Generally it will be an iron casting in the form of a small cylinder (see Fig. [15]), having provision in it for the packing rings P, and the gudgeon pin G, with its fastening screws S1, S2. The piston itself, as we have observed, must be a nice sliding fit in the bore of the cylinder without any shake or side play when there are no packing rings in the grooves. The packing rings are turned to size so as to fit the cylinder exactly and prevent any gas leaking past the piston into the crankchamber. These rings are very light, are made from cast iron, and arranged to break joint, as indicated, by cutting the middle ring in the opposite direction to the two outer ones. Bosses are cast on the inside of the piston and afterwards bored out to receive the steel gudgeon pin or wrist pin G. This pin is best made of plain parallel cylindrical form ground true, and the bosses in the piston should be reamered out to the exact size of the pin. When the pin has been inserted the tapered screws are screwed hard up by means of a special spanner and bear against the pin, preventing it from coming loose or from shaking or knocking. There are many other methods of fixing the gudgeon pin which are not shown here; each has some special point in its favour, but the one illustrated is undoubtedly the best and affords a positive adjustment for wear.
Fig. 16.—Method of fixing the Gudgeon
Pin which allows for Adjustment
after Wear.
An enlarged view of one of the bosses, showing the taper pin in detail and how the split pin Q prevents it from slacking back by contact with the wall of the piston, is shown in Fig. [16]. Sometimes the lower part of the piston is made lighter by drilling holes through the walls. It is very important to reduce the weight of the piston as much as possible, otherwise the engine cannot attain a high speed, so that it becomes essential to bear this in mind when constructing engines for racing purposes. Frequently we find steel pistons used, as they may be made lighter for the same strength, and then steel piston rings may be used; they are not much in favour for ordinary motor-car engines because the steel pistons expand at a greater rate than the cast iron of the cylinder, so that there is more liability to seizure. The crown of the piston is sometimes curved upwards and at other times curved downwards, but more often it is flat as shown in Fig. [17]. The gudgeon pin is sometimes made of mild steel, and the surface is then case-hardened in the centre where the connecting rod end bears. At the present time it is quite as common to find gudgeon pins made of special nickel steel or other steel alloys that do not require case-hardening. On the whole these special steels make the best gudgeon pins and stand the hardest wear.
Fig. 17.—Three forms of Piston Head.
