Piston slap is not an issue you will most likely not experience if you have a modern car with the latest computer diagnostic systems onboard, but it is a recurring issue with old cars.

Modern cars have motion sensors that regulate the air-fuel mix and have knock sensors that will detect detonation.

The first thing you will notice when your piston is failing is a rattling or knocking sound, loss of power, misfiring, and burning oil. Your dashboard will also most likely start showing the check engine light.

What causes piston slap?

Piston Slap

The piston-to-wall clearance can be large enough so that the piston rocks from one side to another to cause the piston slap. Aluminum blocks tend to more easily succumb to this problem than others.

Pistons that experience more cold clearance gain more velocity when the piston moves from the minor to major thrust side and this causes the piston slap.

Modern cars have a knock sensor that senses any piston variations and relays this information to the ECU. The ECU will then pull the ignition timing, and this should protect the car’s engine from false knocking.

An offset pin can also be used to adjust the motion of the piston, and this will prevent it from hitting the cylinder bore due to the reduced force.

Since the major cause of piston slap is the clearance, you can tighten the bore. The gap between the piston and the cylinder bore becomes wider due to wear and tear.

Here is a video of what piston slap sounds like:

What are the purpose of pistons?


In its basic form, a piston is a cylinder that moves up and down inside the engine block. The air and fuel mixture explodes inside the combustion chamber, and this causes the movement. The piston is connected to a wrist pin, and this is further connected to the crankshaft.

Combustion can occur with only one cylinder but modern fast cars have as many as 12. Most cars have four, six, or eight chambers.

Due to the amount of heat produced in the combustion chambers, pistons are created to be lightweight while strong enough to withstand wear and tear. Most of the pistons are made from an aluminum alloy.

In the old times, the pistons were made from cast iron. This meant that they could handle excess heat.

Today, cars handle heat better and aluminum has become the preferred choice for piston design. However, the piston rings are still made from cast iron or steel alloys. To seal the gap between the piston and cylinder, compression ring seals are used. When these rings start to wear, you will notice a bluish smoke being emitted.

The oil ring is a combination of two rings and is used to wipe excess oil from the cylinder wall. When these rings wear out, they become less elastic, and this causes oil to move to the combustion chamber from the crankcase.

Piston diagnosis

Aluminum Piston
Aluminum piston

The check engine light will often come on when the pistons start failing. If the car has onboard computer diagnostic systems, you will get an error code showing a particular cylinder that is misfiring or a knock sensor trouble code.

The reasons for misfiring vary and include: a defective or dirty fuel injector, poor ignition coils, worn spark plugs, or defective ignition cables. However, these do not cause the piston slap, but rather the blue smoke coming from the exhaust.

Snapped timing belt

The timing belt is vital in a vehicle because it helps fine-tune the movement between the pistons and the valves. The two collide whenever the belt snaps. This will cause further engine troubles, and it is something that you need to change immediately.

Burned piston

Dirty fuel injectors can cause a hole to develop on the top of the piston. This can only be detected the moment you open the top of the engine. The use of wrong spark plugs can also cause burned pistons.

Worn piston rings

This is caused when the piston rings wear out, and this causes air to flow in the space between the cylinder and the piston. Oil seeps into this space and goes into the firing chamber; the result is a car that produces white smoke and consumes a lot of engine oil.

How to solve piston slap

While the piston slap is annoying, it does often not cause much damage to the car, and you can continue driving your car for miles if it’s not too worn or burns too much oil. The first thing you will need to do is to check the status of your pistons – are they in good condition? This will require you to get a good mechanic to dismantle the engine for you.

One of the biggest causes of piston slap are the shafts knocking on the cylinders. If this is the case, you will need to get a new and re-bored piston. If you decide to change them, wait until the engine loses compression. You can replace the oversized pistons with two pistons on the passenger side. Replace the rings as well.

Most people have been solving piston slap by having the pistons knurled through a lathe and knurling tool. However, this has caused a myriad of problems like the forming of small dots on the piston’s cylindrical surface.

For some diesel engines, the optimization of the piston pin is known to reduce the noise. But the modification can cause idling and this exuberates the piston slap.

Is piston slap harmful?

In the long run, piston slap is harmful to your engine, as it will probably become more and more noticeable while the cylinder walls and piston clearance gets higher.

There is no clear answer to this, but it will depend on materials used to make the pistons. If you have aluminum pistons with short skirt length, they can damage pistons when you rev up the engine while it is cold.

The reason is that the aluminum pistons have not had enough heat to grow to shape; they will rattle the against motor block’s cylinders. The solution is to wait for the engine to be warm enough before you rev it.

Final Thoughts

Piston slap can be annoying and embarrassing. Imagine going to an important meeting only for your old car to start making rattling noises and emitting bluish smoke from the exhaust. Modern cars do not suffer from piston slap due to internal onboard computers. A simple solution to reduce piston slap is to warm your engine during cold weather before revving it.