Piston Slap Vs. Rod Knock: What Are The Differences?

Both Piston slap and rod knocks create concern about your engine. To remedy these concerns, you need to understand piston slap vs. rod knock. What are the differences between them?

Know that the sound of piston slap seems like air weeping. But the sound of rod knocks seems like two rods are hitting each other. Also, you can drive with a piston slap for a long time which is not a good idea with rod knocks. Driving with rod knocks can end up breaking the rod bearing, which can cause severe accidents.

It is wise to never ignore both piston slap and rod bearing. These will not only cost you money but also can lead to other engine problems like severe oil burning.

Main Summary

Priston slap and road knocks indicate that your engine needs maintenance. Remember that the piston and piston-cylinder slap happens in the piston and piston-cylinder, but road knocks happen in the crack journal. Also, the remedy cost of rod knocks is way higher than the piston slap.

What Is A Piston Slap?

The piston of a car is fitted into the cylinder by keeping some space between the piston and the cylinder wall. This helps to access fuel and air. The loose fit also works better as there is the side-to-side movement of the piston.

But over time, the gap increases due to wear out of both piston and cylinder. It happens due to the fiction of air and fuel. This makes the piston knock on the cylinder wall and make a slapping noise. This is called piston slap.

What Is Rod Knock?

Rod knock is a sound that happens between the rods and the crankshaft while changing the direction. In every crankshaft, a moderate space is left in the bearing. This is to ensure the flow of oil on the bearing surface. This also helps to maintain the overflow of oil from the cracking journal.

With time the bearing wears off and creates more space. This makes the rod bang and creates noise. Due to the winding of the space, oil can’t be a barrier anymore.

Piston slap vs. Rod Knock

Piston slap and rod knocks are both indicators that your engine needs maintenance. It indicates that both engine parts are wearing off and need to be changed as soon as possible. But to identify the problem, you need to learn what their differences are.


Both piston slap and rod knocks indicate engine fault. But they happen due to two different reasons.

Priston Slap:

Piston slap takes place when you try to start a cold engine. Sound can wear off once your engine heats up.

Experts say this happens as the engine gets old and the Piston wears off, creating a broad distance between the Piston and the cylinder wall. When there is a piston slap, your engine will show a “Check Engine” light.

Rod Knocks:

When you drive your car for a long time, it can decay the bearing. Thus Rod knocks happen.

On the other hand, it can also occur from the premature wearing of the bearing and irregular oil change.


The sound of the Piston slap and the Rod knocks constitute a significant way to differentiate between the Piston slap and Rod knocks.

Priston Slap:

The sound of a piston slap is on the flats’ frequency. You will hear the sound when you start a cold engine. Once the engine heats up, the sound might not be there.

Rod Knocks:

Rod noises begin after the engine starts and the vehicle starts moving. Sound is more like two roads banging with each other.


Driving with both Piston slap and rod knock is possible, but this is not something one should be doing as they indicate issues with their engine.

Priston Slap:

Pistons slaps occur at the beginning of driving. When the engine warms up, the sound will not be there. But over time, the sound will get heavier. You can keep going with the Piston slap until you see a burning oil sign.

Rod Knocks:

You can keep on driving even with the Rod knock up to some miles. But the time is limited. If you continue driving, the rod can break and cause a severe accident.

Remedy Cost

The only remedy to avoid Piston slaps and rod knocks are doing maintenance by replacing the piston, cylinders, and bearings. The cost of this maintenance will vary.

Priston Slap:

The maintenance cost of the piston slap is not that much compared to rod knock. Including the parts and labor, it can cost between $100 to $200.

Rod Knocks:

Maintenance of rod knocks is quite expensive. It can cost you between $2000 to $3000, including the labor cost.

The remedy of rod knocks comes with the replacement of gasket, seals, rod bearings, head bolts, and cooler lines. Thus it cost way more than a piston slap.

Comparison Chart

To compare the Piston slap with rod knock, a comparison chart can be constructive. Here is a chart that can help you to summarize the comparison.

 Priston SlapRod Knock
SoundBlunt soundKnocking of metal bars
PartsPiston CylinderRod Bearing and Crack Journal
DrivingPossible until oil burningOnly up to a few miles
Remedy Cost$200 – $300$2000 – $3000

Frequently Asked Questions (FAQs)

1. What is piston slap caused by?

Piston slap is caused when the cold running clearance is ample. While rocking, it slaps the side of the chamber and causes noise.

2. Is Rod knock the same as engine knock?

Rod knocks are a kind of Engine knocks. But all engine knocks are not rod knocks.

3. How long will an engine last with a piston slap?

A few engines will run about 100,000 miles with a piston slap, while others will fall to pieces in 30,000-40,000 miles.

4. How do you know if your engine has a rod knock?

The most recognized rod knock symptom is the sound. With a rod knock set up, you will hear a banging or knocking sound as though somebody is hitting metal on an iron object when you start the vehicle. The noise will enhance as you press up the gas pedal.

5. Does rod knock only happen under load?

Rod knock comes from the engine. It can make a sound when under pressure, but generally, that is not the case.

6. Does rod knock get louder with RPM?

Rod knocking noises are frequently clearly at high rates (more than 2500 mph). Subsequently, padding the gas pedal may set off a clatter toward the back, which happens during quick travel somewhere in the range of 2500 and 3000 rpm.

7. Can rod knock go away and come back?

It’s unusual for a rod knock to go away and then come back. Sometimes after heating up, you might not hear the sound much louder, but it is there and can be heard during circular rotation.


To identify what is wrong with your vehicle’s engine, it is wise to know about Piston Slap Vs. Rod Knock and learn the differences. It helps to recognize the issues with the car and treat it accordingly. Otherwise, you will need to do a thorough checking. This will cost you both money and time.

The severity of rod knock is more than the piston slap. But it is necessary to take care of both problems. Even though the reason for the piston slap can also cause the engine light to blink, the rod knocks need to be recognized only by sound. Therefore, be concerned about any odd sound your vehicle makes while you start driving.

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