Push rods and rocker arms



In the overhead valve design, the valve is in an inverted position, and the engine must be designed to incorporate push rods and rocker arms. Instead of the valve resting on the valve lifter, the push rod rests on it and actuates the rocker arm, and this in turn the valve. The length of the push rod is adjustable to provide proper valve clearance. This, owing to their greater length, averages .008 in. to .015 in., as compared with .004 in. to .006 in. for the L-head engine.

Connecting rod design

As a rule, the connecting rod is forged in the form of an I beam. The upper end of the rod carries the piston pin, and the lower end is designed to be mounted on the throws of the crankshaft. The style of the connecting rod is determined by the method of mounting the piston pin. In the clamp type the pin is inserted in the boss at the upper end which is split on one side. A bolt or cap screw is used to draw this split boss tight onto the pin, and thus the pin is secured rigidly in the rod end. The pin may be made with a slight notch at its centre to receive a section of the clamping bolt, which centres the pin in the rod and prevents end movement.

The second type of rod carries a bronze bushing in the upper end, and the pin is fitted to it so that it floats in this bushing. As a rule, pins for this type of rod are fastened into the piston so that they may not turn nor move endways, but some designs allow the pin to float in the piston-pin bosses.

In an attempt to get the best engine design, connecting rods have been made longer. At the same time, they are much lighter than those used in early engines. The lower end of the Pontiac rod carries two connecting-rod bolts to bolt the cap to the rod. The bearing at the lower end is smaller for some of the lighter rods. Crankshafts were not so heavy in early designs, and rods and pistons were comparatively heavier. The weight of connecting rods is a vital element in reducing the vibration of the engine, as well as in determining its top speed. This is the reason for balancing connecting-rod and piston assemblies to close limits.

Standard practice for the lower end of the connecting rod has always been to make it of the split-bushing type. Common practice spins and sweats the babbitt metal into the connecting rods, since this aids in heat dissipation. Replaceable bearing shells are used in many engines.