A variety of piston pins is used. Some are made to anchor in the piston-pin bosses of the piston, and some are made to anchor in the upper end of the rods. Some have oil holes provided, and others do not. Common practice in piston-pin design seems to be making the pins of large diameter, considering the size of the piston, etc. The next consideration is lightness, and this is secured by making the pin in tubular form. The large diameter insures long wear, and the lightness reduces vibration. The method of securing the pin to the rod or the piston, as the case may be, is rather immaterial as long as the particular fastening device is secure and does not loosen.
Many devices have been evolved to retain the piston pin in its proper position within the piston. When the pin is anchored to the rod, no other provision is needed. It is essential, however, to have the clamping screw properly locked to prevent its loosening. Various devices have been used to secure the pins in position by placing them in the piston-pin boss at the ends of the pins.
These may take the form of soft-metal washers just fitting the pin hole, or they may be in the form of split springs which are sprung into a recessed groove just at the ends of the piston pins. A lock ring fitted to a groove in the upper end of the piston rod and another groove in the centre of the piston pin is another means of retaining piston pins.
It is generally conceded that cast iron is a good piston material so far as long life and expansion properties are concerned. Cast-iron pistons may be plated with tin or other long wearing metal to promote rapid breaking in and long life. The aluminium pistons are claimed to be less given to an accumulation of carbon, owing to better heat conducting properties. They are also desirable from the fact that they materially decrease engine vibration.
An engine equipped with them has good pickup and is flexible. A serious objection to the aluminium pistons, when they are cast and produced along the lines of the cast-iron pistons, is the excessive clearance. Aluminium expands twice as fast as cast iron, and on this account twice the clearance must be allowed when fitting the solid-type aluminium piston to the cylinder bore.
In order to overcome this detrimental feature, the so-called constant-clearance pistons were evolved. These pistons are provided with a split skirt and are fitted to the cylinders with just bare clearance, say .001 in. to .002 in. When the engine warms up the metal expands, but this merely causes the groove to become smaller and allows the piston to retain its close fit in the cylinder bore.
Of course, the ring lands are relieved so that expansion at that point will not cause the piston to grip the cylinder wall. These pistons have overcome the objection to the aluminium piston from the point of expansion. Aluminium pistons of the old solid-skirt type were noisy until they were thoroughly warmed. These pistons are always snug to the cylinder walls. That is the reason for calling this particular type constant-clearance pistons.
Rings in the upper two grooves are compression ; those in the lower two grooves are oil-control rings. Aside from the rings made along conventional lines, there are scores of other designs. There are rings made in one piece, two pieces, three pieces, and four pieces.
They include those which have special oil grooves. They represent a multitude of special joints, all designed to prevent the loss of compression. A number of expanders on the market are designed to be placed back of the ring to keep it in close contact with the cylinder wall at all times. Some of these rings give good results. Many rings are made with square-cut ends. Slots may be cut through the rings for purposes of oil control.