What Is Connecting Rod?- Parts, Function, and Types

A car engine’s efficient functioning depends on the pistons’ reciprocating motion. Pistons are connected to the engine and crankshaft via connecting rods. From transferring combustion pressure to sustaining engine power, connecting rods are vital in the overall process.

Let’s find out more about the working of the engine connecting rod, which makes it an essential performance part of an engine.

What is Connecting Rod?

A connecting rod is an engine component that transfers motion from the piston to the crankshaft and functions as a lever arm. Connecting rods are commonly made from cast aluminum alloy and are designed to withstand dynamic stresses from combustion and piston movement.

The connecting rod provides the mechanical linkage between the piston and the crankshaft and must exhibit properties of high strength, low inertial mass, and uniformity of mass with the other connecting rods attached to the crankshaft.

The small end of the connecting rod connects to the piston with a piston pin. The piston pin, or wrist pin, provides a pivot point between the piston and connecting rod. Spring clips, or piston pin locks, are used to hold the piston pin in place.

The big end of the connecting rod connects to the crankpin journal to provide a pivot point on the crankshaft. Connecting rods are produced as one-piece or two-piece components.

A rod cap is the removable section of a two-piece connecting rod that provides a bearing surface for the crankpin journal. The rod cap is attached to the connecting rod with two cap screws for installation and removal from the crankshaft.

Parts of Connecting Rod

Following are the parts of the connecting rod:

Parts of Connecting Rod
  • Small End: The end at which the connecting rod is attached to the face of the piston pin is known as the small end of the connecting rod.
  • Big End: The end at which the connecting rod is attached to the side of the crank pin is known as big end of the connecting rod.
  • Bush Bearing: Both ends of the connecting rod are fixed with a bush bearing. A phosphor bronze bush is fitted with the solid eye is attached to the small end of the connecting rod. The Big end is attached to the crankpin. The end is divided into two parts and is supported over the crank bearing shell.
  • Bearing Insert: In the big end of the connecting rod, there is a bearing insert that is connected to the bearing cap, it is known as a bearing insert. These are made in two parts that fit together on the crankshaft. This is the position where the connecting rod travels along the reverse direction.
  • Bolt and Nut: After the connecting rod is fitted with the crank at the bottom, both sides of the big ends are fastened by some bolts and nuts. Thus, by combining these all components the connecting rod is ready to use.
  • Shank: Furthermore, each of the bolt and nuts are employed to connect both the connecting rod and bearing cap. A section beam is applied it is known as shank. The section of the rod may be rectangular, tubular, and a circular section.
  • Wrist Pin: The engine piston is connected to the connecting rod with the help of a hollow hardened steel tube called wrist pin. It is also known as gudgeon pin. Wrist pin goes through the short end of the connecting rod and pivots on the engaged piston.
  • Piston: The piston is connected to the crankshaft with the help of a connecting rod, which is usually shortened to the rod or Conrod. The purpose of the piston is to work as a movable plug in the cylinder, which forms the bottom of the combustion chamber.
  • Bearing Cap: Shell bearings have an adjustment for wear, but it controls the running and the side clearance allows the bearing cap to be tightened correctly.

The function of Connecting Rod

A connecting rod, also called a ‘con rod’, is part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft.

The connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an internal combustion engine, it allows pivoting on the piston end and rotation on the shaft end.

The predecessor to the connecting rod is a mechanic linkage used by water mills to convert the rotating motion of the water wheel into reciprocating motion.

The most common usage of connecting rods is in internal combustion engines or on steam engines.

Types of Connecting Rod

Following are the types of connecting rods, used in various types of engines:

  • Plain type rod
  • Fork and blade rod
  • Master and slave rod
  • Billet conrods
  • Cast rods
  • Forged rods

1. Plain Type Rods

The plain type of connecting rod is used in inline and opposed engines. The big end of the connecting rod is attached to the crankpin and fitted with a bearing cap.

The bearing cap is mounted by a bolt or stud at the end of the connecting rod. The connecting rod must be replaced in the same cylinder and in the same relative position to maintain proper fit and balance.

2. Fork and Blade Rods

These types of connecting rods are used on V-twin motorcycle engines and V12 aircraft engines. In each pair of engine cylinders, a “fork” rod is divided into two parts at the big end and a “blade” rod is tapered from the opposing cylinder to fit this gap in the fork.

This system eliminates the rocking couple that occurs when the cylinder pairs are balanced along with the crankshaft.

In the big-end bearings type of arrangement, the fork rod has a single wide-bearing sleeve that extends over the entire width of the rod, including the central gap.

The blade rod then runs directly outside this sleeve, not on the crankpin. This causes the two rods to move back and forth, this reducing the force on the bearing and the surface speed. But, the bearing speed also reciprocates instead of continuously rotating, which is a major problem for lubrication.

3. Master and Slave Rods

Radial engines typically use master-and-slave connecting rods. In this system, the one piston consists of a master rod with a direct attachment to the crankshaft. Other pistons connect their connecting rods to the rings surrounding the edge of the master rod.

The disadvantage of master-slave rods is that the stroke of the slave piston is slightly larger than that of the master piston, which increases the vibration in the V-type engine.

4. Billet rods

Billet connecting rods are designed from steel or aluminum. Compared to other types of connecting rods, they are lighter, stronger, and longer in lifespan.

It is commonly used in high-speed vehicles. It is sometimes designed to reduce stress risers and ease into the natural grain of the billet material.

5. Cast Rods

These types of connecting rods are preferred and designed by manufacturers because they can capable of handling the load of a stock engine.

Cast rods require low cost to produce and cannot be used in applications of high horsepower. The cast rods have a noticeable seam in the middle that separates them from the forged type.

6. Forged Rods

Some of the connecting rods are manufacture by forging. These types of connecting rods are made by forcing a grain of material to the shape of the end. Depending on the required properties the material may be steel alloy or aluminum.

Commonly used steel alloys are chrome and nickel alloy. The end product is not designed to be brittle. Hence, nickel or chrome alloys increase the strength of the connecting rod.

Signs Of A Bad Connecting Rod

A bad connecting rod is a significant problem for an engine because it can no longer perform intake, exhaust, compression or power strokes. Continue reading to learn a few of the signs that a rod is damaged:

  1. Knocking noise: A knocking sound is one of the more common signs of a failing connecting rod, which is why it’s referred to as rod knock. This knocking tends to get louder as you speed up, but it may go away once oil starts to circulate and lubricate the failing connecting rod. Instead of ignoring the loud sound, bring your car to a mechanic for an inspection.
  2. Low compression: Compression refers to a piston’s ability to take in air and fuel and compress them. A lack of compression in one or more of the pistons often leads to misfires or underperformance while driving. Getting a compression test is the best way to see if you have low compression, which a professional can perform rather quickly.
  3. Low oil or oil pressure: Engines tend to lose a whole lot of oil very quickly when a connecting rod fails. This issue can even lead to a drop in oil pressure, meaning the oil isn’t circulating through the engine. Low oil or low oil pressure will cause your dashboard lights to illuminate, so be sure to bring your car into a professional ASAP if that happens.
  4. Visibly bent or damaged rod: The easiest way for a mechanic to tell whether the symptoms above are due to a bad connecting rod is to check under the hood. We emphasize the word mechanic here, because inspecting the rods requires disassembling the engine, which is something only a trained professional should do.

Causes Of A Bad Connecting Rod

Connecting rods can bend or break for a variety of reasons. Here are a few of the most common causes of failure:

  • Flooded engine: An engine that’s filled with fluids instead of just air is known as hydrolocked. When this occurs, connecting rods can bend or break when you start up your vehicle. A hydrolocked engine needs to be drained before you can continue to drive.
  • Over-revving the engine: The harder you rev your engine, the more force gets exerted on the connecting rods. If you make a habit of over-revving, rods are bound to break. Thankfully, newer engine technology prevents over-revving, but it can still occur if you drive an older vehicle.
  • Engine modification: If you upgrade your engine to make your car go faster, be warned! Most stock connecting rods aren’t built to handle heavy modifications and can fail if the upgraded pistons put too much pressure on them.

Should You Drive A Car With A Bad Connecting Rod?

Definitely not. While a bad connecting rod can cause some serious damage in its early stages, the consequences of continuing to drive with a bad connecting rod can be truly catastrophic.

If the bad connecting rod breaks while moving at high speed, its pieces can make a big hole, right through the side of your engine block. At the same time, the piston that the connecting rod was formerly attached to is free to do its own damage at the top of the cylinder, where it can smash into the intake and exhaust valves, bending or breaking them.

This event is known as “throwing a rod.” Congratulations, you have just completely destroyed your engine.

How Do You Fix A Bad Connecting Rod?

There’s no easy way to deliver the sad news. A bad connecting rod is a sign of serious engine damage. Even if you catch it before your engine throws a rod, there will be some major engine rebuilding in store for your car. The possible list of damaged items can include:

  • The connecting rods themselves
  • The connecting rod bearings
  • The crankshaft
  • The pistons
  • The piston wrist pins The cylinder walls
  • The valves
  • Other related parts

Your mechanic will first need to disassemble your engine to the point where the extent of the damage can be assessed. The next step is to compile a list of items that need to be repaired or replaced, plus plenty of labor to go with it. This will be expensive.

If this happens to your car, you may have a difficult decision to make, depending on the news you receive from your mechanic. Based on the extent of the damage, and the state of the rest of your vehicle, your mechanic may offer you one or more of the following choices:

  • Repair your damaged engine and replace the parts that are no longer usable
  • Replace your entire engine with a remanufactured unit, which will come with a warranty
  • Get a used engine from an auto recycling yard – it will not have a warranty
  • Sell your damaged car to a scrap yard now, because the repair costs are much more than your car is worth.

The first option is for an engine with minimal damage. The second option is worth doing if your car is fairly young and has plenty of life left in it.

The third option is an inexpensive way to keep an older car running a little longer, and it will also allow you to sell your car in running condition, which will bring you a higher price.

The final option makes sense if your car is old and worn, and will not be generally reliable just because you replaced the engine. If the last one sounds like your car, it may be time to say goodbye.