Things you must know about connecting rod

A connecting rod which is also called con rod is one of the major parts of an internal combustion engine. Although it’s also used on the steam engine but found great important on vehicles.

A conrod connects the piston to the crankshaft which aids the conversion of reciprocating motion of the piston into rotation of the crankshaft. The part functions as a lever arm and it’s required to transmit the compressive and tensile forces from the piston and rotate at both ends.

connecting rod

Today we’ll be looking into the definition, functions, material, parts, working, and diagram of connecting rods.


Connecting Rod Definition

A connecting rod or conrod converts the linear up and down movement or reciprocating motion of the piston into circular motion of the crankshaft. It experiences tension, compression bending and buckling during the process.

Read Everything you need to know about automobile piston

Conrod Materials

Connecting rods are commonly made from cast aluminium alloy material, which helps to withstand the stresses that occur during the process.

In mass production, connecting rod is usually made of steel. For high-performance applications, a billet is used, which are machined out of a solid billet and not cast or forged.

However, cast iron can be used for cheaper, low-performance applications like motor scooters.

Design Nomenclature

  • A = cross-sectional area of the connecting rod.
  • L = length of the connecting rod.
  • C = compressive yield stress.
  • Wcr = crippling or buckling load.
  • Ixx = moment of inertia of the section about the x-axis
  • Iyy = moment of inertia of the section about y-axis respectively.
  • Kxx = radius of gyration of the section about the x-axis
  • Kyy = radius of gyration of the section about y-axis respectively.
  • D = Diameter of piston r = Radius of crank

Connecting Rod Parts

The connecting rod parts in internal combustion engine consists of big end, rod and small end.

Pistons are connected to the small end of the conrod with a piston pin or wrist pin that can swivel in the piston. This pin provides a pivot point between the piston and the connecting rod. The piston pin is held in place with a spring clip or piston pin lock.

The big end part of the connecting rod is mounted to the crankpin with a plain bearing, which reduces friction. But some smaller engines use bearing to avoid the need for pump lubrication.

Generally, there is a pinhole bored through the bearing on the big end of the connecting rod. It allows lubricating oil to pump into the thrust side of the cylinder wall to lubricate the pistons travel and piston rings.

Finally, on the connecting rod parts, the conrod can rotate at both ends, so that the angle between the connecting rod and the piston can change as the rod reciprocate around the crankshaft.

Read: Understanding the petrol engine

Functions of Conrod

  • The primary function of a connecting rod is to create a link between the piston and crankshaft so as to transmit power.
  • The conrod help to deliver lubricating oil to the cylinder walls and wrist pin or gudgeon pin.

Types of Connecting Rod

Conrod types are based on the material they are made with. Below are the various types of connecting rod available out there:

Billet conrods:

the billet connecting rods are machine from one piece of steel or aluminium. It’s lighter, stronger, durable than other kinds of con rods. Billet types connecting rods are normally used for high-end running vehicles.

The product is sometimes designed to reduce stress risers and ease into the billet material’s natural grain.

Cast Rod:

Cast rod is more preferred by manufacturers as it’s capable of handle a load of a stock engine. The production cost is less expensive but cannot be used in high horsepower applications of over 450 up to 6,000 rpm.

A cast connecting rods contains a noticeable seam down the middle which differentiates it from the forged type.

Forged Rod:

con rods made of forging are made by forcing grain of selected material to the shape of the rod. The material can be steel alloy or aluminium depending on the properties required.

Well, manufacturers of forged rods use different types of steel alloys, chrome and nickel alloys. The end product is not made to be brittle. Nickels or chrome alloy increases the strength of a connecting rod.

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Powdered Metal:

connecting rod made of powder metal is also a suitable choice for manufacturers. It is produced with a powdered mix of metal which is pressed into a mould and heated at high temperatures. It melts the mixture into a solid form.

Light machining might be required but the product comes out of the mould basically a finished product. Powdered metal conrods are less expensive to make than steel and stronger than cast connecting rods.

Read: Classifications of Internal combustion engine

Problems of Connecting Rod

Connecting rod can experience problems while working. This is because it experiences various stresses during in the combustion chamber. Some of these issues can be fixed while some required complete changing of the connecting rod. During the operation, the rod is stretched and compressed, which can cause the con rod to break. The broken rod can completely damage the engine. Below is the failure that occurs on a connecting rod:


This is the main reason why connecting rod break. It often occurs in older engines. Compression and the stretching of the rod during process takes place thousands of time in a minute. This eventually causes wears to the part until it ends up breaking. Lack of oil and present of dirty in the engine can speed this issue.

New engines can also experience fatigue in the connecting if the engine is rebuilt. Well, it happens when cheap parts or the wrong ones are used.


Hydrolock is caused when water penetrates into the piston chamber lead to deformation of the connecting rod. This usually occurs when the vehicle drives through a flooded street.

Little water in the cylinder can cause knocking or tapping sound which can be easily repaired. However, if there is a lot of water in the cylinder, all the space for spark time has been taken which may cause the cylinder rod to bend or snap.

Over Revving:

Over revving is a failure that occurs in new and high-performance engines. If the tachometer shows red, it indicates the condition of the connecting rod is in danger. This is because the forces acting on the con rod increase dramatically at high revolutions.

Pin Failure:

The piston pin that connects the connecting rod to the piston get wear and cause failure. In some engine, it results in catastrophic engine failure. This is when the connecting rod goes the engine block or when the crankshaft is bent.

Read: Types of automobile engine

In some engines, it causes loss of power. If such happen, that is if the engine stopped immediately after the pin breaks. There is a probability that the engine is saved else total breakdown might occur.

That’s it for this article “connecting rod”. I hope you enjoyed the reading, if so kindly comment, share and recommend this site to other technical students. Thanks!

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