil or dirty oil with poor lubrication, or possibly excessive engine carbon buildup affecting piston movement. It's particularly noticeable during cold starts in winter because cold oil has poor fluidity. Another possibility is loose or worn piston pins, which need timely inspection. I've learned that once you hear this sound, you shouldn't delay—check the oil level and replace the old oil. If there's no improvement after a few weeks, it's best to take it to the repair shop. For prevention, I recommend using high-quality engine oil and not exceeding the maintenance intervals.

[SaraRose]

Piston knocking is mainly caused by excessive clearance between the piston and cylinder. Common reasons include wear, oil starvation, or carbon buildup. During cold starts, high oil viscosity leads to poor lubrication, and the wide clearance results in knocking sounds. Long-term neglect can wear cylinder walls, increase oil consumption, and even lead to engine failure. Regular oil changes and carbon deposit removal are recommended to prevent the issue.