Engine pistons are supposed to move in vertical up-down movement when the engine is running. A piston slap will occur when the gap between the cylinder and the piston is too wide causing the piston to move sideways hence, the slap sound. The increase in width is a result of wear and tear. Experts believe that you are likely to spend more on fuel and possible engine problems if you do not rectify the situation on time.

​Next time you hear a knock under the hood of your car, relate it to this question, what is a piston slap? because this is the question you are likely to ask your mechanic upon diagnosis. The next question you will ask your mechanic is this one, is it a problem? Of course, it is a problem because it will cause excessive oil burn, engine misfiring and loss of power.

Now let us take a look at what may be the reason behind the piston knock.


1. Burned Piston

​If your piston looks like it has one side of its top section melted, it simply means that your combustion chamber is overheating. You should check for any reason that could lead to your engine overheating. The number one culprit that you should be looking for, in this case, is the fuel injector that corresponds to that piston position. Use a fuel injector cleaning machine to clean up the injector and do a complete flow test before reconnecting it back to the engine.

Burned Piston

2. Cracked Piston

If the exhaust fumes are not circulating well, this can cause a slow deterioration of the piston. The circulating fumes provide a cooling effect, therefore, controlling the expansion rate of the piston when in action. The intense movement of the pistons can easily cause a crack that may cause damage if not replaced on time.

Cracked Piston

3. Scuffed Piston

This can cause overheating in the combustion chamber due to inadequate lubrication. The clearance between the piston and the cylinder which in today's car models tend to be smaller such that when there is little expansion between the piston and the cylinder, a piston slap will be unavoidable. To reduce this, a lubricant can be used to act as a coating agent when overheating takes place. This method will offer a temporary solution to piston slap because with time the lubricant also may wear out.

For you to be sure that stuffing is the reason for the piston knocking effect, you have to locate the exact position where scuffing occurs which should be on the upper surface and in some cases on the sides. Other marks such as carbon traces on the lower side indicate at some point the piston got overheated. This mark can also be used to point out the lack of lubrication.

If you can spot scuff marks on the edges or corners of a piston, this is indicative of distortion due to boring (hole). Marks on the thrust side show that the wrist pin is ben

Scuffed Piston

4. Serrated Piston

This will happen due to grooves that are formed on the lower side of the piston causing dents and marks on the upper side. Dirt and debris that is the number one cause of these dents. You need to make sure that your air filters are clean and that the engine’s intake system does not take in debris or dirt.

5. Damaged Piston

When your engine overheats too often, your pistons are likely to form grooves on their surfaces. The top most areas of the pistons are affected because they are next to the combustion chamber. Depending on the type of compression rings your engine has, an ideal one should either be made of steel or ductile iron which can withstand temperatures of up to 600 degrees Fahrenheit. Another notable solution is coating the compression springs to make them withstand heat changes and last longer.

 Damaged Piston

6. Destroyed Piston

If a piston gets into contact with a valve, it will break into pieces. This is a likely scenario if the timing belt breaks down. When this is the case you should check the entire engine for debris that may be clogged in the bearings and neighboring pistons. A piston may jump out of it's position if the valve spring fails.

7. Wrist Pin Damage

If your engine does not provide enough lubrication to the pistons, the wrist pins holding the pistons will be damaged or break. The wrist pins can also be broken due to fatigue as a result of over revving the engine.


We now know the reason behind a piston slap. The best time to notice a piston slap is when you start the car. Piston slap may be heard even in cars with good pistons, but the knocking should not continue after the engine warms up. You can take your car for a compression or a leakdown test to confirm what is the exact cause of engine noise.

The next time you take your car for regular service make sure you take this article with you so that you can understand the many confusing reasons as to why your pistons suddenly start knocking. I hope I helped you understand the reasons behind the causes of a piston slap. If you want to help another person know more, please share the article and possibly leave a comment on how you dealt with yours.

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Chief editor at Paul's Gigantic Garage. I came up with this blog to help you to sort out some basic problems your car may be facing and in turn, save lots of cash by some of my DIY tips that I hope will get your car out of its current situation. I also had in mind motor enthusiasts who for the love of cars who are constantly looking for information about cars on the latest trends, questions, fixes, driving, tricks, and accessories.


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