Key Takeaways
The following are the key takeaways from this article:
- A connecting rod connects the piston and crankshaft that converts the reciprocating motion of the piston into the rotary motion of the crankshaft.
- The major parts of a connecting rod include bush bearing, shank and wrist pin, bearing insert, bearing cap, and pinhole.
- Some problems with connecting rods include fatigue, pin failure, over-revving, and hydro lock.
- The symptoms of a bad connecting rod include knocking engine noises, low compression, low oil pressure, and visually damaged rod.
- The cost to resolve the connecting rod can be as less as $10 to replace failed wrist pin and as high as $2800 to replace the faulty connecting rods.
Introduction
Both the piston and crankshaft are important components of an engine. However, these components cannot function effectively without a connecting element between them (1).
It is where the connecting rod plays its role as it connects the piston and crankshaft, converting the reciprocating motion into the rotary motion of the crankshaft. A connecting rod has two ends where the piston side of the rod is known as the small end, and the crankshaft side is known as the big end.
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The parts of the piston, along with its history, common issues, and symptoms, are discussed in this article.
History of Connecting Rod
Connecting rods were designed by the Arab inventor engineer Al-Jazari between 1174 and 1206AD. He designed a machine that helps in connecting the pistons to the crankshaft, known as the connecting rods (2). However, the main purpose of the connecting rod is always the same, i.e., the job of connecting rod is to convert the linear motion of the engine piston into the rotatory motion of the crankshaft rotatory motion.
Functions and Evolvement of Connecting Rod
The connecting rod has two main functions, i.e., it attaches the piston to the crankshaft. The connecting rod is further connected with heavy loads and chambers. The combustion chamber moves in different directions, and high force is exerted on the connecting rods. The speed of the piston varies due to the changing speeds, and the load on the connecting rods varies. Also, the oscillatory motions of the connecting rod help change the shape of the connecting rods (1).
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The construction of the connecting rods in the past is also similar to the previous models. The only change is in the length of the rod. There are two openings in the rod. The small opening has bushes, and the bigger opening consists of bushes.
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The previous model connecting rods were made of steel. Still, nowadays, the connecting rod is usually made of Aluminum as it is light in weight. However, the steel rod requires more power to rotate. Furthermore, the steel can gets easily corroded, and the functioning is then slowed down (1). So, the modern model rods are constructed from Aluminum, but the function is similar.
Major Parts of a Connecting Rod
Although, the connecting rod seems to be a simple component with a simple function of connecting the piston and crankshaft. But several small components facilitate the high-speed motion of the connecting rod. The following are the major parts of a connecting rod:
Bush Bearing
Bush bearing is the main part of the connecting road as it is on both ends of the connecting rod. Therefore, the material of the bush bearing can change at both ends.
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Usually, the bush bearing at the small end comprises phosphor bronze, and a solid eye bush bearing is attached to the small end (1). At the same time, a crankpin bearing is present at the big end. In some engines, the connecting rod at the big end is divided into two parts that are further supported over the crank-bearing shell. The role of bush bearing is to alleviate friction during the motion of the connecting rod. That is why some smaller engines replace bush bearings with a lubrication pump.
Shank and Wrist Pin
The piston is usually connected with the connecting rod with the help of a wrist pin. A wrist pin is a simple steel hardened tube that goes through the small end of the connecting rod and the pivots of the piston (2). The wrist pin is also known as the gudgeon pin. Nuts and bolts are used to connect the connecting rods and bearing cap. The section beam applied is known as the shank. The shank can be rectangular, circular, or tubular.
Bearing Insert
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A bearing insert is present inside the big end of the connecting rod. The bearing insert is usually connected to the bearing cap and has needle bearings.
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Some bearing inserts are made in two parts and fit together at the crankshaft (4). The bearing insert allows the connecting rod to move in the reverse direction.
Bearing Cap
A bearing cap is another major part of the connecting rod. This prevents the wear and tear of the bush bearings within the connecting rod.
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This alleviated the friction and allowed seamless movement of the connecting rod alongside its motion.
Pinhole
The pinhole is bored on the big end of the connecting rod and is bored through the bush bearing. This allows for the lubrication of the side of cylinder walls allowing for the seamless movement of the piston and piston rings.
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Pinhole also allows for setting the right angle between the piston and crankshaft for effective functioning (1).
Common Problems of Connecting Rod
The connecting rods in the engine are used to make a connection between the crankshaft and the pistons. It converts the linear motion of the piston to the rotatory motion. The following issues take place in the connecting rods.
Fatigue
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Fatigue is the major issue that usually occurs in connecting rods. This issue occurs due to the damaged connecting rods.
Due to the piston’s compression, the connecting rods’ metal part is continuously rubbed. Due to the increased friction, the rod becomes brittle and ultimately breaks down. Due to the dirty oil and less oil in the engine, the engine parts are not well lubricated, and these parts become rough. Due to the constant friction, the connecting rods are heated up, and ultimately the efficiency of the engine becomes poor. Thus, the main cause of fatigue is the lack of lubrication and minimum oil. Also, the faulty oil pump is the major reason for the broken connecting rods (3). This poorly affects the performance of the engine.
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The poor performance of the oil filter causes a defect in the bearing lining as the dirty particles get trapped in the filter, and the lubrication phenomenon is slowed down. As a result, noisy sounds are produced in the engines.
Pin Failure
A pin known as the wrist pin in the engine joins the connecting road with the piston. This wrist pin plays an important role in the engine as it helps in converting rotatory motion to linear motion (2).
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The damaged wrist pin poorly impacts the engine’s performance as the connection between the piston and crankshaft is broken due to the pin. As a result, the broken wrist pin leads to engine failure.
Furthermore, less power is generated due to broken pins of the connecting rods and the car failing to accelerate at high speed.
Over Revving
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Over-revving is another major reason for broken connecting rods. In addition, over-revving can impact the engine’s performance and is dangerous for the vehicles.
When the tachometer hits the red button, the connecting rods break down. This happens due to the strong force acting on the connecting rods. The force increases because of increased revolutions. Also, the over-revving impacts the engine’s overall performance as the valve remains open for a long period, and the engine fails to build the required amount of pressure. Besides, the air-to-fuel ratio in the combustion chamber is disturbed, and the spark is ignited after delays. This poorly impacts the health of the engine.
Hydrolock
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Hydrolock is a condition where the connecting rods get damaged and change shape. This happens when the water enters the piston chamber.
This condition usually occurs when the car is driven through deep watery surfaces. When the water enters the piston, harsh sounds are produced in the engine. Due to the water in the engine, it becomes difficult to produce sparks, and the engine’s performance becomes weak. In addition, the water in the engine makes it difficult for the piston to move, so the air compression is slowed down, and a car fails to accelerate.
Symptoms of Common Problems in Connecting Rod
As discussed above, there can be different reasons for a problem with the connecting rod. And it is recommended to get the connecting rod checked if you are facing any of the below-mentioned symptoms:
Knocking Engine Noise
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A knocking engine noise is called the knocking rod sound, indicating a failing connecting rod. The knocking sound indicates that either one or more components of the connecting rod are malfunctioning.
Usually, a deformed rod can make a knocking noise. However, a knocking sound can also result from a malfunctioning bearing in certain cases. The knocking noise can increase with an increase in speed (2). But it will eventually after traveling a few miles. It is because the oil circulates and lubricates the failed bearing. Therefore, it is recommended to pay attention to the knocking engine noise and take the car for a service checkup.
Low Compression
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Compression results from the piston’s ability to compress the air-fuel mixture. The connecting rod, as discussed above, plays a major role in piston movement by setting the right angle.
If one or more pistons misfire in the car, it can lead to an underperforming or misfiring engine. You can take the car to a service center for a compression test (5). The compression test analyzes the firing of every piston and the movement of connecting rods on different computer tests. Low compression can occur for several reasons, including deformed / malfunctioning connecting rods.
Low Oil Pressure
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Engines can lose oil quickly if the connecting rod fails. It is because connecting rod is unable to move the piston correctly that is responsible for collecting the engine oil and sending it back to the reservoir.
Low oil levels can be indicated through the dashboard lights in your car. In some cases, the oil pressure might drop, which would not allow the oil to circulate through the engine due to a bad connecting rod (2). The driver should be aware of the specific warning lights indicated on the dashboard and the OBDII code followed by the warning light.
Visually Damaged Rod
Another symptom of a bad connecting rod is a visibly bent or damaged rod. The connecting rod can melt due to high engine temperature, and a mechanic can check the connecting rod visually to analyze its physical condition (3). Visual analysis of the connecting rod requires engine disassembly, so it is recommended that the drivers take the car to a professional mechanic to analyze the connecting rod.
Conclusion
To conclude, a connecting rod is an important component of your vehicle that supports engine combustion. Although it seems like a simple rod connecting two engine components, it is made up of different small important parts aiding in the seamless movement of the connecting rod. A connecting rod can face different problems over time, such as fatigue, pin failure, hydro lock, etc., indicated by knocking engine noise, low compression, low oil pressure, and damaged rod. Therefore, it is recommended to resolve the connecting rod issue at the earliest to avoid any further engine damage.
FAQs
Depending upon the extent of the malfunctioning connecting rod, there are different symptoms of a malfunctioning connecting rod. Usually, a malfunctioning connecting rod will cause an engine to knock. However, the engine knocking sound can vary depending on the condition of the connecting rod. Further, there can be low compression, engine misfire, and low oil pressure illustrating connecting rod malfunction.
A connecting rod has to bear a tension load made up of cast steel, fabricated steel, or forged steel. In addition, some connecting rods facing compression loadings are made of aluminum alloys. So, it depends upon the type of load on the connecting rod and the material used in its manufacturing.
There is no particular date or mileage after which you need connecting rod replacement. However, some manufacturers recommend an engine overhaul after 100,000 miles, and the mechanic will decide whether the connecting rods require replacement.
