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

Piston slap

Has your car ever started making a rattling or knocking sound? This could be a sign that your piston is failing. Most likely, you will not experience piston slap if you have a modern car. This is because there is a diagnostic system in the newer cars. If you have an older car, you could experience a piston slap. 

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What does the diagnostic system do? Modern cars have motion sensors that regulate the air-fuel mix. These sensors are used to help the computer system detect a problem. So what happens when your piston is failing? You will notice a rattling or knocking sound, misfiring, a loss of power, and oil will start burning. Additionally, you might notice that your check engine light is one. You will want to get this issue checked out as soon as possible. 


What Is Piston Slap?


Piston slap is defined as the rocking of the piston in the cylinder. This means that it is not moving up and down the cylinder as it desires it is being forced to. Why does this happen? This will occur when there is too much room for the piston within the cylinder. The rattling and knocking sound comes from the skirt of the piston hitting the cylinder wall. This will typically occur due to wear and tear over time or as the car starts to age. 


You might be wondering when does this noise typically occur? The noise will happen when the car is idling or overrun. Often the two-stroke single-cylinder race engines will suffer from this. If pistons are set up and machined to the correct specs, slap should not occur. If you hear a noise like this, you should always get it checked out. Ignoring the issue could lead to failure over time or worse problems with your engine. 


What Causes Piston Slap?


Since the piston to wall clearance is large, it can cause the piston rocks to slide from one side to the other. This will cause piston slap. This problem tends to happen to aluminum blocks rather than other materials. If the pistons experience more cold clearance, they will gain more velocity. This ultimately causes a piston slap because the piston moves from the minor to the major thrust side. 


It is important to note that modern cars have a special sensor that will notify the cars ECU system of any piston variations. The ECU will pull ignition timing from its system and protect the car’s engine from false knocking. If you don’t have this system, you can also use an offset pin to adjust the motion of the piston. What will that do? It will prevent the piston from hitting the cylinder. 


What Do Pistons Do?


You might be wondering what is the purpose of pistons? A piston is a cylinder that will move up and down inside the engine block. How does this movement occur? There will be a mixture of air and fuel that will explode inside the combustion chamber and then move the piston up and down. Furthermore, the piston is connected to a wrist pin. This connects to other parts of the engine.


Do you know how many chambers most cars have? The majority of cars have either four, six, or eight chambers. Some of the newer and faster cars have up to 12 chambers. Even though cars have many chambers, combustion can only happen within one chamber. 


There is a lot of heat created in the combustion chambers. The pistons are lightweight so they can move easier. Nowadays, they are often made from aluminum alloy which allows them to move up and down the cylinder easier. Back in the day pistons were made from cast iron. This material was used because it could handle the extra heat. 


Within this system, there are the pistons and the piston rings. Since the piston and the cylinder don’t fit perfectly together, compression ring seals are used to seal that gap. If these rings start to wear out over time, you will notice that smoke will be emitted because they are starting to fail. 


Another part of this system is the oil ring. This part is a combination of two rings. It is used to wipe unnecessary oil from the cylinder wall. This is important so you don’t get an oil buildup within the engine. If these rings wear out, oil will start to leak into the combustion chamber. This could lead to very bad problems in the future. You want to make sure you get this fixed as soon as possible so you don’t completely damage your engine. 


Diagnosis Of Piston Slap 


If you have pistons that are failing, often your check engine light will turn on. This will turn on to notify you that there is an issue somewhere throughout your car. If the car has a diagnostic system, there will be an error code. This error code will help you detect what the issue is coming from. The error code will show which cylinder is misfiring. Sometimes there could be a false reading because of a broken sensor. These sensors are critical in telling the system whether there is an issue or not. The sensors will notify the diagnostic system and then that system will turn on the engine light.


Why do misfires occur? This could happen for many different reasons. Some of these reasons include bad ignition coils, bad spark plugs, a dirty fuel injector, or a bad spark wire. It is important to note that those parts won’t cause the piston slap. Those parts will cause blue smoke to be emitted by the exhaust system though. If this blue smoke is produced, it can be bad of the environment and bad for you to breathe in. This smoke often has harmful gases. 


Snapped Timing Belt: Do you know what a timing belt is? The timing belt is a critical part of every car. It helps fine-tune the movement between the pistons and the valves. So you might be wondering what happens if the belt snaps? Well, the pistons and valves will crash into each other. This is not good for your engine system. It will cause more engine issues and you need to get this problem fixed as soon as possible. 


Worn Piston Rings: Wear and tear occur over time with almost any part in your engine system. The piston rings will start to wear out over time if they have not been replaced or properly maintained in a long time. When this happens, air will flow in between the spaces of the cylinder and the piston. 


Oil will also start to leak into these spaces and travel down to the firing chamber. If this occurs, this is very bad. The car will produce white smoke and take in a lot of engine oil. You will damage your engine system if you let this go on for too long. It is dangerous to have oil in places where it shouldn’t be. Always take your car into a mechanic if you notice an issue with your car engine. 


Burned Piston: It is important to make sure you properly maintain your car to try and avoid faulty car parts and repairs. If your fuel injections accept too much dirt into them, holes will start to develop on the piston. The dirt will wear down the material and create holes. You will be able to detect this problem if you open up your engine. If you find holes in your piston, your system will not work properly and the piston slap could occur. You need to get this problem resolved immediately. 


How Do I Fix Piston Slap?


Now you understand what piston slap is but how can you resolve the problem? The first thing you will want to do is check your pistons. You need to determine what type of condition they are in. Most likely you will need to see a good mechanic so they can take apart the engine for you to look at the pistons. It is always smart to have a professional take apart your engine because they know what they are doing and they know how to put it back together.


Piston slap occurs from the skirts knocking on the cylinders. If this is the root of your problem, you will need to get a new piston. You need to make sure the engine loses its compression before you replace the pistons. It is always smart to replace all of the rings as well because those could be malfunctioning as well. You might as well replace all of that while the engine is taken apart. 


It is important to note that piston slap is very annoying because it causes loud noises, but it won’t initially cause too much damage to your car if you get it fixed right away. The good news is if the pistons are too worn down you can drive your car for a few miles as long as you don’t notice burning oil. You can not ignore this problem though because you will damage your engine if you don’t fix this. Oil will eventually leak into your engine which could completely damage it. Always go get the proper diagnosis from a mechanic if your engine is making noises because piston slap could be occurring. 


Is Piston Slap Bad?


If you let piston slap occur for too long, it is very harmful to your engine. The longer you wait to fix this issue, the more noticeable the problem will become. Your cylinder walls and piston clearance will continue to get higher. Furthermore, if your pistons are aluminum, they can damage pistons when you rev up a cold engine.  


You might be wondering why do aluminum pistons cause this? This happens because the aluminum pistons have not been able to hold in enough heat yet to grow into the proper shape. If this happens, they will rattle against the motor block’s cylinders. To solve this problem, you will need to wait to rev your engine until it is warm. 


Piston slap can be very embarrassing because of the loud noises that come from your engine. Can you imagine driving a coworker to work and your car starts making loud rattling noises and emitting smoke? This will happen if you experience a piston slap. You should always get your car checked by a mechanic the moment you hear noises from your engine. The longer you wait, the more damage will occur to your engine. You want to preserve your car’s engine because it is very expensive to repair if you completely damage it. 




Do you hear a rattling noise coming from your engine? It is critical that you get it checked out as soon as possible. If you continue to drive with a piston slap, you could damage your engine which will lead to worse problems. Don’t continue to drive until you get a proper diagnosis because it puts you and other individuals on the road in harm’s way. 


Now you have to decide whether you want to fix your car or not. Do you think your car is worth the money it costs to fix this problem?  Have you been spending a lot of money replacing different parts in your car lately and is your car aging? You might want to consider not spending the money to repair your car. Don’t feel pressured by a mechanic because you have other options!


Our company sells refurbished cars and car parts at the best price. We will give you cash for your car so you can put that money towards a newer car. Are you interested in selling your car to us? Head over to our website and get a quote. You can also call us at any time and we will answer any of your questions or concerns.  


Car repairs can add up and cost a lot of money. If your car is starting to age and you feel like it is not worth it to fix, you don’t have to spend the money. You can sell your car to us instead. Contact us today to get a quote!


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