Gudgeon Pin/Wrist Pin: Function, Construction, and Installation
What Is a Gudgeon Pin?
A gudgeon pin, also known as a wrist pin, is an important part of a car engine. It creates a connection between the connecting rod and the piston and provides a bearing for the connecting rod to pivot upon as the piston moves.
Gudgeon pins serve the above functions with a combination of other parts such as with wheels or cranks. In the first designs of engines which also included steam powered models or large stationary ones, the gudgeon pin was positioned within a sliding crosshead that connected to the piston with the aid of a rod. The term gusgeon refers to a type of pivot or journal.
Canada and the United States prefer using the term “wrist pin”, but the United Kingdom uses “gudgeon pin”.
Construction of Gudgeon Pin
The gudgeon pin or ‘wrist pin’ is one of the most well-known parts of the car engine. It is usually a short hollow forged rod made from high strength and hard alloys of steel which can be removed from the rod and piston, or crosshead.
In the case of small-sized automotive engines, high revs tend to level pin designs with small offsets, greatly increasing complexity.
The gudgeon pin operates at some of the highest temperatures in the engine and experiences harsh temperature lubrication challenges due to its location, in addition to needing to be lightweight and compact so as to fit within the diameter of the piston, not significantly increasing the reciprocating mass.
The need for lightness and compactness requires a small diameter rod which is heavily sheared and bent, perhaps the most highly pressure loaded of any bearing in the entire engine from the standpoint of a shear and bending moment.
The problems stated above are solved by the bearing construction and its features designed for internal combustion engines. It is among the most highly engineered components exposed to rotating motion.
Design Options
The gudgeon pin can be placed in the engine in either a semi-floating or a fully floating configuration, depending on engine design. It permits the connecting rod to turn freely about its axis during engine operation.
Specialized versions like those used for racing cars are manufactured because of the exceptional mechanical properties needed for high-performance engines that create some unique demands.
Semi-Floating
In the semi-floating setup, the piston pin is normally attached to the piston by an interference fitting the journal to the piston. As a result, the small end bearing of the connecting rod acts, by itself, as a bearing.
Under this configuration, the only bearing that can be (and often is) fitted is at the small end. If necessary, this bearing surface is provided either by electroplating the small end bearing journal with an appropriate metal or, more typically, by putting a sleeve bearing or needle bearing into the small end eye that is bonded to the aperture of the small end.
Fully floating
All types of bearings are required in fully floating configuration; both in relation to the eye of the small end and the gudgeon pin and the journal in the piston. The gudgeon pins are usually secured by circlips.
In this instance, there are no parts with interference fit, therefore, the pin freely rotates within only bearing surfaces. The average speed of rotation at each of the three bearings is reduced to one half, the amount of loading borne divided by a bearing, which is normally three times longer than a semi floating one with an interference fit into the piston.
During routine maintenance like oil changes and tune-ups, a mechanic visually inspects the engine for obvious signs of wear and damage. A problematic component, such as a gudgeon pin, may lead them to advise undergoing a replacement.
Lopsided or noteworthy wear will be noticed, as it may suggest there’s a problem with the engine that requires immediate attention. Shifting focus from minor to serious engine problems is dangerous, as heaving failures of components or the engine can eventually result.
Though costly, repairs are more economical when addressing the issue in the moment instead of waiting for a subsequent breakdown.
