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Gudgeon / Piston Pin: Function & Design (Explained)

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Last Updated on: 7th September 2023, 12:37 am

One engine part that often goes unnoticed is the gudgeon pin, also known as the wrist pin or piston pin.

It is a small yet crucial part that ensures the operation of the engine.

In this article, we’ll explain the gudgeon pin, exploring its construction, its purpose and more.

gudgeon pin diagram

What Is the Gudgeon Pin and It’s Purpose

The gudgeon pin is a cylindrical rod that serves as a connection point between the piston and the connecting rod within an internal combustion engine.

This linkage allows the piston to pivot on the pin, transmitting the force generated during the combustion process to the connecting rod and, subsequently, to the engine’s crankshaft.

It’s responsible for converting the linear motion of the piston into the rotational motion of the crankshaft.

The gudgeon pin resides within the small end of the connecting rod, while its opposite end is secured to the piston.

It’s is subjected to immense mechanical forces, heat, and vibrations, making its design and material selection crucial for the overall performance and longevity of the engine.

As the piston moves up and down within the cylinder, this piston pin ensures smooth and controlled motion, reducing friction and wear.

This, in turn, contributes to reduced energy loss and increased power generation.

Design and Materials

Gudgeon pins are typically made of high-strength materials such as steel, alloy, or even titanium.

Precision is important when manufacturing these pins. Tolerances are kept exceptionally tight to ensure optimal fit and motion.

Advanced machining techniques, including CNC machining, are employed to achieve the required accuracy.

Gudgeon pins often undergo surface treatments such as nitriding or chrome plating to enhance their durability and resistance to wear, corrosion, and heat.

There are different types of gudgeon pin designs, such as the following:

  • Full-floating
  • Semi-floating
  • Press-fit

Full-floating pins, for example, are entirely free to move within both the piston and connecting rod, reducing friction and wear.

Semi-floating pins strike a balance by allowing movement in one component while being fixed in the other.

Semi-Floating

In the semi-floating setup, the wrist pin and the piston are typically connected through an interference fit with the journal inside the piston.

In this arrangement, the small end bearing of the connecting rod serves as the sole bearing component. In this setup, only the small end bearing requires a bearing surface if necessary.

This layout is typically more complex to manufacture and service due to the involvement of two bearing surfaces or inserted sleeves, which complicates the design.

Additionally, precise alignment of the pin is necessary to ensure the central position of the small end’s eye.

This design was more common for single-cylinder engines, particularly when dealing with engines that had long cylinder blocks and crankcases, due to thermal expansion considerations.

Fully-Floating

In the fully floating configuration, bearing surfaces are present both between the small end’s eye and the gudgeon pin, and between the pin and the piston’s journal.

The gudgeon pins are usually held in place using circlips.

No interference fit is employed in this setup, and the pin essentially “floats” on the bearing surfaces.

This arrangement results in a halved average rubbing speed for each of the three bearings, and the load is distributed across a bearing that is typically about three times the length of the semi-floating design with an interference fit with the piston.

gudgeon pin, piston and connecting rod diagram

Functioning

As the piston is driven downward by the force of the expanding gases during combustion, the gudgeon pin pivots in the connecting rod’s small end, translating the piston’s linear motion into rotational motion.

This rotation drives the connecting rod, which, in turn, converts the piston’s motion into the rotary motion required to turn the engine’s crankshaft.

In addition to its mechanical function, the gudgeon pin contributes to the engine’s overall cooling.

It conducts heat away from the piston to the connecting rod, which is then dispersed into the engine’s lubricating oil.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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