Piston Parts and Functions Explained

Piston parts, unlike designs and material, do not vary across different pistons. They remain the same with differences only in their sizes.

This article explores the automotive piston assembly components, detailing the function of each.

To make identifying each part easy, we added images in the descriptions.

Piston components extend all the way from the part bordering the combustion chamber to the end that connects to the crankshaft. Collectively, these parts make the piston assembly.

These are the components we will focus on. Before proceeding, here is a diagram showing all piston parts.

Now, on to a detailed description of each component.

1. Piston Ring

Piston Ring Definition

Piston rings are pieces of split rings that mount on a recessed area of the piston. There are usually 3 piston rings in a typical car engine. The number varies, and a piston can even have one ring. The areas or surfaces between these rings are called piston ring lands. The ring mounting grooves are designed to maintain the piston ring position, and you may hear something like tapered construction.

The split design of the piston ring has several benefits. It allows for spring action, which helps the rings to maintain the correct piston ring gap. The split also makes piston ring installation easy. To ensure spring constant under heat, load, pressures, and other conditions, manufacturers prefer cast iron or pieces of steel for piston rings material.

Piston Ring Function

The primary function of piston rings is to seal off the combustion chamber and regulate lubricating oil usage. The rings also serve to conduct heat away to the cylinder bore. As mentioned before, most vehicle engine pistons feature three rings; two upper compression rings and a lower oil ring. For clarity, the different rings are explained below.

• Compression ring– this is the top side ring and nearest the combustion chamber. It is also called the gas or pressure ring. The ring prevents combustion gases from leaking. Compression rings also help transfer heat from the piston to the walls of the cylinder.
  
  
• Scraper/wiper ring– this is located between the compression and oil rings. It features a tapered surface and serves the function of both rings: to seal the combustion chamber and to wipe oil off the walls of the piston cylinder.
  
  
• Oil control ring– the piston oil ring is the lower ring on the piston. It consists of two thin surfaces, with holes all around. The slots allow oil to flow back into the sump. As its name suggests, the function of the piston oil control ring is to remove excess oil from the cylinder walls. It does that as the piston works back and forth.
  
  
  

2. Piston Skirt

Piston Skirt Definition

The skirt of a piston refers to the cylindrical material mounted on the round section of a piston. The part is usually made from cast iron material due to its excellent wear resistance and self-lubricating properties. The skirt contains the grooves to mount the piston oil ring as well as compression rings. Piston skirts come in different styles to suit specific applications.

Piston Skirt Function

The skirt guides the piston as it travels up and down the cylinder. Its design helps the piston to overcome the side forces created by the changing angle of the connecting rod. If the skirt is worn, obtaining a proper piston seal for efficient combustion would prove difficult.

The piston would also rock in the cylinder uncontrollably and cause piston slap. When that happens, you may hear the infamous piston slap noise, especially during cold starts. Piston slap will often go away after the engine has warmed. That is because the resulting resulting expansion closes the gap between piston and cylinder.

If the noise does not stop, the cylinder may need to be tightened, among other measures. Otherwise, the problem does not present much danger, and you may drive your car if the noise only appears when starting the engine. 

There are two main types of piston skirts:

• Full skirt

It is also known as solid skirt. The full skirt features a tubular shape. It is commonly used in the engines of large automobiles.

• Slipper skirt

The type of piston skirt is used on the pistons of motorcycles and some cars. It has part of the skirt cut away to leave only the surfaces on the back and front of the cylinder wall. This helps to reduce weight and minimize the contact area between the cylinder wall and the piston.

3. Piston Pin

Piston Pin Definition

Also known as wrist pin or Gudgeon pin, the piston pin is the hollow or solid shaft  in the skirt section. The piston rod pivots on this pin, held in the piston ring bushing. For tensile strength, piston pins are usually built from alloy steel and machined to fit the piston bearing. Holes in the connecting rod deliver oil to the wrist pin, helping to reduce friction.

Piston pin assemblies and mounting styles vary. They can be categorized into 3 designs: free to revolve in both piston and connecting rod, clamped to connecting rod, and rigidly mounted to piston bosses.

Piston Pin Function

A piston pin forms the connection or pivotal point of the pistons and connecting rod. They provide bearing support and help pistons to function properly. In other words, the pin facilitates the back and forth movement of the piston.

As we have seen, piston pins use three methods to mount on the piston assembly. These give rise to the following types of pins.

• Stationary/fixed pin– the pin attaches to the bosses of the piston via a screw. The piston rod then pivots on the pin.
  
  
• Semi floating– the pin attaches to the connecting rod in the middle, and the pin ends move freely within the piston bearing and at the bosses. 
  
  
• Full floating– in this pin type, the pin is not attached to the pin or piston connecting rod. Instead, it is secured by plugs, clips, or snap ring attached to the piston bosses. The pin can then oscillate at the bosses as well as the rod.
  
  
  

4. Piston Head/Crown

Piston Head Definition

Also known as the piston crown or dome, the head of a piston is its top surface. It is the part that comes into contact with the combustion gases. As a result, it gets heated to extremely high temperatures. To prevent melting, piston head parts are made using special alloys, among them steel alloy.

A piston head is usually built with channels and cavities. These help to create a swirl that improves combustion. Different piston head types are used in different engines. Reasons for the differences vary. The preferred piston head design depends on many factors, such as expected performance and engine type.

Piston Head Functions

The crown, as it is also known, forms the surface that takes in the pressures, temperatures, and other stresses of the expanding gases. Among the purposes it serves are:

• Creating swirl to make combustion uniform and regulate knocking
  
  
• Act as a heat barrier between the combustion chamber and the lower piston parts
  
  
• Contain the pressures resulting from knock in the cylinder
  
  
  

5. Connecting Rod

Connecting Rod Definition

Often shortened to conrod or rod, the connecting rod is one of the most important piston parts. It links the piston to the engine’s crankshaft and moves the piston in and out of the combustion chamber. Connecting rods have to bear a lot of mechanical strain and should be sturdy enough. For this reason, the parts are mostly forged, although casting is also frequently used.

Automotive piston manufacturers often prefer steel to manufacture these rods. Alloy steel is also a popular conrod material, especially for high-performance engines. For milder engines, aluminum may be preferred for its lightweight nature. The rods of small engines such as scooters can even be made from iron.

Connecting Rod Function

The conrod rotates the crankshaft, producing the motion that enables a vehicle to move. In some engines, the piston rod features a hole or bore to deliver lubricating oil to the cylinder walls and wrist pin. Manufacturers make conrods in various designs. Versions include cracked joint, milled joint, straight and angled separation rod, and the parallel and tapered rod design.

The connecting rod is divided into several parts. They are:

• Small end– it is the smaller end of the rod. It consists of the rod eye and piston bushing. The small end links to the piston via the piston pin.
  
  
• Big end– the big end is the part opposite the small end of the conrod. It connects to the crankshaft and features a slit design to allow it to be mounted.
  
  
• Connecting rod beam– this is the section between the small and big parts of the rod. Usually a double T construction, the beam may or may not contain an oil bore to channel lubricant to the cylinder.
  
  
  

6. Connecting Rod Bolt

Connecting Rod Bolt Definition

Also in the list of piston parts is the conrod bolt. These bolts clamp the rod to the crankshaft. The bottom end of rod bolts bolt are rod caps and bearings, held in place by a nut. A cotter pin on the nut prevents the assembly from coming undone.

Conrod bolts are usually made from steel. In applications where weight reduction is an essential feature, aluminum bolts care common. The rod can also be made from nickel. Nickel connecting rod bolts are stronger and mostly used in heavy-duty rods.

Connecting Rod Bolt Function

As we have seen, the bolts secure the connecting rod to the crankshaft. They help the rod to withstand the strain caused by the rotating crankshaft. In their absence, the rod would break down affect the working of other engine parts. The rods guide every piston stroke, ensuring smooth engine operation.

Conrod bolts are made with the ability to bend a little under the action of the piston and crankshaft movements. That protects the rod caps from collapsing due to the extreme strain caused by the moving crankshaft and piston.

Connecting rod bolts come in a variety of designs. They can be hexagonal, round, flat or an embossed design. Some come threaded others unthreaded. Threaded bolts make the best parts due to their firmer grip.

7. Piston Bearings

The bearings are piston parts that are located at the points where pivotal rotation takes place. The are usually semicircular metal pieces that fit in the bores of these points. Piston bearings include the shells found at the big end where the rod connects to the crankshaft. There are also bearings at the small end where the rod links to the piston.

Piston bearings are usually manufactured using composite metals like lead copper, silicone aluminum, and others. The bearings are often coated to improve hardness and enable them to bear the load of piston and connecting rod movements.

Piston Parts Questions

1. Do engine pistons have return springs?

No, they do not. A piston return spring is not required in these pistons. However, you will find the springs in drum brake assemblies. Here, they help to retract the brake shoes when you release the brake pedal.

2. What is piston liner?

It is a kind of liner installed on the walls of an engine cylinder. Usually made of superior material than that of the cylinder bore, the liner provides a hard-wearing surface. Car owners install it to either protect the cylinder or repair it when rebuilding the engine.

Because the liner is replaceable, using it increases engine life. A piston liner is also called piston sleeve and does not form part of piston assembly. You will also find it going by the name cylinder liner or sleeve.

3. What piston parts can be replaced?

Most piston assembly components can be changed. They include piston rings, piston bearings, piston rod bolts, and several other parts. These can be purchased individually or as a piston kit. 

Piston rings wear quickly. They are one of the most replaced piston parts. When damaged, these rings cause various piston and engine problems. That can lead to extra costs. Given that the piston ring price is only a few hundred dollars, replacing these parts can save you from expensive repairs.

4. What is the cost of replacing piston parts?

The amount can exceed $1000 or even $2000. Piston kit price or that of individual components may be low, but not so the amount you pay a mechanic to do the job. Replacing anything hidden within the engine block involves several hours of work, which is why it costs a lot.

Add that to the fact that there is usually a piston set to change, and the amount can be pretty high. You can choose to replace the piston or piston parts yourself, of course. However, you would need the right tools, which include the following:

A piston ring compressor to install the rings, a micrometer to piston measurement, and a feeler gauge set to measure piston and piston ring clearance. You may also need a piston ring filer to hone the rings to the correct specifications. You also need to understand how to replace the piston correctly. (we have a whole article dedicated to piston installation process). There are many piston types on the automotive piston market.

To find the right one for your engine, scour the websites of manufacturers. They usually provide an automotive pistons catalog. These contain invaluable information regarding the specific piston you may be looking for. That includes piston size chart for piston to cylinder clearance, piston depth chart, and more. You will also have the chance to piston price across sellers.

Conclusion

Havinglooked at piston parts, you now know how each component looks like as well as its location on the piston assembly. We have more articles on the automotive piston and you can browse through them. The articles cover several topics, including piston measurement when rebuilding your engine. And if looking for the right piston for your car engine, we have a full-length article about piston types.