Some piston rings are coated with a soft material to assist them to ‘bed in’. New rings and cylinders have very small irregularities and when these wear off, the rings will provide a better seal. To assist this process, the rings can be coated with phosphate, graphite or iron oxide. These are relatively soft substances which wear more quickly than the ring material. The coatings also absorb oil and help with ring lubrication. This prevents scuffing of the cylinder walls.
To prevent wear, the face of the piston ring can be coated with chromium. This is a relatively hard material, but it operates well against the cast-iron cylinder walls without scuffing.
The piston pin secures the piston to the small end of the connecting rod and also provides the wrist-like action that is needed between the two parts.
There are different methods used to retain the piston pin, but two that are commonly used are shown in Figure 6.21. These are:
2. Floating pin. The piston pin is arranged to float in both the eye of the connecting rod and the piston. Circlips in grooves in the piston bosses retain the piston pin and prevent it from coming into contact with the cylinder walls.
There are variations of the designs shown. With some floating piston pins, thrust pads of soft metal are fitted in the ends of the pins. The pin floats in the piston and in the connecting rod. The thrust pads are allowed to come into contact with the cylinder wall, but being soft and lightly loaded, they do no hanu.
With another design, the piston pin is clamped in the connecting rod. The eye of the rod is split and a bolt is used to form it into a clamp. This design increases the mass at the small end of the connecting rod, which is not desirable because it adds to the mass of the reciprocating parts.
Most piston pins depend on splash and oil mist for lubrication, but some piston pins are pressure- lubricated. For this, an oil passage is drilled the full length of the connecting rod. Oil from the connecting- rod bearing passes through the passage to the piston pin.
Most engines have a connecting rod with a ‘spit hole’, or oil jet, in the crank-pin end. This hole lines up with the oil hole in the crank-pin once on each revolution of the crankshaft. When this occurs, a squirt of oil is directed onto the cylinder walls as shown in Figure 6.22. This also lubricates the piston pin.