Dario, the gudgeon pin has to be strongly located axially. Probably most of us have seen what happens when the pin moves axially: you get a set of neat-looking keyways down the bore, due to contact from the sides of the end of the pin. Even if you catch it very early that will call for a rebore, but often the wear exceeds the rebore limit by the time you find out about the problem.
There have been several ways of locating the pin, used on various engines over the years. Back in the 1930s it was fairly common to have a "little-end bolt", screwed from one side of the connecting rod to the other, and passing through a recess ground in the center of the pin. This was expensive, it weakened the rod in a critical place, and it increased reciprocating weight, so it was a poor solution. Later, circlips took over, except for the odd exception when they put aluminium "slippers" at each end of the gudgeon pin, so the slipper rode against the bore and kept the pin away from it. (The 1960s Simca Aronde was an example of that one.) Car engines normally use either circlips, or a shrink fit between the connecting rod and the gudgeon pin. Before that became fashionable some car engines had a bronze bush in the connecting rod, and circlips at the ends of the pin, so the pin could make its own choices whether it turned in the rod or the piston. That was a particularly poor solution, used in an engine that was famous in Australia (the Holden "grey motor" of 1948-1963). The result was insufficient bearing area in both piston and rod, and a whole lot of engines with "gudgeon knock" due to mechanics reaming the bush in the rod oversize, to make it easy to assemble the engine.
To get a good design, the engineer must provide enough bearing surface either in the rod or the piston, not in both. Shrinking the rod onto the pin is one good solution to this problem: all movement is in the piston, and the rod is reduced in width to maximize the bearing area in the piston. Also, the piston runs a lot hotter than the rod does, and aluminium has a high coefficient of thermal expansion, so it is very difficult to keep the pin from rotating in the piston even if you wanted to. Another good solution, though an expensive one, for very small engines, where bearing area is hard to come by, is to fit a needle roller bearing to the connecting rod, then make the bearing area in the piston considerably smaller than it would otherwise have to be.
