valve operation. If the inlet valve is operated by suction only, the spring is comparatively light and the exhaust valve has the strongest spring. When mechanical valves are employed and both are the same, the exhaust and inlet valve springs are of equal strength to facilitate replacement and to make them interchangeable during manufacturing.

Q. How strong should valve springs be?

A. Valve springs should be just strong enough to hold the valve firmly against its seat and to return it to a closed position at the maximum speed of the motor just as soon as the cam roller leaves the raised portion of the valve operating cam. If valve springs are too strong, considerable power will be consumed in raising them, whereas if too weak the valve operation and consequently the engine action will be erratic.

Q. Name advantages of enclosed valve gear.

A. On modern motors the tendency is to enclose the valve operating plungers and springs in chambers formed in the cylinder castings by means of cover plates. This is desirable practice because it keeps dirt and grit away from the valve operating plungers and valve stems and insures superior quietness of operation. (Fig. 23.)

Q. What is the piston and of what material is it made?

A. The piston is a reciprocating cylindrical member that moves in the cylinder and which transforms the power of the explosion to mechanical energy. Pistons are usually made of close grained gray iron of approximately the same mixture as the cylinder iron though where great lightness is desired, as on aeroplane motors, steel may be employed. (Fig. 45.)

Q. What are the piston bosses?

A. The piston bosses are projections cast inside of the piston to hold the wrist pin or member that supports the upper end of the connecting rod. (Fig. 46.)

Q. What are the piston rings?

A. A piston is usually provided with a series of grooves in which rings of cast iron are mounted to form a packing. The piston must

be a free fit in the cylinder in order that it will not expand unduly when heated and bind. For this reason the packing rings are depend

ed upon to keep the exploding gases from leaking by, and as they have considerable elasticity they conform to the cylinder bore and fit it very closely. As they are narrow they do not have much bearing surface on the cylinder and do not offer undue friction if properly lubricated. The piston rings are usually placed at the top of the piston, as shown at Fig.

45, though in rare instances they have been applied to the bottom of the piston, as outlined at Fig. 46.

Q. Name common form of piston rings.

A The piston rings may have a uniform thickness at all points and the inside be concentric with the outside or they may be thinner at one side than the other and the inner circle of the ring be placed eccentrically in relation to the outer periphery. The eccentric ring is the type more generally used, as it is believed to have a more uniform expansion when heated than the concentric form. The piston ring is always thinnest at the point where it is split.

Q. Why are piston rings split?

A. Piston rings are split for two reasons, the most important being to obtain sufficient elasticity so that they may be sprung in place in the piston grooves and also that they may expand sufficiently to take up the space existing between the piston and cylinder walls.

Q. Why are piston rings carefully machined?

A Piston rings are carefully machined so they will be true to cylinder bore and fit the grooves of the piston accurately and at the same time they must have a smooth surface where they contact with the cylinder walls in order to reduce friction to a minimum. After piston rings have been in use for a time the surface becomes covered with a hard glaze and there is practically no friction between the ring and the cylinder.

Q. What is the difference in size between piston diameter and cylinder bore?

A. The piston is always made smaller than the cylinder by a few thousandths of an inch. The difference between diameter and bore is regulated by the size of the engine and the methods of cooling employed. On ordinary automobile engines the piston has three diameters, being smallest at the top and largest at the bottom. The amount of taper of the piston walls is so slight that it is not noticeable except with delicate measuring instruments. The piston of a five inch bore engine would be .007" smaller at the top than the cylinder or 4.993" in diameter. At the middle point approximately on the center line of the piston bosses the piston would be 4.995" in diameter or .005" smaller than the cylinder bore. At the bottom the piston would be about .003" smaller than the cylinder bore.

Q. Why is this difference in size necessary?

A. The piston is made smaller than the cylinder bore in order to prevent friction between piston and cylinder walls as they become heated in operation and to provide a space between the reciprocating and fixed members or an oil film

Q. What is the wrist pin?

A. The wrist pin is a short bar of steel of circular or tubular cross section placed between the two piston bosses to which the upper or small end of the connecting rod is fastened.

Q. How is the wrist pin secured in piston bosses?

A. The wrist pin is usually a push fit in the piston bosses and is kept from moving out of place by some form of lock which passes through both piston boss and wrist pin. The simplest form of lock

is a common set screw which bears against the wrist pin or some form of taper pin or split cotter, passing through the piston bosses into the wrist pin interior. (Fig. 46.)

Q. What is the connecting rod?

A. The connecting rod is the member that forms the connecting link between the reciprocating piston and the rotary crankshaft. It describes a rotary movement at its lower end and oscillates at its upper end. (Fig. 47.)

Fig. 47.-Connecting Rod Assembly and Bushings Used on Crank Shaft Main Bearings.

Q. What determines connecting rod length?

A. The length of the connecting rod depends entirely upon the stroke of the engine. Long stroke motors have longer connecting rods than short motors. A connecting rod is usually equal to 2.5 times the stroke of the piston in length.

Q. What materials are used in connecting rods?

A. Connecting rods are invariably made of steel drop forgings at the present time, though formerly steel, bronze, and even malleable iron castings were used.

Q. Name two common connecting rod types.

A. The common forms of connecting rods used on automobile

motors are shown at Fig. 48. type and the lower cap is an connecting rod proper and is attached thereto by means of two or four bolts. The form outlined at B is a modified marine type and in this the lower cap is hinged at one side and attached by means of a bolt at the other. (See also Figs. 45, 47.)

That at A is known as the marine entirely separate member from the

B-Hinged

Fig. 48.-Outlining Method of Constructing Connecting Rod Big Ends.
A-Marine Type with Completely Removable Cap.
Type.

Q. What is the upper connecting rod bearing made of?

A. The upper connecting rod bearing is usually a bushing of hard bronze forced into the boss at that end of the connecting rod. It bears on and runs in connection with a case hardened steel

wrist pin.

Q. What is the lower connecting rod bearing made of?

A. The lower connecting rod bearing, which encircles the crank pin of the crankshaft, is usually composed of some anti-friction metal, such as Babbitt on the cheap light engines or Parsons white bronze or similar alloy on the more expensive motors. On light en