Piston Slap – What Is It & Why Does It Occur?

Car Pistons

If your vehicle is making a knocking or rattling sound, it might be a piston slap. In many cases, this frightening sound indicates that one or more pistons are failing. However, you probably will never hear this noise in a newer vehicle, as it almost only happens on the older car models.

We discuss what piston slap is and what might cause it. We will also look at how to repair a piston slap. 

What Is Piston Slap?

Piston Slap
Piston slap occurs when the clearance between the cylinder wall and the piston is too large

Piston slap happens when the piston rocks in the cylinder. Instead of solely moving up and down as it should, it rocks back and forth. In many cases, it means that too much room exists within the cylinder for the piston. When everything operates normally, the piston should be held in place by the walls. As the skirt of the piston hits the cylinder walls, knocking and rattling occurs. 

As cars age, piston slap presents itself because of normal wear and tear. In many cases, the noise is louder during idling. As we told you, it is very rare with piston slap in newer car models, so therefore we strongly recommend doing a proper diagnose before taking the engine apart.

RELATED: What is Rod Knock?

What Causes Piston Slap?

Worn Piston

The main cause of piston slap is worn pistons or cylinder walls, creating a too big distance between the piston and the cylinder wall.

When the piston to wall clearance becomes too large, the piston begins rocking side to side, creating what is known as piston slap. It happens more frequently with aluminum blocks than iron. Additionally, there might be louder piston slap when the engine is first started because of extra cold clearance. 

In modern vehicles, the ECU alerts if the piston variations are off at all. Sadly, the only way to deal with a piston slap is to figure out what is causing it and repair the problem.

RELATED: 6 Causes of a Knocking or Pinging Car Engine

Check Engine Light

If the piston begins to fail, you might see the Check Engine Light come on. With modern vehicles, the onboard diagnostic system sends out an error code when something is wrong.

With an OBDII scanner, you should be able to see the exact cylinder that is affected. However, knocking can also be caused by other issues, such as a dirty fuel injector or trouble in the ignition system. 

Burnt Piston

Fuel injectors can get dirty over time. If left unchecked, the dirty fuel injector can lead to a hole in the top of the piston.

Additionally, using the wrong spark plugs can lead to burnt pistons. The only way to see this problem is to open up the top of the engine.

Piston Slap Fix

Understanding what piston slap is helps you figure out how to fix it. You will need to determine what has caused the problem to completely repair it. Start by checking the pistons to see what condition they are in. This requires you to take apart the engine. If you don’t have the experience for this, you want to take your vehicle to a licensed mechanic.

In most cases, the piston slap is caused by this vital part knocking on the cylinders. In this case, the only way to fix the problem is to get a new piston. Prior to this fix, you need to perform an engine compression test. If there are other issues with the engine or if your vehicle is older, it might not be worth replacing the piston. 

If you decide to move forward with a piston replacement, make sure you swap out the rings for new ones. Defective piston rings could have led to some of the problems you are experiencing. Plus, now would be the time to replace the piston rings because the engine will already be apart.

Replacing a piston can cost $1,000 or more, making it a very expensive repair. That’s why many people with older vehicles choose not to replace the piston at all. At first, the bad piston is going to create a lot of noise, but it might not lead to permanent damage. If you plan to keep your vehicle, you will want to get the piston replaced before it leads to more severe engine damage.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.