LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:44 PM |
1. Valve Train Components
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Train Components
Valve trains refer to the mechanisms that control the gas flow into and out of cylinders through the opening and closing of the intake and exhaust valves. Valve trains in four-stroke-cycle engines include a crankshaft-driven camshaft operating at half the engine speed. This next section examines the principal components of the valve train mechanisms.
©2026 - CDX |
1.1. Poppet Valves
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Poppet Valves Poppet valves are the most common type of valve used in diesel engines. A large head and a long stem characterize poppet valves. The head of the intake valves is usually larger than exhaust valves because pressure forcing the air charge into the cylinder is much lower than forces pushing exhaust gases out Figure 25. On diesel engines, the valves are often designed in two pieces that are spun-welded together. The weld, though durable under most operating conditions, can break when valves become overheated. The valve head will separate from the stem and the head will embed itself in a piston if a weld failure occurs Figure 26. The valve stem is typically chrome-plated steel to reduce its requirement for lubrication and to minimize wear and friction. The head is made of specialized alloys to withstand high temperatures and mechanical forces that slam the head into the valve seat. These materials improve the valve’s wear resistance, the ability to withstand high temperature, and the absence of lubricant when using dry fuels, such as natural gas. Stellite ® , Crutonite ® , or Inconel ® are alloys commonly used to make the valve head. These materials are made from a combination of chromium, nickel, tungsten, and cobalt to give the metal great strength as well as corrosion and wear resistance at high temperatures. Crutonite ® is the latest valve material developed with less than half the amount of costly nickel as traditional high-temperature alloys. |
FIGURE 09-25: Nomenclature associated with poppet valves. Note that valve seats are typically cut with three angles to improve gas flow across the seat. |
FIGURE 09-26: A broken weld of a two-piece valve is evidenced by a valve head embedded in the top of a piston. Overheating a valve can cause the weld to break. |
FIGURE 09-27: Two-valve engines use a larger intake valve and a smaller exhaust valve. Fuel distribution is not symmetrical and creates more noxious emissions compared with a four-valve engine. |
Two and four valves per cylinder are common, but a number of engines may use three and five valves. An extra intake valve, usually the third valve (such as used in the Caterpillar 3126B or C9), is used to improve intake airflow for better combustion efficiency, power density, and lower emissions. Four smaller intake and exhaust valves rather than two valves—a single large intake and a single exhaust valve—are used in central direct injection combustion chambers to accommodate a centralized location for the injector nozzle. A centrally located injector provides better uniform fuel distribution in the combustion chamber for lower emissions Figure 27. This emissions reduction feature of four-valve cylinder heads explains why most diesel engines now use four valves rather than two. The downside of four-valve technology is that it adds more complexity to the valve train, and smaller valve heads with less mass cannot withstand intermittent peak exhaust temperatures as well as a larger valve head used in two-valve engines. Note that in two-valve engines, the injector is offset to one side of the combustion chamber and leaned over to make room for the larger single exhaust and intake valves.
©2026 - CDX |
1.2. Solid and Hollow Valves
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Solid and Hollow Valves
|
FIGURE 09-28: Sodium is used to fill hollow exhaust valves. Sodium turns into liquid at high temperatures, enabling it to better conduct heat from the head to the valve guide. |
©2026 - CDX |
1.3. Valve Face Angles
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Face Angles
Valve faces are machined at an angle so that they may wedge themselves into the valve seat and improve sealing. Valve face angles are measured from the horizontal. A 45-degree angle is common and is better at centering the valve at closure while wedging the valve tighter against the valve seat. Some manufacturers use alternate angles. Flatter angles of 30 or 40 degrees can be used on diesel exhaust valve faces to provide for better heat transfer. The valve face and seat angle also shape the valve face-seat gas flow characteristics. For example, some exhaust and intake valve face angles are flatter, using between 20–40 degrees to permit better flow over the edge of the seat when it begins to open. This flatter angle also creates less wear, resulting in fewer overhead adjustments.
©2026 - CDX |
1.4. Valve Guides
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Guides
Valves are located in guides that are either integral, meaning they are cast into the cylinder head, or removable, meaning they can be replaced Figure 29. Premium engines have replaceable valve guides that can be easily serviced during overhaul. The condition of a valve guide is critical to oil consumption control. A limited amount of lubrication oil must pass between the guide and the valve. In some engines, a single spiral groove may be machined to allow some oil into the guide. Guide material, such as sintered iron made of compacted iron particles, can better retain oil in pores for lubrication while maintaining narrow valve stem-to-guide clearances. However, oil can coke, or carbonize, in guides, causing abrasive wear of the guide. This means valve stem-to-guide clearances must be kept very narrow to prevent oil from accumulating in the guide Figure 30.
FIGURE 09-29: Valve guides can be either cast into the cylinder head or made replaceable from an iron alloy insert pressed into the cylinder head. |
FIGURE 09-30: Coking oil in a valve guide can cause premature valve stem and guide wear. |
Similarly, worn guides can allow oil to be drawn down the guide and leave thick carbon deposits on the back of the valve head, restricting gas flow. Oil can also enter the combustion chamber or exhaust stream because of the Venturi effect. This happens as a low-pressure area is created at the base of the valve guide by high-velocity gases that draw oil into the exhaust or intake stream, causing high oil consumption that may be accompanied by blue smoke Figure 31. Older diesel engines, particularly turbocharged engines, did not use seals on the intake guides because the intake manifold was pressurized and had little or no vacuum. However, positive umbrella seals are now used on all valve guides and stems to minimize the entrance of oil into the guide Figure 32. The high temperatures of the guide can cause oil to coke and thus accelerate abrasive wear. Oil entering the exhaust stream can contribute to engine emissions.
FIGURE 09-31: Valve stem seals are used to minimize the entry of oil into the valve guide. Worn guides can allow oil to enter the exhaust or intake stream. |
FIGURE 09-32: A positive valve stem seal prevents oil from entering the valve guide. |
| Technicians Tip | ||
| Exhaust smoke color is an important diagnostic observation. Color depends on the type and size of particles that the light passes through when visible smoke appears in exhaust haze. Large-size particles make smoke appear either as gray or black, depending on the smoke density. However, blue haze is created by vaporized oil and fuel particles, which are some of the smallest particles in the exhaust. The color is called Tyndall blue, named after the scientist who also discovered the greenhouse effect. The blue color of microscopic-size tobacco smoke is another common example of Tyndall blue. As particles increase in size, the light wavelength making up the blue color of light disappears and smoke begins to appear white. To determine whether a white cloud or haze appearing from the exhaust is smoke or steam, carefully observe how it dissipates. Steam or water vapor will rise and dissipate quickly, especially in warm temperatures. Smoke tends to stay closer to the ground and requires more time to dilute with a light wind. |
| Technicians Tip | ||
| Worn guides and missing valve stem seals lead to high oil consumption and blue smoke during deceleration. Oil in the guides can carbonize, accelerating stem wear. Also, any oil consumption in an engine leads to formation of particulate emissions. Long exhaust systems, such as those found on school buses, create a longer and more intense period of low pressure at the valve guide in the exhaust port, especially during deceleration. More oil can be dragged down the stem during deceleration and increase oil consumption, which is almost always higher in school buses when compared to other types of chassis. |
| Technicians Tip | ||
| Combustion quality in diesels at idle is often poor due to low combustion pressures and temperatures. Engine slobber, a residue consisting of unburned fuel, soot, and engine oil, can coat the back face of valves during prolonged idle. When left to cool, slobber will solidify on the back face of an open valve, preventing the valve from closing during engine start-up. This phenomenon is very common in cold-climate conditions when subzero temperatures do not rise above double digits. The result of a valve becoming heavily covered in engine slobber after shutdown is catastrophic engine failure during cold start-up. Solidified slobber prevents the valve from closing and results in piston-to-valve contact during cold start-up conditions. |
©2026 - CDX |
1.5. Valve Seat Inserts
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Seat Inserts
All premium diesel engines use replaceable valve seat inserts Figure 33. Typically, these are manufactured from material such as Stellite ® or Iconel ® . Replaceable inserts eliminate the need for complex machining processes to install a new valve seat into a damaged or rebuilt head. Their wear-resistant hardness and heat resistance add considerable durability and prolong an engine’s operating life. Valve seats have a minimum of three angles to ensure good gas flow and proper transfer of heat from the valve face. A narrow 45-degree seat promotes clean valve seats and good sealing. Three angle cuts on the seat provide improved gas flow and a narrow contact between the valve face and the seat to promote heat transfer Figure 34.
FIGURE 09-33: Replaceable valve guides and hardened valve seats are used on almost every heavy-duty diesel engine. They are pressed into the cylinder head. |
FIGURE 09-34: Valve seat inserts are made from extremely durable materials to prolong engine life. Valve seats and guides are replaceable during the remanufacturing of cylinder head components. |
Typically, valve face-to-seat contact width must be adjusted to within 0.0625–0.125" (1.5875–3.175 mm) for optimal heat transfer. Too wide a seat contact diminishes pressure per square unit of area and will result in less heat transfer from the face to the seat. This may seem counterintuitive, but high- pressure contact transfers heat better than low-pressure contact. To achieve this, a bottoming cutting tool opens the valve seat below the narrow face-to-seat contact line visible on the valve face. A topping cutting tool opens the valve seat above the 45-degree face-to-seat contact line.
©2026 - CDX |
1.6. Valve Lapping
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Lapping
Because contemporary valve head and seat alloys, such as Stellite®, are so hard, it is very difficult to machine them. Instead, the valve parts are precisely cast and finished with some grinding, which means minimal machining is required. It also means that field servicing of valves is not practical apart from using some abrasive lapping compound to seat new valves in a cylinder head or to check valve seat-to-face contact. Lapping a valve involves applying some abrasive compound to the valve face and rapidly turning it back and forth in its seat using a stick with a suction cup attached to the valve head Figure 35. After the valve is lapped and cleaned, pencil marks or a purpose-made machinist dye is applied to the valve face. The valve is reinstalled, and the contact pattern is checked to verify that a uniform contact line is centered in the valve face Figure 36.
FIGURE 09-35: To help seat a valve, abrasive lapping compound is used in the face contact area while the valve is rapidly twisted back and forth using a lapping stick. |
FIGURE 09-36: To check the width and uniformity of the valve face-to-seat contact, vertical pencil markings are made around the circumference of the valve face. The valve is placed back in the seat and turned several times before removing and rechecking the contact pattern. |
©2026 - CDX |
1.7. Valve Protrusion
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Valve Protrusion
|
FIGURE 09-37: Measuring valve protrusion or intrusion is critical after replacing valves and seats. Too much protrusion will allow contact between the piston and the valve; too little will prevent proper gas flow. |
©2026 - CDX |
1.8. Valve Springs
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
Valve Springs
Valve springs hold the valve closed on the seat and must maintain tension on the valve train when the valve is open. If inertia of the valve train is great enough, weak valve springs will allow the valve to stay open after the cam rotates to its inner base circle, which should normally allow the valve to close. This condition is known as valve float . Valve springs with more tension may seem to be a good solution; however, excessive spring tension will slam the valve against the seat. High spring tension will also prematurely wear the valve face and seat, potentially stretch the stem, and wear cam lobes and other valve train components due to high valve opening pressures. HD diesel engines do not rotate fast and generally require only a single valve spring. High-speed automotive diesels will use two valve springs. An inner spring, on some high-speed diesels, helps prevent valve float Figure 38. During service, valve springs should be carefully inspected for free length, cracks, nicks, pitting, straightness, and spring tension. A valve spring testing apparatus, which operates like a weigh scale, is used to check spring tension. The spring is first compressed to a height specified by the manufacturer. Then the spring scale reports the valve spring’s tension in units of pounds, ounces, or grams weight. For example, if a spring is compressed to 1" (2.5 cm), a corresponding manufacturer’s specifications would indicate that an acceptable valve spring pressure of 150 lb (68 kg) or greater is required. If spring tension falls below this pressure (e.g., 145 lb [66 kg]), the valve spring must be discarded Figure 39.
FIGURE 09-38: Parts of a valve mechanism. This valve from a high-speed diesel uses dual springs to prevent valve float. Most low-speed, HD diesels use only one. |
FIGURE 09-39: A valve spring tester measures a spring’s free length and compressed tension. |
©2026 - CDX |
1.9. Valve Cooling
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Valve Cooling Because exhaust gas temperatures can momentarily exceed 1200° F (649° C) as they exit the valve port, valve damage can occur if heat is not removed from the valve. Excess heat must be removed to prevent burning, breakage of the valve stem and head where they are welded, and seat warpage. If the seat is warped, burned, or damaged, the result can lead to valve leakage. Good seat contact provided by a three-angle valve cut of the valve head is important for good heat transfer from the valve to the seat. Also, mechanically rotating the valve slightly with an automatic mechanism each time it opens can eliminate localized hot spots that can lead to valve burning. Valve rotators are commonly installed on either the bottom or the top of at least the exhaust valve to produce this rotation Figure 40. Intake valves are cooled by intake air entering the cylinder and flowing over the valve face and seat. During the valve overlap period, cooler air passes over the exhaust valve face and seat, passing out of the cylinder with exhaust gases. |
FIGURE 09-40: The top of a dual valve spring. The inner spring rubs against the outer spring to minimize valve float, and the rotator turns the valve each time it opens and closes to help cool the valve. |
©2026 - CDX |
1.11. Valve Actuating Mechanisms
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Valve Actuating Mechanisms Transfer of movement from the camshaft to the valves is performed through the valve train. For in-block camshaft engines, the camshaft initiates valve action by transferring force through the cam follower and then to the pushrod, rocker lever, and, finally, the valve. Multivalve setups will use a valve bridge to open two valves with one rocker lever Figure 41. Most of these pushrod-type valve actuating mechanisms will use a roller-type cam follower or lifters and roller rocker levers to reduce friction. This reduction in friction not only increases the durability of the components but also can add 1.5–3% higher power output and reduced fuel consumption. Reducing valve actuation friction is especially important in diesel engines because of very high loading pressure on the camshaft by the exhaust cam followers. This high loading pressure occurs when the exhaust valve opens against cylinder pressure at the end of the power stroke. Cylinder pressures remaining at the end of the power stroke are still high, and tremendous force must be exerted by the camshaft to open the valve and begin the exhaust stroke. The combination of pressure and friction in the exhaust valve train causes premature valve train wear and camshaft failures unless extra features are designed into the exhaust valve train mechanisms. Excessive cam lobe wear is measured using a dial indicator when the camshaft is still in the engine Figure 42. The amount of lift each lobe has is compared against specifications. A worn exhaust will produce an engine misfire or popping sound through the intake air system when the intake valve opens because the exhaust gas cannot leave the cylinder through the exhaust port. |
FIGURE 09-41: Valve bridges actuate two valves with a single rocker lever. Older bridges were adjustable. Newer engines use nonadjustable bridges. |
 |
 |
 |
||
| FIGURE 09-42: Camshaft lobe wear is measured either on the engine with a dial indicator or on the bench. | ||||
Roller-type followers and larger-diameter and wider cam lobes are used in combination with exhaust rocker arm ratios to reduce the pressure acting on the camshaft during valve lift, which increases the durability of exhaust valve train components. Roller-type lifters and followers reduce friction between the camshaft and the follower. When hip or flat lifters are used, the cam may have a special angle ground into the lobe to rotate the lifter as the cam wipes along the bottom of the lifter Figure 43. This rotation minimizes premature lifter wear. Offsetting the lifter slightly from the lobe centerline will also cause lifter rotation Figure 44.
FIGURE 09-43: A comparison of a hip and a roller mechanical lifter. The fork used on the roller lifter prevents the lifter from spinning in its bore. |
FIGURE 09-44: Hip valve lifters follow the cam profile and have a centerline slightly offset from the cam lobe centerline. When the cam turns, the lifter rotates on its axis, reducing friction between the lobe and the lifter. |
| Technicians Tip | ||
| Adjustments to valve bridges are generally not performed during scheduled valve and injector adjustments. Whenever they are adjusted, they must never be loosened or tightened while placed on the valve stems. Pulling a wrench to loosen the locknuts will bend the valve stems, causing the valves to stick open. Always remove the bridge and loosen it in a soft-jawed vise. After setting proper clearances on the engine, the bridge is removed again and tightened in a vise. The locknut should be only lightly tightened while on the engine and only firmly torqued while in a vise. |
©2026 - CDX |
1.12. Pushrods
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Pushrods Pushrods transmit the cam actuation force from the followers to the rocker levers. To reduce weight and inertia at high speeds, longer pushrods are usually hollow. When servicing, the pushrods should be checked for wear at each end as well as bending caused by miss-adjustment or high-speed valve float. Some pushrods will use a hardened end at the rocker lever to withstand greater friction at this point. If the weld at the ball socket end fractures, oil can enter and accumulate inside the pushrod. The easiest way to check for oil entering the pushrod is to listen to the sound made when the pushrod is dropped. A pushrod filled with oil will make a distinctively different thud-like sound when it is dropped and will not bounce as much as a good pushrod. Shorter, solid pushrods capable of transmitting larger forces with less flexing are also commonly used with high-mounted camshafts Figure 45. Solid, short pushrods allow for higher injection pressures and reduced injection lag. |
FIGURE 09-45: A high-mounted camshaft is located as close to the cylinder head as possible to shorten the length, inertia, and weight of the pushrods. Solid, short pushrods will not flex easily. |
©2026 - CDX |
1.13. Rocker Arms
LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train. |
|
Rocker Arms Rocker arms convert the linear change of the cam lobe profile to the reciprocating motion used to open and close the valves. The arms may be constructed of stamped, cast, or forged steel. A roller is incorporated into the levers of overhead cam engines to reduce friction. A much wider roller is used on the exhaust valve rocker lever because cylinder pressures are higher when the exhaust valve opens, and the loading of the roller is much greater than on the intake valve. The roller for an injector lobe is the widest because injection pressures can reach as high as 33,000 psi (2275 bar) on engines such as the Cummins ISX. Hardened wear pads may be welded to the arm, or the arm may be induction hardened for increased durability. If rocker arms are of different lengths on the engine, the exhaust rocker lever is often longer than the intake lever Figure 46. This is due to the cam profile of the exhaust cam lobe. A less aggressive profile with a smaller lift is used on the camshaft’s exhaust lobe to minimize wear. A low rocker arm ratio means a greater mechanical advantage is used on the exhaust lever to multiply the cam lobe lift and open the valve more rapidly. This translates into less friction on the exhaust cam lobe. A larger exhaust rocker arm lever ratio is not a rule, though—there are many exceptions. |
FIGURE 09-46: This Detroit Series 60 diesel uses a rocker arm with two levers to actuate the valves. Note that the injector and exhaust valve rollers are wider due to the greater loading forces. |
Crossheads, Yokes, and Valve Bridges
To actuate more than one valve with a single rocker lever, most four-valve engines use a valve crosshead to exert the camshaft force against the valve stem. Crossheads are also called yokes and bridges . There are two varieties of crossheads: nonadjustable, guideless crossheads and adjustable crossheads that use a guide pin to locate the device on the cylinder head deck.
©2026 - CDX |