Piston Slap – What Is It And Should You Worry About It?

Are you hearing a knocking or rattling noise from your engine? You may have a piston slap. What’s a piston slap, you ask? Well, that’s what we’re going to discuss here. We’ll be talking about what a piston slap is, whether or not you should worry about it, and everything else you might need to know about your engine’s pistons.

• Piston slap
• Symptoms
• Fix
• Other engine knock causes
• Conclusion
• FAQs

Car Piston

Before we got to the piston slap, first you will need to understand what a piston is. It’s basically a disk or a thick metal plate that fits inside your engine’s cylinder. The pistons are connected to a rod that connects them to the crankshaft. As the piston moves up and down, it will move the crankshaft which will then transfer that rotation to your wheels via a driveshaft. To really understand what a piston is for and how it works, we’ll have to talk about how an internal combustion engine in your car works:

What Is A Piston

Modern internal combustion engines (or ICE for short) are now four-stroke engines. This means it goes through four stages in each cycle: intake, compression, power, and exhaust. During the first stage, your engine will take in fuel and air into the cylinder via the intake valve. During this stage, the piston will move downwards to create suction inside the cylinder.

Afterward, the cylinder will begin the compression stroke. At this stage, the piston will move upward, compressing the fuel and air mixture and preparing it for combustion, which will begin the power stroke. At the power stroke, the fuel and air mixture will make a controlled explosion.

If you have a gasoline engine, then the combustion happens with the help of a spark plug. The spark plug creates a small electric spark at its tip to trigger the explosion. Meanwhile, a diesel engine makes combustion by simply using the high compression it has without the help of a spark plug.

When the fuel and air mixture is ignited, it creates an explosion and pushes the piston downward again. This will then turn the crankshaft that powers your car, which is why it’s called the power stroke. Finally, your piston will move upward again to push out the burned gas via the exhaust valve. Afterward, the process begins again from the intake stroke.

You can learn more about how an internal combustion engine works by watching the video below:

What Is A Piston Slap

Okay, now that you know how an engine works, it will be easier to understand what a piston slap is and why it happens. The pistons in your car fit very snuggly in the cylinder, which means there is very little clearance between the pistons and the cylinder walls. This is fine since pistons require little side-to-side movements when they go up and down. This also helps to prevent the fuel and air mixture from escaping.

Over time, the piston and cylinder wall may wear down due to the heat and friction. When they wear down, there will be more room between the piston and the cylinder wall. Once this happens, the piston will rock in the cylinder, and the skirt (that’s the cylindrical wall of the piston) will slap the engine’s cylinder wall.

This will then create that rattling or knocking noise you hear from your engine. This mostly happens when the engine is idling or during the overrun (that’s when you let go of the throttle and the engine’s RPM goes down).

Some engines are more susceptible to piston slap, especially engines with aluminum pistons or cylinder blocks. Aluminum is a lighter material than iron which is why it’s often used in some performance cars to save weight. It’s also better at transferring heat to the coolant, which helps to keep the engine cool. However, they’re not as strong as cast iron which is heavier but more durable, that’s why aluminum engines are more susceptible to piston slap.

Bad Piston Symptoms

There really aren’t any other symptoms of piston slap other than a knocking noise coming from your engine. You might notice a drop in performance or poor fuel economy, but this is unlikely to happen. In fact, if you have a relatively modern car, it’s unlikely you will experience a piston slap.

This is because modern cars now have an ECU module or Engine Control Unit. The ECU uses a variety of sensors to adjust many things about the engine to make sure it can run smoothly. For example, if the engine isn’t getting enough air, it will compensate by injecting more fuel into the engine so it can still work. You will still need to fix the air intake problem, but the engine will work just fine for a period of time.

One of these sensors includes a knock sensor, which is basically like an ear for the ECU that listens for irregular sounds and vibrations coming from your engine. The ECU will then use the information to adjust the engine’s ignition timing to eliminate the irregular knock. We’ll talk more about why engine knocks might occur later on.

Also, the ECU will send a signal to turn on the check engine light when something is out of the ordinary. This means it will notify you of a problem and you can fix it before the piston slaps even occur.

Piston Slap Sound

The only way to verify that you have a piston slap is to dismantle your engine and check the condition of your pistons. This will take a lot of time and money, so we don’t actually recommend doing this unless the knocking noise is accompanied by other symptoms. For example, a snapped timing belt may be caused by piston slaps. But most of the time this is because of age or water pump seizure. Also, if the timing belt in your car is over 10 years old, then it’s already time to replace it anyway.

Other than dismantling the engine and checking the pistons physically, there’s really nothing else you can do to verify a piston slap.

Is Piston Slap Bad

There’s a lot of debate online on whether or not the piston slap is actually bad. But the general consensus is that the piston slap isn’t really something you will need to worry about, at least not immediately. You can still drive with a piston slap, but it may be uncomfortable and annoying if you can hear that knocking noise from the engine. They may cause further damage to the engine in the long run, but it really isn’t a big problem.

We recommend paying attention to your car’s exhaust gas. A piston slap may wear down the piston rings more quickly, which can lead to an oil leak into the engine. When oil leaks into the engine’s cylinder, it will be burnt along with the fuel and air mixture, which creates blue smoke from your exhaust pipe. When this happens, you will need to replace the piston and the rings. Leaving it unfixed will cause more damage to other components such as the catalytic converter.

How To Fix Piston Slap

As mentioned, piston slaps aren’t an immediate cause for concern. But once you see other symptoms such as smoke from the exhaust, then it’s time to fix your engine. At this point, your only real option is doing an engine rebuild. This process involves dismantling the engine, replacing all the damaged and worn-out parts with new ones, and then rebuilding it again.

Obviously, engine rebuilds are expensive. You can get away with just replacing a few parts to save costs. For example, if you only need to replace the piston rings, they’re only around $200 a set and it’s even cheaper for some cars. However, because of the labor cost, this will still cost you anywhere between $1,000 – $2,000.

Dismantling and rebuilding an engine is a long and laborious process and takes 10 hours to do on average. This is why we recommend you do a full rebuild so that you replace other necessary components as well while you’re at it. With this, your engine will be as good as new.

Naturally, this is not an option for everyone, since engine rebuilds can cost as high as $4,500 and even more in some cars. If this isn’t an option, then you’re better off selling your car and putting the money towards a new car. You will have to sell it at a lower price since the car is technically damaged. But at least this will put cash on your hands.

We don’t recommend doing an engine rebuild yourself to save money unless you’re a trained professional mechanic.

Engine Rebuild

Before doing an engine rebuild, you will need to assess whether it’s actually worth it or if will you be better off just selling the car? If the cost of the rebuild isn’t far off from your car’s resale value, we would actually suggest just selling it as-is. But if it’s still worth it, or if you still want to keep your car for sentimental reasons, we have a few tips.

The first is obviously to shop around so you can get the best possible estimate. However, this is a very invasive and costly repair job, so it’s also important to have an auto repair shop that you trust do it for you. This is so that you can be sure of the fact that the repair shop won’t charge you for anything you don’t need.

It’s also a good idea to remind the mechanic to contact you before they replace or repair a part that isn’t in the original estimate/quotation. This way, you can approve or disapprove of the job they’re about to do. You also won’t be surprised by unmentioned repair jobs when you receive the bill.

Finally, if you have a good knowledge of engine parts, it might be cheaper for you to source the replacement parts yourself. Auto repair shops may charge a premium over certain parts, so you might be able to find parts for cheaper elsewhere. Keep in mind that not all repair shops are willing to work on your car if the part doesn’t come from them.

What Causes Rod Knock

Piston slap isn’t the only cause for engine knocks. There are a few reasons why you’re hearing knocking and rattling noises from your engine. Some are minor issues with fairly easy fixes, some might be more serious. Here are other possible causes of engine knocks:

1. Low Octane Fuel

An octane rating is the measure of a fuel’s ability to resist premature detonation or combustion of the fuel and air mixture in your engine. Using fuel with a lower octane rating than recommended will combust the mixture when it isn’t supposed to. This will then create a knocking or pinging noise in your engine.

If you recently filled your car with fuel that has a lower octane rating than the manufacturer’s recommendation, fill it up with the correct or higher octane fuel the next time you make a stop. This should eliminate the knocking noise.

You can also use an octane booster to pair with lower octane fuel to improve performance. But often they will cost just as much as using high octane fuel, so we don’t really recommend this.

2. Carbon Deposits

The combustion process in the engine will leave carbon deposits on the engine valve, spark plugs, and other components involved in the combustion process. Over time, the carbon deposit will build up and obstructs the combustion process. This will then reduce the amount of volume inside the cylinder and increases the compression, which will result in a knocking noise.

Most of the time, you can resolve this issue by using a fuel injector cleaner (and learn how to clean fuel injectors). They usually cost around $20 a bottle, and using them is very simple. You will need to have a nearly empty fuel tank, and then pour the appropriate amount of fuel injector cleaner. The manufacturer will specify how much you will need. Then top up the fuel tank with your regular fuel. Some cleaners may require you to idle the car for some time, so be sure to check their instructions before using it.

If the fuel injector cleaner doesn’t work, you may need to do a carbon cleaning. But a carbon cleaning can cost anywhere between $350 – $900. We don’t actually recommend doing this unless you feel you need to.

3. Incorrect Spark Plug or Spark Plug Gap

Your car’s manufacturer will often recommend what type of spark plugs you should use for your car. This is because a spark plug has a heat range, which is the amount of heat it can withdraw from the combustion chamber or your cylinder. If you use the wrong type of spark plug, it won’t work properly and can lead to a knocking noise.

Additionally, a spark plug has a gap (once you learn how to gap spark plugs) between the center and the ground electrode, which will need to be set correctly. If it’s too narrow then the spark might not be powerful enough to ignite the fuel and air mixture. Too wide and the spark might misfire rapidly or even not fire at all.

Check if your car is using the correct spark plug and if has the correct gap. This video below can teach you how to gap your spark plug correctly:

4. Bad Ignition Timing

If you’re using the correct spark plug with the correct gap, then you may have an ignition timing issue. Your car’s ignition system has a timing, that is, at which point in the engine’s travel the spark plugs will fire to ignite the fuel and air mixture. And much like telling your crush that you like them, the timing has to be correct. Otherwise, the spark plug won’t fire correctly leading to multiple detonations. This in turn will create an engine knock.

5. Too Lean Fuel and Air Mixture

The fuel and air mixture in your car has to be set correctly depending on the condition. It can’t be too lean or too rich or it may affect the car’s operation and even damage parts. A faulty oxygen sensor, fuel injector, or MAF sensor may cause your car to have a mixture that’s too lean. This is because it’s feeding incorrect information to the ECU. This then leads to the ECU making incorrect adjustments to the mixture.

A lean fuel mixture means the mixture has more air than fuel. And if it’s too lean, the mixture won’t burn fast enough which can lead to multiple detonations and engine knocks.

6. Faulty Knock Sensor

As mentioned, the knock sensor notifies the engine control unit if it detects any irregularities in the engine’s sound and vibration. The ECU will then make the appropriate adjustments to correct the problem automatically without any help from the driver. Convenient, isn’t it?

Obviously, if the knock sensor goes bad, it won’t be able to send the correct information to the ECU. This in turn leads the engine not be able to fix some of the minor problems. If this is the case, you will need to replace your engine’s knock sensor. This typically costs between $250 – $400 depending on the car’s make and model, and this includes labor.

7. Worn Rod Bearings

As previously explained, the piston in a car is connected to the crankshaft by using a connecting rod. The rod connection to the crankshaft has bearings to facilitate smooth piston movement. These bearings will wear out or come out of position over time. When it does, the piston will then rattle against the crankshaft which creates a knocking sound.

When this happens, you will basically need an engine rebuild since these parts are deep within the engine. This means a rod-bearing replacement job is very expensive and can cost anywhere between $1,000 – $2,000. This is why rather than replacing an individual part, it will be good to replace all the necessary and worn-out parts in an engine and do a full rebuild. It’ll cost more, but you can expect the engine to be trouble-free afterward.

8. Bad Belt Tensioner or Pulleys

Sometimes engine knocks aren’t actually coming from the engine itself. Instead, the sound you’re hearing may be coming from the engine’s serpentine belt that is connected to several pulleys. This belt has to be under the right amount of tension for it to be able to work properly. If there isn’t enough or there’s too much tension, you may hear a rattling or slapping noise that can be mistaken for a knocking engine.

The serpentine belt may also wear out over time. If you see any damage to the belt, then it’s time to replace it. This usually costs between $100 – $200 for most cars including labor. If you don’t see any damage, then what you need to do is readjust the tension of the belt. You should also check if any of the pulleys are bent.

Piston Slap Vs Rod Knock

If your engine is knocking, it might just be a false knock. A false knock is when the engine makes a knocking noise but there isn’t really a problem with the engine. Some engines are susceptible to this such as the LS1 engines in a Corvette. This often happens during a cold start, why does this happen? The reason has to do with metal expansion and contraction.

The pistons in your car will expand when it’s hot, and contract as it cools down. Because of this, the pistons are designed to be slightly smaller than the cylinder bore (the diameter of the cylinder). This will allow the piston to contract when it’s hot and fits perfectly in the engine’s cylinder during operation.

Since the piston is smaller when it’s cold, the piston and cylinder wall has more clearance. This causes the piston to slap the cylinder walls during a cold start, creating that knocking noise. Once the engine gets up to operating temperature, the piston will expand and fits the cylinder, eliminating the knock. Since there isn’t any problem with the engine, and it’s just how it was engineered, it’s a false knock.

Learn more about false knocks and how to diagnose them in the video below:

Piston Slap: In Conclusion

To recap, the piston slap is a knocking noise that happens when the pistons slap into the cylinder wall because of excess clearance between the two components. In most cases, a piston slap isn’t a cause for concern as it won’t immediately damage other components. And replacing the pistons just because of a piston slap isn’t worth it because it’s very expensive since you’ll have to rebuild the engine.

If you hear knocking sounds from your engine, make sure it isn’t something else that’s causing it. If it’s caused by bad rod bearings, a faulty knock sensor, or bad ignition timing. But if it’s a piston slap, then you generally don’t need to worry unless it’s accompanied by other symptoms such as blue exhaust smoke.

Piston Slap Essential Knowledge

1. Piston slap is a phenomenon where the rocking of a piston in a cylinder bore causes unwanted noise, especially on cold start.
2. Piston slap can be caused by several factors, such as short piston skirts, larger piston-to-wall clearances, and aluminum blocks.
3. Piston slap can also be more prominent in high-performance aftermarket applications, such as supercharged, turbo, nitrous, and endurance engines, which require wider tolerances and more clearance.
4. JE Pistons recommends a unique coating called “Perfect Skirt” to eliminate piston slap, which reduces the necessary piston-to-bore clearances and stabilizes the piston in the cylinder.
5. Perfect Skirt is a patented coating that is screen-printed and cured to the piston skirt, which permanently bonds a solid film lubricant to the bare aluminum and reduces running friction.
6. Perfect Skirt also helps prevent “false knock,” a performance-killer that can result in acoustic knock detection, where the rocking of a wide-clearance piston triggers the knock sensors in an engine, causing the ECU to pull timing, boost, or both.
7. False knock can be hamstrung by JE’s Perfect Skirt, which keeps the piston centered in its bore and prevents the knock sensors from detecting noise.
8. Perfect Skirt is designed to conform to the cylinder bore, reducing piston-to-bore clearances, and stabilizing the piston in relation to the cylinder axis.
9. Perfect Skirt is recommended for engines like the Gen III/IV LS and Gen V LT, VR38DETT, 2JZ-GTE, and 4.6/5.4/Coyote.
10. JE’s Perfect Skirt coating is designed to reduce noise, stabilize the piston, improve ring seal, wear, and general longevity of the piston.

FAQs On Piston Slap

If you’re still curious to learn more about a piston slap, our FAQs here might help…

What Is Rod Knock

Rod knocks are deep, rumbling, knocking sounds emanating from the engine. Oftentimes, it’s an indicator that there’s some wear and tear or physical damage on your engine’s rod bearings. It’s possible that there’s too much clearance between the connecting rod bearings, causing it to move excessively. It’s worth mentioning that the aforementioned rod clearance is crucial, as it needs to fit within a set tolerance. Otherwise, and while your pistons are moving, this excess motion may cause the connecting rods to impact or rub against the engine’s crankshaft. Rod knocks are usually caused by premature wearing, such as due to running with low or contaminated motor oil.

What Does A Rod Knock Sound Like

Rod knocks, as their name implies, sound like hard knocking from the engine. It can sound like a loud bang or knock within the engine as if two metal objects are hitting each other at speed. Usually, this knocking noise will get louder when you rev the engine high up and let off the gas. If you do hear this knocking sound coming from the engine, it’s best that you stop driving immediately. To be clear, rod knocks aren’t always a sign that your engine will be destroyed at an instant. However, driving even just a few minutes with rod knock present can severely increase the chances that your engine will blow. If not, it’ll cause further internal damage to the engine.

What Causes A Rod Knock

In most cases, rod knocks are caused by premature wear and tear of the connecting rods. This, in turn, is caused by poor maintenance, such as not changing the oil regularly or running your car with a low level of oil. Other than that, it may also be caused by using the wrong type of oil or driving around with low oil pressure. In short, engine oil without its lubricating abilities can lead to a build-up of sludge and corrosion within the engine, causing rod knocks. Additionally, rod knocks may also be attributed to a bad timing chain tensioner, faulty main bearings, worn-out water pump bearings, cracked flywheel or flexplate, as well as malfunctioning alternator rotor bearings.

What Causes Piston Slap

The primary cause of piston slap in your engine is due to either worn-out pistons or even damage to the cylinder walls. In essence, this contributes to adding more clearance and space between the pistons and cylinder walls. When there’s too much of a gap or clearance between them, the pistons would essentially rock side-to-side, thus, hitting the cylinder walls. This is how piston slapping sounds occur. Piston slaps are more frequent with engines that feature aluminum blocks rather than cast iron. Moreover, you might notice how piston slaps are much louder during a cold start, as the colder temperatures contribute further to this excess clearance between the pistons and cylinder walls.

How Does A Piston Work

Pistons are moving disks, fully enclosed within the cylinder of your engine, and further reinforced by piston rings to ensure that it’s gas-tight. The void between the top of the cylinders and the pistons is what we call the combustion chamber. This is where air and fuel enter, and are subsequently ignited by the spark plugs. The force of this explosion alone would push the pistons downward. In essence, it transfers the impact of this combustion and expanding gases to the crankshaft, where the pistons are connected to. The pistons’ reciprocal motion translates into rotation motion with the crankshaft, which then provides energy to the transmission and onto the driven wheels to move your car.

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