How to Solve Engine Piston Slap?

[McThomas]

Engine piston slap can be resolved by the following methods: 1. If knocking sounds occur in low-temperature environments after starting the car and disappear as the temperature rises, this type of piston slap can be temporarily ignored, and driving can continue; 2. If the knocking sounds persist or become more pronounced after the engine reaches normal operating temperature, the driver should avoid high-speed driving, stop to inspect promptly, and prevent potential traffic accidents; 3. If severe cylinder ovality, scoring, or excessive clearance between the piston and cylinder is found after removing the cylinder head and extracting the piston, replacement is necessary—replace the cylinder liner, piston, and piston rings; 4. If connecting rod deformation is detected, replacement is strongly recommended.

[JoshuaDella]

Engine knocking is no trivial matter and must be addressed promptly. I dread hearing that metallic tapping sound, which is usually caused by excessive clearance between the piston and cylinder wall. It's most noticeable during cold starts, right? Don't rush to dismantle the engine just yet—check these three things first: Has the engine oil been left unchanged for too long and become too thin? Low-quality oil can't form a proper oil film. Are the spark plugs the wrong model? Incorrect heat range can easily cause detonation. For older vehicles, check the wear on the crankshaft bearings—excessive clearance can lead to insufficient oil pressure. If the knocking persists even after the engine has warmed up, head to the repair shop immediately and use a borescope to inspect the cylinder walls for scratches. If the wear is severe, you'll need to bore the cylinders and replace the piston ring set.

[SanBenjamin]

Last week, my neighbor spent 5,000 yuan on a major engine overhaul for this exact issue. Engine knocking is most commonly caused by long-term use of low-octane gasoline. Insufficient gasoline octane rating leads to detonation, where the fuel ignites before the piston reaches top dead center. The solution is actually quite straightforward: First, fill up with two tanks of 98-octane gasoline to flush the fuel system, and add some fuel system cleaner to remove carbon deposits. If you still hear metallic knocking sounds during hard acceleration on the highway, there's an 80% chance the connecting rod is deformed. I've dealt with engine knocking issues three times myself, and each time it was ultimately resolved by replacing the piston pin and connecting rod bearings. Remember to have the technician measure the piston ring end gap - if it exceeds 0.5mm, they should be replaced, otherwise oil will sneak into the combustion chamber and cause blue smoke.

[LaSkylar]

An experienced mechanic taught me this method to identify knocking types: knocking at idle indicates piston skirt wear, knocking during acceleration indicates detonation, and knocking during deceleration indicates loose small end of the connecting rod. The cheapest solution I've tried is switching to a 40-weight full synthetic oil to increase oil film strength for an old engine. If the knocking occurs when the engine is cold but disappears when warm, it's likely due to excessive piston cold clearance—replacing it with a thicker piston pin can help. Never overlook coolant temperature issues, as a clogged radiator causing overheating can also lead to detonation sounds. If you hear continuous abnormal noises, shut off the engine immediately. Last year, a car was driven until it seized, ultimately requiring even the crankshaft to be replaced.