What's Piston Slap and Why Does It Happen? What is Piston Slap?

A piston slap could be the cause of your car making an unusual sound like a knocking, rattling or popping noise. This can be a sign that one of your pistons has failed. You will probably not hear it in an older vehicle as this sound is rarer on older models.

This article will discuss the causes of piston slap and how to prevent it. Also, we will discuss how to fix a piston slippage. Let’s begin with the basic information.

What is Piston Slap?

Piston slap occurs when the piston moves in the cylinder. The piston does not move as smoothly up and down, but instead it rock back and forth. It can also indicate that there is too much space within the cylinder to accommodate the piston.

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If everything works normally, you should keep the piston in position by using the walls. When the piston skirt touches the cylinder walls it will knocking or rattling. 

Piston slap develops as cars age due to normal wear and tear. Many times, the noise during idle is more noticeable. It is rare to hear the noise when you are idling in modern cars. We recommend that you do a thorough diagnosis before removing any engine parts.

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Why does Piston Slap happen?

Wearing pistons, or cylinder walls is the main reason for piston slap. They create a gap between the piston and the wall of the cylinder. Piston slap occurs when the distance between piston and wall becomes too great.

Aluminum blocks are more likely to experience this than iron. Because of the extra cold clearance, you might hear a louder piston click when your engine starts. 

The ECU in modern cars will alert you when the piston variation is off. Unfortunately, there is no way to fix a piston slippage other than to find the cause and correct it.

Related: Six Causes for a Pinging, Knocking Car Engine

1. Verify Engine Light

The Check Engine Light might come on if the piston is beginning to fail. Modern vehicles have an onboard diagnostic system that sends out error codes when there is a problem.

An OBDII scanner should allow you to pinpoint the cylinder affected. Knocking could also occur due to other problems, like a dirty fuel injector and a trouble with the ignition. 

2. Burnt Piston

Fuel injectors can get dirty over time. A dirty fuel injector could lead to damage at the top of your piston if it is not taken care of.

Also, burnt pistons could result from using incorrect spark plugs. To see the problem, you must open the top of your engine.

How to fix a piston slap

It is important to understand what the problem is and then figure out how it can be fixed. In order to fix it completely you'll need to find out what is causing the issue. Check the pistons first to determine if they are in good condition. You will need to disassemble the engine. If you don’t have the experience for this, you want to take your vehicle to a licensed mechanic.

Most cases of piston slap are caused by the vital component knocking on the valves. The only solution is to replace the piston. Before you can do this, it is necessary to conduct an engine compression test. The piston may not need to be replaced if you have other engine problems or your vehicle has an older model. 

You should replace the piston rings if you are going to do a replacement. Some of your problems could have been caused by defective piston rings. The engine is already apart so it would make sense to change the piston rings now.

The cost of replacing a piston may be $1,000 to $1000, so it can become a costly repair. That’s why many people with older vehicles choose not to replace the piston at all. While the noise from a bad piston will initially be annoying, it may not result in permanent damage. It is important to have the piston changed if your plan is to maintain your vehicle.