Valves: Valves are classified into two types: intake valves and exhaust valves. Their function is to open and close the intake and exhaust passages; they consist of a head and a stem. The head serves to open and close the passages, while the stem guides the valve's movement. Intake valves are typically made of standard alloy steel, whereas exhaust valves are made of heat-resistant alloy steel, as the head of the exhaust valve comes into direct contact with combustion gases and is subjected to intense heat.
To ensure a tight seal and effective heat transfer between the valve head and the valve seat, a conical mating surface is employed between the two, which is subsequently ground to a precise finish. The angle formed between this conical surface and the flat top surface of the valve head is known as the valve face angle; common valve face angles are 30° and 45°. The valve stem acts as the guiding component for the valve's movement. Typically, the tail end of the valve stem features a machined groove designed to accommodate the tapered keepers (valve locks).
Valve Seats: A valve seat is a circular bore machined directly into the cylinder block (in side-valve configurations) or the cylinder head (in overhead-valve configurations); it mates with the valve to provide a sealing function. In some designs, the valve seat is manufactured as a separate ring-often made of wear-resistant alloy cast iron-which is then press-fitted into the cylinder block or cylinder head.
Valve Guides: The function of the valve guide is to ensure the valve undergoes correct axial movement while simultaneously facilitating the indirect transfer of heat from the valve stem to the surrounding water jacket. To facilitate ease of repair and replacement, valve guides are manufactured as separate components and then press-fitted into the cylinder block (or cylinder head). When press-fitting a valve guide into the cylinder block (or head), specific interference fit tolerances and insertion depths must be maintained to ensure optimal heat transfer.
Valve Springs: The primary function of valve springs is to ensure a tight seal between the valve and its seat. Additionally, they serve to dampen and counteract the inertial forces generated by the valve and other associated valvetrain components, thereby preventing any disruption to the proper operation of the valve timing mechanism.
Valve Lifters (Tappets): These components transmit the lifting motion from the camshaft to the valve (in side-valve configurations) or to the pushrod (in overhead-valve configurations), thereby controlling the opening and closing of the valves.
Valve Pushrods: In overhead-valve valvetrain systems, the pushrods transmit the lifting motion from the valve lifters to the rocker arms. The pushrod is a straight rod constructed from hollow steel tubing, with ends of differing shapes welded to each extremity. The upper end features a concave spherical socket, into which the spherical tip of the rocker arm's adjustment screw seats; the lower end features a convex spherical tip, designed to insert into the concave spherical seat of the tappet.
Valve Rocker Arm: Its function is to redirect the motion of the tappet and transmit it to the valve. It is an unequal-length, double-armed lever featuring a circular bore at its center. The end of the longer arm presents a curved working surface that contacts the tail of the valve; the end of the shorter arm contains a threaded hole for mounting an adjustment screw and locknut, used to regulate the valve clearance. The central section houses the rocker arm bearing, which is fitted with an internal bronze bushing.
Rocker Arm Shaft: This is a hollow cylindrical shaft mounted to the cylinder head via several support pedestals. The rocker arms are sleeved onto the shaft, allowing them to pivot in an arc-like motion. The hollow interior of the shaft communicates with the main oil gallery, thereby supplying lubricating oil to the valve train components.
Camshaft: Used to control the timing of valve closure and the variation in valve lift for each cylinder. It also drives various accessories, such as the oil pump, fuel pump, and distributor. It is manufactured as a single integral unit comprising intake cams, exhaust cams, journals, a gear for driving the oil pump and distributor, and an eccentric lobe for actuating the fuel pump rocker arm.
Timing Gears: The camshaft is typically driven by the crankshaft via a pair of timing gears. The smaller gear is mounted on the front end of the crankshaft and is known as the crankshaft timing gear. The larger gear is mounted on the front end of the camshaft and is known as the camshaft timing gear. The gear ratio between the large and small gears is 2:1, ensuring that for every two full rotations of the crankshaft, the camshaft completes one full rotation.
To ensure precise valve timing and ignition timing, corresponding meshing marks are inscribed on both gears. Furthermore, to limit the axial thrust (longitudinal shifting) of the camshaft-which may occur during operation in response to changes in engine speed-an axial thrust limiting device is incorporated into the assembly during installation.
