[vilant]

Valves and seats
Engines can also be classified by valve arrangement and valve train configuration. The various valve train configurations may be grouped into 2 categories based on the location of the valves. The first category is the group with valves located beside the cylinders and pistons. The second category is the group with the valves located over the pistons in the cylinder heads.
Engines with valves in the cylinder block are known as flathead engines. There were 2 types: T-head and L-head (they got their names from an imaginary letter formed from the piston and valve heads). I won’t get into to detail about these because they are obsolete.
Engines with valves in the cylinder head come in 2 groups. The first group has their camshafts located in the cylinder block, they are known as overhead valve (OHV) engines. The I-head engine is an OHV engine with the camshaft in the cylinder block. The camshaft operates the valves through the lifter, push rod, and rocker arm. It gets its name from the imaginary letter formed by the piston and the valve (see figure 17a).The second group has their camshafts located in the cylinder head, they are known as overhead camshaft (OHC) engines. Here are the configurations of OHC engines:
1-Single overhead camshaft – this configuration has one camshaft operating both intake and exhaust valves. There are 2 types of configurations; one that operates the valves directly through the lifters and one that operates the valves through rocker arms (see figure 17a).
2-Double overhead camshaft- this configuration has 2 camshafts, one for intake valves and one for the exhaust valves. They operate the valves directly through the lifters.

Another type is F-head engines, which are a combination of the 2 valve arrangements. In this engine the intake valves are the overhead type located in the cylinder head. The exhaust valves, however, are located in the cylinder block. This configuration is not used anymore.


Added 4/29/13
Each cylinder in a 4 stroke cycle engine must have one intake and one exhaust valve, or on the LT-5, 2 intake valves and 2 exhaust valves. The valves that are commonly used are of the poppet design. The word poppet is derived from the popping action of the valve. Poppet-type valves are made in the following 3 basic shapes: the mushroom, semi-tulip, and tulip (see figure 18). The valve shape is dependent upon the requirements of its engine and combustion chamber shape.
Construction and design considerations are different between intake and exhaust valves. The difference is based on their temperature operating ranges. Intake valves are cooled by the incoming fuel mix and exhaust valves are subjected to intense heat from the burnt gases that pass by it. The temperature of the exhaust valve can be in excess of 1300F (704.04C). Intake valves are made of a nickel chromium alloy and exhaust valves are made of silichrome alloy. In certain heavy-duty and most air-cooled engines, the exhaust valves are hollowed out and filled partially with metallic sodium. The sodium, which liquefies at operating temperatures, splashes between the valve head, where it picks up heat, and the valve stem, where the heat is transferred to the valve guide. Some exhaust valves use a special hard facing process that keeps the face of the valve from taking on the shape of the valve seat at high temperatures (see figure 18).
Valve seats are very important, they must match the face of the valve head to form a perfect seal. The seats are made so that they are concentric with the valve guides; that is, the surface of the seat is an equal distance from the center of the guide all around. There are 2 common angles used, when machining the valve seat, 30* and 45*. The LT-5 valve seat angle is 44*. The face of the valve is usually ground with a one-half to 1 degree difference to help the parts seat quickly. The LT-5 valve face angle is 45*. In some cases, a small portion of the valve seat has an additional 15* degree angle ground into it to narrow the contact area of the valve face and seat. By reducing this contact area, the pressure between the mating parts is increased, which makes a better seal (see figure 18). Valve seats can be either a part of the cylinder head or separate inserts. Valve seat inserts are generally held into the head with an interference fit. The head is heated in an oven to a uniform high temperature and the seat insert is shrunk by cooling it in dry ice. While the 2 parts are at opposite temperature extremes, the seat insert is pressed into place.
Valve guides are the parts that support the valves in the head. They are machined to a fit of a few thousandths of an inch clearance with the valve stem. This close clearance is important for the following reasons:
1- It keeps the lubricating oil from getting sucked into the combustion chamber past the intake valve stem during the intake stroke.
2- It keeps exhaust gases from getting past the exhaust valve stems and into the crankcase area during the exhaust stroke.
3- It keeps the valve face in perfect alignment with the valve seat. Valve guides may be cast into the head or they may be removable (see figure 18). Removable valve guides are usually press fit into the head.

Added 5/6/13
The valve assembly is completed by the spring, retainer, and seal. Before the ring and retainer fit into place, a seal is placed over the valve stem. This seal acts like an umbrella to keep the valve operating mechanism oil from running down the stem and into the combustion chamber. The spring, which keeps the valve normally closed, is held in place by the retainer. The retainer locks onto the valve stem with 2 wedge –shaped parts called valve keepers.
It is common in heavy-duty applications to use valve rotators. The purpose is to keep carbon from building up between the valve face and seat, which could hold the valve partially open. The release-type rotator releases the spring tension from the valve while open. The valve will then rotate from engine vibration. The positive rotator is a 2 piece valve retainer with a flexible washer between the 2 pieces. A series of balls between the retainer pieces roll on the machined ramps as pressure is applied and released from the opening and closing of the valve. The movement of the balls up and down the ramps translates into rotation of the valve.
The camshaft provides for the opening and closing of the engine valves. The tappets or lifters are the connecting link between the camshaft and the valve mechanism. Camshafts are usually made from cast or forged steel and the surfaces of the lobes are hardened for longer life.
The camshaft is supported, and rotates, in a series of bearings along its length. These bearings are pressed into their mountings and are made of the same basic construction as crankshaft bearings. In some cases, like the LT-5, when the engine is constructed of aluminum, the camshaft is supported directly in its mountings and no bearings are used. The thrust, or back and forth movement, usually is taken up by the thrust plate(s), which bolts onto the front and or rear of the engine block. The LT-5 employs bolt-on retainers and thrust washers to prevent thrust movement. The drive gear or sprocket is bolted onto the front of the camshaft. There are 3 basic configurations for driving the camshaft (figure 19):
1-Gear drive- A gear on the crankshaft meshes directly with another gear on the camshaft. The gear on the crankshaft is usually made of steel, and the camshaft gear may be steel (for heavy-duty applications), aluminum, or pressed fiber (when quiet operation is a major consideration). The gears are helical in design because they tend to push the camshaft rearward during operation to help control thrust.
2-Chain drive- Sprockets on the camshaft and the crankshaft are linked by a continuous chain. The sprocket on the crankshaft is usually made of steel, while the camshaft sprocket may be steel (for heavy-duty applications), aluminum, or aluminum with nylon covering the teeth (when quiet operation is a major consideration). Since the LT-5 has 4 camshafts, each side is connected to the crankshaft by a sprocket assembly. You can see a cut-out picture of one side of camshafts in figure 3. There a 2 common types of timing chains. One is a silent link type chain that is used in standard and light-duty applications. The other is a roller-link chain (which may have a single or double row of links), which is used in heavy-duty applications.
3-Belt drive- Sprockets on the crankshaft and camshaft are linked by a continuous neoprene belt. The belt has square-shaped internal teeth that mesh with the sprockets. The timing belt is reinforced with nylon or fiberglass to give it strength and prevent stretching. This drive configuration is limited to overhead camshaft engines.

Most engines with chain or belt-driven camshafts use a tensioner. The tensioner pushes against the belt or chain to keep it tight. This helps to keep it from slipping, provide more precise valve timing, and compensate for component stretch and wear. Belt-driven configurations use a spring-loaded idler wheel. Chain-driven configurations usually use a fiber rubbing block that is either spring-loaded or hydraulic. The hydraulic tensioner works by the same principle as a hydraulic lifter, we’ll get to them shortly. The hydraulic tensioner is more desirable with rubbing blocks because it doesn’t exert excessive pressure, resulting in longer component life.
The camshaft and crankshaft must remain in the same relative position to each other. Because the crankshaft rotates twice as fast the camshaft, the drive sprocket or gear on the crankshaft must be exactly one-half the size of the drive sprocket or gear on the camshaft. For the camshaft and crankshaft to work together properly, they must be in the proper initial relation to each other. This initial position between the 2 shafts is designated by marks called timing marks. These 2 timing marks are aligned at the time of assembly.
Camshafts can also have auxiliary functions, like driving other engine components. There are sometimes gears machined into the camshaft that will drive the oil pump and distributor. There also may be an extra lobe on the camshaft to drive the fuel pump.
Tappets (or lifters) are used to link the camshaft to the valve mechanism. The bottom surface is hardened and machined to be compatible with the surface of the cam lobe. There are 2 basic lifter classifications:
1-Mechanical tappets- Mechanical lifters are simply barrel-shaped pieces of metal. On flathead engines, there is an adjusting screw mechanism to set the clearance between the tappet and valve stem. Some lifters may also have wider bottom surfaces. These are called mushroom tappets. Another variation is the roller tappet, which has a roller contacting the camshaft. This type of lifter is used mainly in heavy-duty applications to reduce wear (see figure 20).
2-Hydraulic tappets-The hydraulic lifter is popular in overhead valve engines (and used on the LT-5), it uses oil under pressure to automatically maintain zero clearance in the valve mechanism or as stated in the LT-5 FSM “The hydraulic lifters maintain zero lash between the camshaft lobes and valve stems.” The lifter body, which contacts the cam lobe, is hollow. Inside the body lifter there is a plunger that operates the valve mechanism. Injecting oil into the cavity under the plunger will regulate its height, thereby adjusting valve mechanism clearance. The hydraulic tappet (see figure 20 hydraulic tappets) operates as follows: oil supplied by the engine lubrication system reaches the lifter body and enters through passage (A). The oil then passes through the passage (B) to fill the plunger. The oil then passes through passage (C) where it pushes the check valve off its seat to enter the cavity under the plunger. As the oil fills the cavity, it pushes the plunger up, to where it contacts the valve mechanism. When the cam lobe pushes the lifter body up, the oil is trapped in the cavity and cannot escape because the check ball seals the opening. This trapped oil then becomes a solid link between the lifter body and the plunger. The constant pressurized supply of oil will maintain the zero clearance.

The face of the tappet and the cam lobe are designed so that the tappet will rotate during operation. This is done by machining a slight taper in the cam lobe that mates with a crowned lifter face. Using this type of design causes the tappet face to roll and rotate on the lobe, rather than slide. This rotating will increase component life (see figure 21).
The valves in overhead valve and overhead camshaft engines can use additional components to link the camshaft to the valves. Overhead valve engines use push rods and rocker arms. Overhead camshaft engines use various configurations of rocker arms (the LT-5 does not however).

Push rods(figure 21)- are usually constructed of hollow steel. Rocker arms (figure 21) are made from steel, aluminum, or cast iron. The most common are stamped steel, which is lightweight, strong, and cheap to make. They usually pivot on a stud and ball, though some engines use a shaft arrangement. Aluminum rocker arms are used generally on small high-speed applications. In some applications, like competition, the aluminum rocker arms will be pivoted on needle bearings.

Adjusting valve clearance on solid-tappet, valve-in-head engines is usually done by a screw on the rocker arm. On overhead valve (or push rod engines) there is usually a screw-type adjustment where the push rod actuates it. The adjusting screw can be either the self-locking type or have a jambnut to lock it. A few engines are equipped with adjustments on an adjustable mounting pivot. By turning the adjusting screw the height of the rocker arm changes. On overhead camshaft engines the camshaft is positioned directly over the top of the valve stems. On these engines the valve clearance is adjusted by putting shims between the cam lobe and the lifter. Various thicknesses of shims are used to obtain the desired clearances.