Piston Slap vs. Rod Knock – Comparison

Piston Slap vs. Rod Knock – Comparison

Many things can be responsible for strange sounds coming from the engine. If the noise is tapping or knocking, you should suspect a piston slap or rod knock.

If you are new to the auto world, you may be confused about pinching down the problem; you are not alone here.

Rod knock vs Piston slap

How can you differentiate what may be responsible for this unwelcome noise coming from your cars’ engine?

Come with us as we attempt to provide helpful answers to the question, giving you hints while at it on how to resolve the issue.

What Is A Piston Slap?

Piston slap may be responsible for the knocking or rattling sound you hear in your car’s engine, giving you a sign that the engine may be failing.

We can define a piston slap as the rocking sound emanating from the piston in the engine’s cylinder.

The piston’s normal movement should be up and down motion, but a piston slap occurs when the piston starts moving sideways, hitting or slapping the body of the cylinder.

Piston slap may not immediately damage your car, but you must not ignore the noise.

Suppose left unattended for too long. It can damage the engine.

It can cause oil to leak into the engine, damaging it. The noise will increase as the problem persists, and you will not be able to ignore it at all.

When Does It Occur?

A piston slap occurs when you what to cold-start your engine, and it is expected to subside once the engine starts running and warms up.

It usually happens when the car engine overruns or runs idly. Experts say that the piston slap is due to the wearing and tearing of the cylinder as it ages.

One will expect this to be the case as the engine gets older.

Studies show that the problem usually comes up in aluminum blocks more than in cylinder blocks made of cast iron.


A piston slap is a sign that the piston is failing. The “Check Engine” indicator light will warn you about this.

A knocking noise when you start the engine cold coupled with the “check engine” indicator light is a good sign that you are having a piston slap problem on your hand.

You can also get an error code on your car’s diagnostic system if your car has one.

The error code will give a more specific direction on where the problem is coming from, making it a better way to detect piston slap.

The error code can tell you the particular cylinder affected. However, a broken sensor can give you a false reading.

The sensors are responsible for notifying the diagnostic system, which then turns on the engine light.


As we mentioned earlier, your engine can be more susceptible to a piston slap as it gets older. Poor maintenance can equally cause the problem.

You can prevent piston slap by replacing the pistons or piston rings when they wear.

Failure to do this will cause air to flow between the piston and cylinder, opening the door to oil leakage.

The oil will then travel down to the firing chamber, which can be bad for the engine.

Only purchase quality spare parts for your vehicles since bad piston rings and pistons can subject your car to this problem.

You should consider visiting a good mechanic if you do not have the required knowledge to resolve the problem.

The mechanic may have to dismantle the engine to check the piston. The ultimate solution is to buy a new piston to replace the damaged one.

To avoid future malfunctioning, you should consider replacing all the rings at once.

Piston slap vs. rod knock comparison


Rod-knock is a sound produced when rods in your car engine knock against the crank while changing its rotational direction.

The noise produced by the rod tends to increase in frequency when the engine is under load. How can you know if it is not a piston slap?

Noise due to piston slap will only occur when you start the engine cold but will stop once the engine warms up.

If the noise persists after the engine warms up, it is probably due to rod knocking.

There is a bearing between the connecting rod and the crankshaft. The bearing or bushing can fit in the crank journal via the clearance tolerance in the engine.

The clearance is small but permits the engine oil to flow on the metal surface, protecting it from wear and tear.

Despite the preventive design, the bearing sill wears off after some time.

When Does It Occur?

The bearing tends to wear because of old age and prolonged use; this leads to the knocking sound you hear from the working engine. The bearing does not wear quickly.

You might have to drive the car for hundreds of thousands of miles before the bearing shows signs of wear. Consequently, rod knocking should not occur often.

If rod knocking occurs when the car hasn’t covered much distance, it is usually due to premature wear.

Premature wear can be engine overheating, low oil pressure, low oil level, and dirty motor oil.

An irregular oil change is yet another major cause of premature wear, which causes rod knocks.


When you start hearing a knocking sound in your car engine while warm and running, it is most probably caused by rod knocking.

The noise announces itself by making a knocking or banging sounds like a metal object on an iron door.

The noise increases upon increasing the car’s speed while in motion.

Check the oil light and “check engine” light indicators too.

If the indicator lights come up as the knocking sound persists, it indicates a possible rod knocking problem due to an oil pressure issue.

You can confirm that the problem is due to rod knocking if the oil light and the “check engine” light go off on their own after some minutes.

The engine oil removed from the engine will contain metal shaving to help confirm rod knocks.


One of the first moves against rod knocks is to drain the oil. You can do this by placing a catch pan under the draining plug.

When replacing the oil, make sure the replacement has the same viscosity rating as the previous one or use the oil recommended for that particular engine.

You can insert the plug after draining the oil, but make sure the draining is complete.

You can also change the oil filter and torque it to the recommended specifications.

Failure to put the remedies in place on time can be detrimental to the lifespan of your car’s engine.


A piston slap occurs when you cold-start the engine but stops as the engine gets warm.

A rod knock on its part will persist even after the engine warms up, making it easy for beginners to distinguish between the two.

Both problems can have far-reaching adverse effects on the car’s engine, necessitating a quick resolution.

It would help if you replaced the piston ring to prevent further damage from the piston slap. You should not delay replacing the bearing to stop rod knocks.

You should also use oil with the correct viscosity rating and pour the recommended quantity of oil.