What are pistons in a car engine?

This blog post will answer the question, ‘What are pistons in a car engine?’, and cover topics like components that make up a piston, what is the role of a piston in an internal combustion engine, and maintenance of piston and piston rings.

What are pistons in a car engine?

A piston is a mushroom-shaped component that moves in an upward or downward manner inside the engine cylinder, thus forming the lower section of the engine combustion chamber. It also plays a vital role in the combustion of air-fuel mixture to produce power for the car.

The piston is linked to the crankshaft by means of a connecting rod. The connecting rod has two ends, the smaller end is what is attached to the piston and is fixed thus allowing only linear motion. The larger part attaches to the crankshaft and is not fixed, hence allowing circular motion.

The basic requirements for a properly functioning piston are as follows:

  • Must give gas-tight seal to the cylinder through bore
  • Must be able to slide freely
  • Must be lightweight
  • Must be sturdy and long-lasting

Components that make up a piston

These are the following components that make up a piston in an engine

Piston crown

The surface or the topmost part of a piston is called the piston crown. It is this surface that comes into contact with the air-fuel mixture undergoing combustion and bears the brunt of all the heat and pressure.

Piston skirt

Along with the vertical movements that a piston undergoes during combustion, there are also horizontal forces that affect it. This horizontal force stems from the circular motion of the connecting rods to move the crankshaft, therefore causing frequent angle changes.

The piston needs to be held in an upright position and protected from hitting the cylinder walls on the side too hard. That is why it has a protective surface on the sides called a piston skirt that absorbs most of the vibrations from the horizontal forces.

Piston rings

A typical piston is made up of three rings. The first two rings around the piston are called the compression rings and the third ring is known as the oil control ring.

The main function of the compression rings is to act as a tight seal that prevents the hot combustion gases to escape from the small opening between the piston and the cylinder walls.

The piston gap is usually about 0.2mm or even less. The compression rings do not act like piston skirts, instead, they use a thin film of oil to run. The compression rings allow the heat from the piston to escape into the coolant present via the presence of water jackets.

The combustion chamber is almost always being sprayed with oil from either the apertures in the connecting rod or the cylinder. The oil control ring on the piston mainly works to eliminate any excess oil from the piston and maintain the optimum level necessary for combustion.

Gudgeon pin

The gudgeon pin, also known as the wrist pin, is what connects the piston to the crankshaft. The pin is made to pass through the fixed smaller end of the connecting rod and latches onto the piston, like a pivot.

These pins are hardened steel parallel spindles fitted through the piston bosses and the small end brushes to allow the connecting rods to swivel. It is made hollow for making it lightweight as it is a reciprocating part.

What is the role of a piston in an internal combustion engine?

The piston has the following roles to play in an internal combustion engine:

  • The hot combustion gases that are produced at high pressure inside the combustion chambers are used as the driving force for the piston movements which in turn, helps drive the wheels.
  • The combustion cycle starts with the inlet valve permitting the entry of the air-fuel mixture into the cylinder.
  • The piston reacts to this by moving downwards followed by compressing the air-fuel mixture as it moves upwards.
  • The compressed air-fuel mixture is then ignited by the spark plugs.
  • This ignition is done at very high pressure and the impact makes the pistons move downwards to transfer that power to the crankshaft via the connecting rod.
  • As discussed earlier, only the lower end of the connecting rod moves in a circular motion while the upper end stays fixed and restricted to linear motion.
  • The power is then transferred to the crankshaft that further sends it to the car wheels via a transmission system and the driveshafts.

Maintenance of piston and piston rings

The maintenance of pistons and their rings can be carried out as follows:

  • Firstly remove the piston and rod assemblies from the engine and separate the two
  • Remove the rings from the pistons using a special ring tool
  • Remove the carbon deposits and varnish carefully from piston surfaces
  • Clean the ring grooves with a clean-out tool
  • Examine the piston carefully for wear, scuffs, cracks, damaged skirts, and worn out ring grooves
  • Check the piston diameter if it is the same as the original measurements
  • Check the fitting of the piston pins in the piston bushings
  • The piston rings need to be checked for tension, scratches, and wear
  • New rings should be installed depending on their condition
  • Piston rings must be fitted to the cylinder and ring grooves in position
  • Resizing collapsed or worn-out pistons is not recommended by automotive manufacturers as excessive resizing can weaken the pistons.

Research shows the features and ways to use a piston ring compressor, which is a special tool used to compress the piston rings in the event of a piston reinstallation.


This blog post addressed the question, ‘What are pistons in a car engine?’.

We came across the various components that make up a piston, and the role that a piston plays in an internal combustion engine. Furthermore, the maintenance procedure for pistons and their rings has also been discussed. Please feel free to comment on the content or ask any questions in the comments section below.

Frequently Asked Questions (FAQs): What are pistons in a car engine?

What causes engine piston damage?

If a car is running on a low-quality fuel, it can result in the formation of cracks on the piston and can lead to poor combustion and problems with the recirculation of exhaust gases.

How many pistons are in a car engine?

A typical car engine has four to six cylinders but the count can be as high as 12 depending on the number of cylinders and performance requirements.

How much does it cost to replace pistons in a car?

Piston replacements can be pretty expensive, with an estimate of more than 1000$, and might hit 6000$ depending on the damage done and the store that replaces it.