The fork-and-blade rod assembly is used primarily in V-type  engines. The forked rod is split at the crankpin end to allow  space for the blade rod to fit between the prongs. A single two- piece bearing is used on the crankshaft end of the rod. This  type of connecting rod is not used much on modern engines. The piston of a reciprocating engine is a cylindrical member  which moves back and forth within a steel cylinder.  [Figure 1-13]  The piston acts as a moving wall within the  combustion chamber. As the piston moves down in the  cylinder, it draws in the fuel/air mixture. As it moves upward,  it compresses the charge, ignition occurs, and the expanding  gases force the piston downward. This force is transmitted  to the crankshaft through the connecting rod. On the return  upward stroke, the piston forces the exhaust gases from the  cylinder and the cycle repeats. The majority of aircraft engine pistons are machined from  aluminum alloy forgings. Grooves are machined in the  outside surface of the piston to receive the piston rings, and  cooling fins are provided on the inside of the piston for greater  heat transfer to the engine oil. Another method of minimizing the adverse effects on engine  operation is to use a compensated magneto. In this magneto  the breaker cam has a number of lobes equal to the number  of cylinders on the engine. To compensate for the variation  in piston position due to link rod angularity, the breaker cam  lobes are ground with uneven spacing. This allows the breaker  contacts to open when the piston is in the correct firing position.  This is further outlined during the discussion on ignition timing  in Chapter 4, Engine Ignition and Electrical Systems. Pistons may be either the trunk type or the slipper type.  [Figure 1-14]  Slipper-type pistons are not used in modern,  high-powered engines because they do not provide adequate  strength or wear resistance. The top of the piston, or head,  may be flat, convex, or concave. Recesses may be machined  in the piston head to prevent interference with the valves. Modern engines use cam ground pistons that are a larger  diameter perpendicular to the piston pin. This larger diameter  The knuckle pins are of solid construction except for the  oil passages drilled in the pins, which lubricate the knuckle  pin bushings. These pins may be installed by pressing into  holes in the master rod flanges so that they are prevented  from turning in the master rod. Knuckle pins may also be  installed with a loose fit so that they can turn in the master rod  flange holes, and also turn in the articulating rod bushings.  These are called full-floating knuckle pins. In either type of  installation, a lock plate on each side retains the knuckle pin  and prevents a lateral movement. Plain-type connecting rods are used in inline and opposed  engines. The end of the rod attached to the crankpin is fitted  with a cap and a two-piece bearing. The bearing cap is held  on the end of the rod by bolts or studs. To maintain proper  fit and balance, connecting rods should always be replaced  in the same cylinder and in the same relative position. 1-12 Figure 1-13. A piston. Figure 1-12. Elliptical travel path of knuckle pins in an articulated  rod assembly.  Fork-and-Blade Rod Assembly Pistons Piston Construction Knuckle Pins Plain-Type Connecting Rods 1 3 7 2 5 6 4