INTRODUCTION - PISTONS AND CONNECTING RODS

1.2 8v

CONSTRUCTION SPECIFICATIONS

The connecting rods are made from ductile cast iron. The piston pins are fixed and counter-matched with the connecting rod.

COMPOSITION

The connecting rod comprises a casing or stem, with a rectilinear axis, which rigidly connects the small end, hinged to the piston pin, and the big end, hinged on the crank pin with bearings in between.

The big end is divided into two sections for fitting reasons: the fixed part is in one piece with the casing and the moving part is connected to the former by studs.

Since the connecting rod is subjected to compression and combined compressive and bending stress forces it tends to become distorted; as a result the connecting rod body should be made in such a way as to combine lightness with maximum resistance and is therefore constructed with a double T section.

The connecting rod is the main element connecting the piston with the crankshaft.

CONSTRUCTION SPECIFICATIONS

The pistons are made from silicon aluminium alloy and are divided into three size categories with an arrow stamped on the crown to indicate engine rotation direction to ensure correct installation.

COMPOSITION

The piston comprises two parts:

• Head (1) or area of the piston rings, with a diameter slightly smaller than that of the cylinder to allow the thermal dilation of the metal.
• Skirt (2) or guide pad for the small end that supports the side thrust.

The skirt contains two hubs or bosses that house the piston pin.

COMPOSITION

These pistons contain a steel ring incorporated through fusion between the skirt and the head; this has the task of preventing the expansion of the skirt in the direction transverse to the piston pin axis.

OPERATION

It is the component that receives the engine impulse directly from the pressure of the combustion gas, via the connecting rod, transmitting the drive force to the crankshaft.

It should meet the following requirements:

• high mechanical resistance that allows it to support the explosion pressures (~35 kg/cm²) and the dynamic stresses;
• good resistance to heat and to wear;
• good thermal conductivity, to transmit the heat from the hot area to the cold area underneath;
• lightness and quiet operation when hot and when cold.

For the specific pressure at the skirt to be low in the working direction and consequently for low consumption for both the skirt and the cylinder liners, the piston pin must be offset (by about 3% of the bore) in relation to the piston axis, on the opposite side to the direction of rotation.