Valve: There are two types of valves: 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 opens and closes the intake and exhaust passages, while the stem guides the valve's movement. Intake valves are made of ordinary alloy steel, while exhaust valves are made of heat-resistant alloy steel because the exhaust valve head is in direct contact with the combustion gases and is subject to severe heat.
Valve Seat: The valve seat is a circular seat hole machined directly into the cylinder block (for side-mounted valves) or cylinder head (for top-mounted valves). It mates with the valve to provide a seal. Some valve seats are made of wear-resistant alloy cast iron and machined into a seat ring, which is then embedded in the cylinder block or cylinder head.
Valve Guide: Its function is to guide the valve to make the correct axial movement and indirectly transfer the heat from the valve stem to the water jacket. For ease of repair and replacement, the valve guide is manufactured separately and then pressed into the cylinder block (or cylinder head). When the valve guide is pressed into the cylinder block (or cylinder head), a certain interference fit and insertion depth should be maintained to ensure good heat transfer.
Valve spring: Its function is to ensure a tight seal between the valve and valve seat, and to mitigate and overcome the inertial forces generated by the valve and other transmission components, preventing disruption of the normal operation of the valve train.
Valve tappet: It transmits the camshaft's pushing motion to the valve (side-mounted) or pushrod (overhead-mounted) to control the valve's opening and closing.
Valve pushrod: In an overhead-mounted valve train, it transmits the tappet's motion to the rocker arm. The pushrod is a straight rod made of hollow steel tubing, with different shaped ends welded to both ends. The upper end is a concave spherical shape, where the ball head of the rocker arm's adjusting screw rests; the lower end is a round ball head for insertion into the tappet's concave ball bearing.
Valve rocker arm: Its function is to change the direction of the tappet's motion and transmit it to the valve. It is a double-armed lever of unequal length with a central hole. The long arm has an arc-shaped working surface at its end that contacts the valve tail; the short arm has a threaded hole at its end for installing adjusting screws and lock nuts to adjust valve clearance. The middle section houses the rocker arm bearing, which contains a bronze bushing.
Rocker Arm Shaft: This is a hollow round shaft mounted on the cylinder head with several supports. The rocker arm is fitted onto the rocker arm shaft and can swing in an arc on the shaft. The shaft's inner bore communicates with the main oil passage, supplying lubricating oil to the valve train.
Camshaft: Used to control the closing time and opening degree of each cylinder's valves. It also drives the oil pump, fuel pump, distributor, and other accessories. It is a single unit consisting of the intake camshaft, exhaust camshaft, journal, gears driving the oil pump and distributor, and an eccentric wheel driving the fuel pump rocker arm.
Timing Gear: The camshaft is typically driven by the crankshaft through a pair of timing gears. The pinion gear is mounted at the front end of the crankshaft and is called the crankshaft timing gear. The large gear is mounted at the front end of the camshaft and is called the camshaft timing gear. The ratio of the large to small gears is 2:1, ensuring that for every two rotations of the crankshaft, the camshaft rotates one revolution.
To ensure correct valve timing and ignition timing, meshing marks are engraved at corresponding positions on both gears. To limit axial movement of the camshaft caused by changes in engine speed during operation, axial limiting devices are installed during installation.
