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The piston pin is an element of the cylindrical crank mechanism, representing the axis of movement of the connecting rod in its connection with the piston and thus provides a movable, articulated connection of the connecting rod head and the piston.

During engine operation, both the piston and the gudgeon pin are subject to severe stress. The finger is affected by the pressure on the compression stroke of the fuel-air mixture, the force of the expanding gases after their ignition, inertia, bending force, etc. Given these loads, hardened steel is the material for the piston pins. Compared to standard versions, Uprated engines receive a piston pin with a harder alloy steel surface.

Structurally, the piston pin has an internal hole to facilitate the part. Low weight means less inertial losses and increased motor efficiency. At the same time, the finger must remain strong and wear-resistant under extreme stress conditions. In mass production, the piston pin bore has a simple cylindrical shape. This solution allows you to reduce costs during the manufacturing process.

Another option for the inner hole of the pin is the execution of open outward holes cones (a version with or without a conical bridge). The tapers taper in the middle of the piston pin.
Such solutions can effectively reduce the weight of the part and maintain the required strength but have a high cost for mass production. For this reason, fingers of this type are usually used on sports and high-powered engines, in which the final manufacturing cost is of secondary importance.

There are two types of piston pins according to the installation method:

  • fixed piston pin;
  • floating piston pin;

The fixed pin assumes a tight interference fit for secure axial retention. Such fastening means no rotation of the pin at the installation site (the upper connecting rod head or the holes of the piston bosses).
The fixed connection is usually realized in the upper connecting rod head. With this mounting method, the piston pin rotates in the holes of the piston bosses, where a certain gap is provided. The installation of a fixed piston pin during the repair of the internal combustion engine becomes possible after significant heating of the connecting rod.

A floating pin means that the part is installed with a certain clearance in the upper connecting rod head and the piston bosses. In this case, the piston pin is fixed using special retaining rings.

The specified circlips of the floating piston pin are installed in the grooves made in the piston bosses. Also, in motors with a floating piston pin, a bronze bushing is installed in the upper connecting rod head to minimize friction.

Floating piston pins need to be lubricated as the pivot area to the connecting rod head is heavily stressed. The connecting rod head and head bushing have a special hole through which engine oil penetrates from the inner cavity of the piston. Concerning this cavity, lubricant is supplied therefrom the oil scraper piston rings through special holes in the piston or can be injected using oil nozzles of the engine lubrication system.

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