Pins are usually hollow and made of alloy steel, machined, hardened, and precision-ground to fit the bearings. Their construction provides maximum strength with minimum weight. Some pins are chromium-plated to increase the wearing

Figure 4-15.Cross section of a semifloating piston and connecting rod assembly.

qualities. The pins are lubricated by splash from the crankcase, by oil forced through drilled passages in the connecting rods, or by the use of piston oil spray nozzles.

Piston pins must be secured in position so that they do not protrude beyond the surface of the piston or have excessive end-to-end motion. Otherwise, the pin will tend to damage the cylinder wall. Piston pins may be secured in the connecting rod assembly in one of three ways: (1) rigidly fastened into the piston bosses, (2) clamped to the end of the rod, or (3) free to rotate in both piston and rod. When piston pins are secured by these methods, the pins are identified as (1) stationary (fixed), (2) semifloating, and (3) full-floating, respectively.

The STATIONARY pin is secured to the piston at the bosses, and the connecting rod oscillates on the pin. Since all movement is by the connecting rod, uneven wear may occur on the contacting surfaces in this type of installation. For this reason, use of this type of pin is not typical in Navy diesel engines.

SEMIFLOATING pins are secured in the middle to the connecting rod (fig. 4-15). The ends of the pin are free to move in the piston pin bearings in the bosses.

FULL-FLOATING pins are not secured to either the piston or the connecting rod. Pins of this type may be held in place by caps, plugs, and snap rings, or spring clips which are fitted in the bosses. (See fig. 4-12.) The securing devices for a full-floating pin permit the pin to rotate in both the rod and piston pin bosses. Of the three types of piston pins, the full-floating piston pin is the most common.