What Is Piston Slap and How Can You Get Rid of It?

Vehicle engines produce dozens of sounds. Some of them are normal, while some of them aren’t.

Many car owners dread hearing knocking and pinging noises from their engines because they’re most likely due to an underlying issue that may cost a lot of time and money to fix.

Aside from these two, there’s another kind of noise you should be on the lookout for一the piston slap.

What Is Piston Slap?

Piston slap is a type of engine noise that occurs when there’s a large cold running clearance or piston-to-wall clearance that causes the pistons to rock from side to side, slapping the side of the cylinder in the process.

Note that the pistons should only move up and down. They shouldn’t have any other motions aside from this.

In most cases, this sound can be heard when the engine is cold. The noise usually decreases when the piston expands during engine operation.

What Causes Piston Slap?

An undersized or improperly shaped piston or an oversized cylinder bore are probable reasons why a slapping noise could be heard from the piston.

These issues are often the result of a design flaw, but regular wear and tear from these mechanical components can also lead to the same outcome. This is why so many pistons have graphite-coated skirts to minimize wear that causes piston slap.

Should You Worry About a Piston Slap?

Internal combustion engines and compressors have a certain amount of piston slap, but it’s an entirely different story when the slapping noise gets loud enough that it’s heard from inside the cabin when the music is on.

There’s been some debate about the severity of a piston slap. Some drivers argue that it doesn’t harm the engine and there’s nothing left to do but to live with it.

Two-stroke single-cylinder engines are known to have piston slap, and some run fine for years without mechanics trying to get rid of the noise.

There are, however, other drivers who strongly disagree with leaving the piston slap as it is and recommend taking the affected vehicle to a mechanic to avoid possible engine damage.

Effects of Leaving a Piston Slap Unaddressed

A loud piston slap can be dangerous for the engine to a certain extent. Here are some of the possible repercussions of ignoring this noise:

Premature Wear of Piston Rings

Piston rings have three major functions in an engine:

• Form a sliding combustion chamber seal to prevent high-pressure combustion gases from leaking past the piston (although this isn’t a perfect seal, so there is some blowby)
• Prevent engine oil from getting into the combustion chamber
• Transfer some of the piston heat to the cylinder wall

Most engines have two types of piston rings一one for compression and another one for oil control.

Compression rings form a seal between the moving piston and the cylinder wall, which lets the engine get maximum power from the combustion pressure.

Meanwhile, the oil control ring makes it possible for the engine oil to return through the expander and openings in the piston.

Too much piston slap can cause the piston rings to wear prematurely. Once this happens, the vehicle can exhibit signs of acceleration and fuel economy problems.

Like other engine troubles, it’s important to address this one immediately to avoid other problems from arising.

Burning Engine Oil

Burning engine oil is another repercussion of ignoring the piston slap. This happens when the piston rings fail and they let engine oil seep into the combustion chamber.

Engine oil that leaks into the cylinder will be burnt with the air and fuel mixture, producing blue smoke that might come out of your exhaust pipe.

How to Fix Piston Slap

In most cases, a piston slap is a result of a manufacturing defect. There’s a possibility that the pistons or bore didn’t have the right specifications, and servicing the vehicle or rebuilding the affected parts is the only way to get rid of the noise. Extreme wear on the piston skirt is another possible cause, particularly on high mileage engines.

The Science Behind Pistons

The piston plays an important role in pressurizing the burned gasoline in the combustion chamber and, by its connection to the crankshaft with the rod, provides turning torque that drives the vehicle.

First, the piston transfers the force of the combustion to the crankshaft via the connecting rod.

Then, together with the piston rings, it seals the combustion chamber. The piston seals the compressed air during the engine’s compression stroke, and the combustion gases during the power stroke.

Lastly, the piston transfers heat from the combustion chamber to the cylinder walls.

Piston Operation

The piston always starts at the top of the cylinder. It accelerates as it moves downward, reaching a maximum velocity before it’s halfway down (intake stroke, intake valve open).

As the crankshaft rotates 180 degrees, the piston passes bottom dead center, the intake valve closes, and the piston moves upward to compress the air-fuel mix, which is already in the cylinder above the rising piston.

The piston accelerates the same way when it moves downward, then reverses again, the exhaust valve opens, and the piston rises again, this time forcing the inert exhaust mix out of the chamber. The exhaust valve begins to close, but right before it does, the intake valve opens, purging the rest of the inert exhaust mix out of the chamber, and the four stroke cycle repeats.

As a result, the piston produces large inertia forces.

Under high speeds, however, it’s ideal to produce less inertia for a more efficient engine operation. This is why pistons are made to be as light as possible but strong enough to do the heavy lifting in an engine.

Piston Weight Principles

The only way to achieve a balanced engine is if all the pistons have the same weight.

Replacing old pistons with new and heavier ones will most likely put additional inertia loads on the rod bearings, leading to rod bearing failure.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Machine shops will weigh and balance the pistons for a fee, but usually, piston weight doesn’t vary much from piston to piston in the same set or size.

See also  Low Compression Symptoms, Plus FAQ

Keep in mind that the replacement pistons should have the same weight as the old ones. It’s also ideal to replace all the pistons at the same time instead of replacing them on separate occasions.

Key Takeaways

A piston slap is a type of engine noise that occurs when a piston rocks from side to side instead of only going up and down.

In most cases, a piston slap is a result of a design flaw, but it can also be attributed to worn-out parts.

Lastly, there’s only a certain amount of piston slap that’s considered normal. If the piston slap is loud enough for you to hear from inside the cabin, it might be best to have your vehicle checked by a mechanic.

Where to Get High-Quality Pistons for Your Vehicle

Any strange sound coming from the engine can cause trouble for car owners. There might be something wrong with the engine, or a component near it is causing issues. For piston slaps, it’s the latter, which is why it’s crucial to get a high-quality piston replacement as soon as possible once the problem arises. Thankfully, CarParts.com has you covered.

Our website has everything you need to get your vehicle back on the road in a jiffy. Simply enter your ride’s year, make, and model into our vehicle selector to view direct-fit replacement parts for your daily driver. You can also use the search filters to find products according to your preferred brand, price range, quantity, and more.

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