The Valves

What do they do?

       The valves allow the air/fuel mixture into the cylinder, and the exhaust gas out of the cylinder.

Valve Design:

        The type of valves used in almost all engines are "Poppet Valves". The matching angle of the face and seat, means the more pressure is created in the cylinder, the better the valves seal.
        Most engines use one intake valve, and one exhaust valve per cylinder, but an increasing number of high performance engines use four, or even five valves per cylinder. We'll talk about them later.

        Cylinder head design, and how fast the fuel/air mixture can flow through the ports, is extremely critical to engine performance. To make more power, an engine must burn more fuel, and the more efficient the engine is at filling its' cylinders with mixture, the more power it will put out. At high speed, the mixture is traveling through the ports at as much as 150 miles per hour, so any restriction will decrease the power output at high RPM. The ports should be as straight as possible, with few corners and protrusions as possible. The ports should also be as large as possible to flow more air. Head design has a great bearing on power.

        One of the weakest links in the design of a high performance engine is the type of valves that are used. When you think about it, we are designing a port and manifold as straight as possible, to get as little restriction as possible, and get the fuel/air mixture flowing as quickly as possible, to get as much mixture into the cylinder as possible, so we get as much power as possible, and then we design a system so that when the valve is open, the valve head nicely blocks off the flow of mixture. It seems counter productive. Maybe one day, a better valve system will be invented. Until then, we are stuck with the Poppet Valve system.

Valve Parts:

        The valve stem slides back and forth in the valve guide. The face closes onto the seat, creating the sealing effect. Fuel/air mixture travels through the intake port on its' way into the cylinder , and exhaust gas travels through the exhaust port on its' way out into the exhaust system. The rocker arm, or the lifter pushes on the tip to open the valve. The neck separates the valve's head from its' stem. The margin is the edge of the valve head. The valve springs push on a retainer to keep the valves shut. Spring pressure on the retainer also keeps the locks in place.

History

       In a modern car engine, the valves are located in the cylinder head. They are either overhead valve, or overhead cam. We'll discuss them later, but first, we'll look at the history of the valve train.
        Early engines were not capable of making a great deal of power,or turning at high RPM, so port and head design was not that important. The bottom end couldn't take the RPM anyway.

T-Head Engines

        The first car engines had both valves in the block. The intake valve on one side of the cylinder, and the exhaust valve on the other, with separate cams for intake and exhaust. This was called the   "T head" engine because the combustion chamber formed a letter "T". It was characterized by low compression, and poor air flow, and therefore low power. The T-head engine was used from the first days of the four stroke, or Otto cycle engine in the 1880's , until the early 1920's.

        Early cars had little need for efficient head design, and high horsepower. How fast did you really want to go on unpaved, potholed roads, with wooden spoke wheels, and virtually non-existent brakes on the rear wheels only? None-the-less, as soon as there were two cars on the road, two things happened; they raced; and, they got into an accident. The photo shown below, is from the 1903 Paris-Madrid race. The car is a de Dietrich with a 5,797cc, or 360 cubic inch "T head engine" that produced 30 hp at 1200 RPM. There were cars of that era that had engines as large as 20 liters or 1220 cu.in., and had only 60 hp. They had compression ratios as low as 4:1.  Note the chain driven wheels, and riding mechanic.  As technology got better, the riding mechanic was no longer needed to fix the car, and became the race driver's second set of eyes. One driver in the 1919 Indianapolis 500, realized that he was carrying around 200 lbs. of extra weight by having a riding mechanic along, just so he could see backwards, and replaced him with a mirror. He won the race, and the next year, all cars had mirrors, and the riding mechanic became history. The rear view mirror has been with us ever since.

L - Head Engines

        As fuel technology became better, it was possible for engines to have higher compression ratios.
It was also cheaper to produce an engine with both valves on the same side of the cylinder, since only one cam was needed, so the "L-Head" engine came into being. This engine had its' valves in the block still, but now they were both on the same side of the cylinder. This raised the compression ratio, and therefore the power, by using a smaller combustion chamber. It did, however, cause the mixture to go through a great number of corners, each one a restriction, to get into, and out of, the cylinder. It was first used around 1910. The last car to use an L-Head, or "Flat Head", or "side valve"  engine was the Rambler American in 1965. It had 199 cubic inches, and 95 horsepower. 

        Probably the most famous flathead engine of all time, was the Ford flathead V-8 which was produced from 1932 until 1953. It powered more hot rods during the 1940's and 50's than any other engine. In car use, it only ever made 125 horsepower, but this was in an era when Chevrolet didn't even make a V-8 engine. The hot setup for the flathead, though, was Ardun heads, which were overhead valve.
        The Ford flathead had serious design problems; such as overheating. The exhaust gas had to go through the water jackets to get out of the engine, and put a great deal of heat into the coolant.
A picture of the Ford flathead V-8 is below.

        Ford flatheads were produced by the millions from 1932 to 1953. The flathead, or "L-head" engine still remains in lawnmower power plants. Both the Tecumseh, and Briggs and Stratton, are "L-head " engines.

F-Head Engines

        For a short period of time from the mid 1950's until the mid 1970's, a few manufacturers, primarily Rolls Royce, Jeep, and Rover, produced engines with their intake valves in the head, and their exhaust valves in the block.

        The same camshaft was used for both the intake and exhaust valves, but pushrods and rocker arms operated the intake valves. This design was never very popular, and was superceded by the overhead, or "I-head" design. These engines were characterized by low horsepower, but high torque at low RPM. They were also brutally reliable.

I-head engines

        The I-head, or Overhead valve engine has been around for a long time. It has been produced since the 1920's, but its' advantage of small combustion chambers, and therefore high compression, was not realized until fuel technology had advanced enough. This happened during the late 1940's.
Nowadays, all car engines are "I-head" design.

... So the four different types of valve configuration are:

...We're only interested in I-head engines.

         I-head engines are made in two types; overhead valve, and overhead cam.

Overhead Valve Engines

        The overhead valve engine still has its' camshaft in the block like the flathead, but it uses a pushrod to extend the motion of the cam, to a rocker arm, which re-directs the motion to the valve.
Overhead valve engines, which were the high performance engines of the past, now power the family cars and mini-vans of today. They generally are large displacement, low performance engines, and make their power at low RPM. Below is a drawing of an overhead valve engine.

Overhead Cam Engines

        During the past twenty years, or so, one of the major goals of the car manufacturers, has been to get better gas mileage. Large displacement, low RPM engines, as a rule, don't get good gas mileage, when compared to smaller, lighter, higher RPM engines. Cars have become lighter, and engines smaller, all in the quest for better gas mileage. For an overhead valve engine to turn at high RPM, it needs stiff valve springs to close the valves at a high rate of speed. This causes the cam and lifters to wear out prematurely, or requires the use of roller lifters, which are expensive. The easiest way to solve the problem of making the valves close fast enough, is not to put heavier valve springs on, but to eliminate the source of the problem; get rid of the push rods, and the rocker arms. This requires moving the cam up to the head, hence the name "Overhead Cam". This requires the use of a long chain, belt, shaft , or series of gears to drive the camshaft. (the cam is now a lot further away from the crankshaft than it was when the cam was in the block) What an OHC engine does is eliminate reciprocating weight. This allows the engine to operate safely at higher RPM. Below is a drawing of an overhead cam engine.

        Notice how much simpler the OHC engine is compared to the OHV engine.

        .... both Overhead Valve and Overhead Cam engines are considered "I-Head" engines.

Combustion Chambers

        The combustion chamber is a space at the top of the cylinder to allow the burning of the fuel to take place. The shape of the combustion chamber, and the way the valves are placed in it, has a great deal of bearing on the power characteristics of the engine. The two most common shapes for the combustion chamber are: the wedge head, and the hemi head. Remember, both the wedge, and the hemi, can be either overhead valve, or overhead cam, but both are "I-head" engines.

The Wedge Head

        Both the intake and exhaust valves are side by side, this causes the fuel/air mixture to have to go through a relatively large number of sharp corners, each one of them a restriction. This slows down air flow, and therefore reduces power output. The spark plug is at the extreme edge of the combustion chamber, this causes the time needed for the mixture to burn to be relatively long, and at high piston speed, can reduce power.
        A type of wedge head has been produced which is more efficient at air flow than the standard wedge head is the canted valve, or 'porcupine' head. The valves are still side by side but are not parallel to each other. The angle on the valves makes for less restrictive air flow into the cylinders, and therefore more power. Engines which use this design are; Chevrolet's mark IV and V, 396, 427, 454, and 502 cu.in. V-8; and Ford's 351 Cleveland, and 429, and 460 cu.in. V-8's.  

The Hemispherical (Hemi) head

        The hemi head engine is the ultimate in combustion chamber design. The combustion chamber is very compact, the spark plug is centrally located, and the flow of air is almost straight into the combustion chamber making its' volumetric efficiency (ability to fill its' cylinders) very good, for high power output. Probably the most famous hemi head engine of the past was the legendary Chrysler hemi head V-8, which was produced from the mid 1950's until the early 1970's. It started out as 355 cu.in., was enlarged to be 392 cu.in, and ended up as 426 cu.in. Right now, many manufacturers produce hemi head engines. They are usually used in sports oriented cars because their performance is so good. Why are they not used in all cars if they're that good? Because they are expensive to build. The machining processes to build a head with valves at opposing angles, are more complicated than making them at the same angle, as in most wedge head engines.
        The centrally located spark plug means there is only a short distance for the flame front to travel. This means a short burn time, and therefore higher speed. there is also less chance of pre-ignition of the fuel/air mixture.

The Valves Themselves

        Poppet type valves are not the most efficient of valve designs. They are just the most cost effective, and therefore are used in cars which are designed to be mass produced, so us common folk can afford them. Other types of valves that have been used are; slide valves, sleeve valves, and rotary valves. They were used in the high performance piston engines used in aircraft, at the end of World War Two. Some had as much as 3,500 horsepower. Money and cost were no object, the government was paying the bills.   
        The intake valve is always the larger of the two valves. Why? When you think about it, the mixture inside the cylinder is burning, so more exhaust gas is going out, than there were intake gasses going in. So it makes sense that the exhaust valve should be the larger of the two, but it isn't. Why not? What's forcing the exhaust gasses out of the cylinder; the piston, remember? It's forcing the gasses out and it's a pretty positive force. What's forcing the intake gasses in? Atmospheric pressure, and it's pretty whimpy, only 14.7 pounds per square inch. So the exhaust valve can be quite small and it won't make that much difference, but a small intake valve makes a huge difference in the power output.  

  
        Of the two valves the intake valve has the easy job. Every time it's open, it gets a cool breeze of fuel/air mixture cooling it down. Every time the exhaust valve is open, it gets the fires of hell going by, so it gets extremely hot. No direct cooling is available, so the only time the exhaust valve has a chance to cool down is when it comes in contact with the valve seat. The seat is relatively cool, the coolant runs through water jackets in the head, so the seat may operate at around 200 degrees F. the exhaust valve, on the other hand, may operate at temperatures of as much as 1500 degrees F. At this temperature the valve may be glowing red hot! There is a great deal of difference in temperature between the exhaust valve, and its' seat. Metal likes to transfer from a hot surface to a cool one, so on a microscopic, molecular size level, little pieces of steel move from the hot valve face, to the cool valve seat. Over a few thousand miles, this is not a problem, but after extended mileage, it creates leakage between the valve face, and seat; and, a loss of compression. See the diagram below.

        After a period of time, and the effects of mileage and wear, the engine's valves no longer seal, and it needs a "valve grind". The head must be disassembled, and a new surface must be put on the valve face, and seat. This is done on a valve grinder. The state of the art machine, is the Sioux 2001 valve grinder. The broken lines below indicate the new, re-surfaced, face and seat.

        Notice in the above diagram, that the margin becomes narrower when we remove material off the face to re-surface it. If the margin becomes less than 1/16" for an exhaust valve, or less than 1/32" for an intake valve, we must discard the valve and replace it. It just doesn't have the thermal mass to withstand the punishment it will go through, and will fail prematurely. The seat surface becomes more receded into the head, the more material we take off. If the seat becomes too receded, engine performance will suffer because of a loss of compression, and shrouding of the valves. A new seat must be pressed into the head, or the head must be replaced.

Tetra-ethyl Lead

        Early engines had a lot of problems with loss of compression due to rapid valve wear. the valve surface just didn't last very long. In 1935, the Ethyl corporation started marketing a fuel additive to gasoline refiners  which made the valve surfaces last much longer. It was known as "tetra-ethyl lead", or just plain "lead".  It also increased the octane rating of the fuel. What it did was coat the surface of the exhaust valve with lead. This acted as a high temperature lubricant, slowing down the transfer of molecular sized particles from the face to the seat. The way it worked, was it coated everything that got really hot, with lead. It also coated the piston, combustion chamber, spark plug, and exhaust system, with lead. This lead powder was an abrasive, so it caused the rings to wear prematurely. It was also caustic, so it caused the spark plugs, and exhaust systems to wear out prematurely. Lead powder also got in the oil, contaminating it prematurely, requiring more frequent oil changes. In short, the lead helped the valves last longer, at the expense of the rest of the engine. It also meant gasoline refiners didn't have to refine their product as precisely, they just had to add lead to it.
        It became apparent, by scientific research during the 1960's, that lead was also an environmental hazard. It got into the atmosphere from car exhaust, was absorbed by the clouds, and therefore the rain had lead content. The lead contaminated rain fell on the grass which absorbed it. The cows ate the grass, and we drank the milk the cows produced. Tests on humans revealed an alarming level of lead poisoning, and it was causing learning disabilities, and birth defects. Something had to be done! Every lifeform on earth was being contaminated by car exhaust. The newly formed Environmental Protection Agency (EPA) in the United States, started putting pressure on the car manufacturers to build engines that didn't require leaded fuel, and in 1971 cars built in North America, no longer needed lead in their gasoline.
        As of 1975, most north American cars came equipped with catalytic converters in their exhaust systems. Catalytic converters convert carbon monoxide and unburned hydrocarbons, two pollutants in the exhaust stream, to carbon dioxide, and water. They do this by passing the exhaust gas through pellets, or a ceramic honeycomb, of platinum and palladium which is glowing red hot. Because lead coats everything that gets hot, it coats the catalyst too. The converter is useless after one tankful of leaded gas. If the car is equipped with an oxygen sensor, it too becomes useless after one use of leaded gas.
        In 1986 the federal government in Canada outlawed the sale of leaded gas because of environmental concerns. At the time there was huge public outcry that older cars would be useless, because they needed leaded gas. This, in fact, has not happened, and older cars are quite capable of running just fine on unleaded gas. In fact, older engines seem to last longer. Leaded gas is still available in the United States, but is totally unnecessary.

        The Ethyl Corporation, no longer making money off of Tetraethyl Lead, is now trying to push Manganese fuel additive on the petroleum industry, even though every car manufacturer says it will ruin their cars' emissions control system.

Making Engines run on Unleaded

         In 1971, most north American car manufacturers, were designing their engines to run on unleaded, or at least low lead fuel. Certain design changes were made, to prevent the valves from burning out prematurely. How did they do it?
        First, they made the valves out of more heat resistant material, especially the exhaust valves. The exhaust valve was faced with a material called stellite and extremely hard alloy of steel. This resisted the transfer of small particles from the face to the seat.
        Secondly, the manufacturers made the valve stem hollow, and filled the stem with sodium. Sodium is a metal which has very good heat transfer qualities. While the valve is open, the sodium absorbs the heat in the valve head. When the valve is closed, the sodium, allows that heat to escape to the valve guide. This increases heat transfer, and allows the valve to run cooler, making it last longer.

        The third way the car builders made the valves last longer, was to harden the valve seats. this was either done by installing a separate valve seat of hardened steel; or by induction hardening the iron seats already there.
        Lastly, the engine builders installed rotators on the exhaust valves. By allowing the valve face to return to a different place on the seat every time it closed, valve, and seat life was tripled.
        In actual fact, because of the advancement of metallurgy from 1935 to the 1960's, leaded gas had probably been unnecessary for years before the low lead engines were produced. It was only the public's perception, and the fact that lead also was a cheap  and easy way of increasing octane, that kept leaded gas in use as long as it was. Environmental concerns aside,  lead in fuel had to go anyway, because it ruined catalytic converters, and oxygen sensors.

Valve Contact

        The matching angle of the valve face and valve seat, on Poppet valves, is most commonly 45 degrees, but occasionally you will find a valve angle of 30 degrees.
        If the valve face and seat angles are the same, then when the valves are re-faced, they must be "lapped". This means a small amount of abrasive compound is spread on the face, and the face and seat are rubbed across each other, to "wear in " the contact area.
        Interference angle is a slight difference of 1 degree in the face and seat angles. If an interference angle is used, the face and seat don't need to be lapped.
        The width of the contact area between the valve face and seat is also very important. If the contact area is to narrow, there is not enough heat transfer, and the valve will burn out. If the contact area is too wide, there is carbon build up, and there is not enough pressure between the face and seat to seal. An intake valve should have between .070" and .090" seat width, and the exhaust valve should have between .080" and .100" seat width. The face should contact the seat about 1/3 of the way up the valve face from the outside edge.

Seats and Guides

        When a cylinder head is made of iron, separate seats and guides are not really needed. The iron is strong and long wearing. But when heads are made of aluminum, as a growing number are, the material is not really very strong, and would wear out very quickly. The use of aluminum, as head material, demands the use of separate, replaceable seats and guides, made of stronger materials such as steel, brass, or bronze.

Valve Springs

        Almost all engines use springs to close their valves. The only exception is the Ducati motorcycle, which uses one cam to open the valve, and another cam to close the valve. This is called a Desmodromic valve gear. Ducati also makes engines with a conventional valve train. In all other engines, the valve spring acts on the retainer, which is locked to the valve stem with split keepers, to close the valve.
        The job of the valve spring is very difficult. It is compressed, and expanded over 70,000 times per hour at 80 KPH. During its' lifespan, a valve spring may have been compressed end expanded trillions of times. When a piece of steel is bent back and forth enough times, it breaks. Valve springs are made of a type of steel which resists this, and are heat treated in such a way to keep it from happening; but, occasionally a valve spring will break, usually with catastrophic results. The tension is released from the retainer, and the split keepers fall out,  allowing the valve to drop down inside the cylinder, where the piston smashes it back up into the combustion chamber. Even if this never happens, at the very least, after a hundred thousand miles or so, a valve spring will lose some of the tension it had when it was new. When this happens, the spring will no longer close the valve fast enough, especially at high RPM, so the engine will no longer "rev up", and power loss results. This is called "Valve Float", or the valves float open. Compression is lost, and therefore, power is lost too. Valve springs should be checked for tension with a spring tension tester when the head is dis-assembled for a valve grind. The tension problem can be cured by replacing the valve springs with new ones.  
        Smaller diameter valve springs use a damper on the inside of the spring to prevent the spring from vibrating. This is not required on larger diameter valve springs.
       

High Performance

        Some engines use double valve springs where a smaller diameter spring is inside the outer one.
Other engines even use triple valve springs. Don't forget that the valve spring not only closes the valve, but also pushes all the other parts such as the retainer, rocker arm, pushrod, and lifter, back against the cam; so increasing valve spring tension increases camshaft and lifter wear dramatically.
        As a racer, or hot rodder, you might assemble an engine with heavy duty, super high tension valve springs, but don't expect the cam and lifters to last very long. Since you probably don't mind re-building an engine between racing seasons, this is probably acceptable, but a manufacturer could not offer a warranty on a car  with an engine that needed to have its' cam and lifters replaced every year. He'd go broke.
        Rather than increasing valve spring tension, the same result of high engine RPM can be obtained by decreasing the weight of the valve train. The springs have less work to do because the valve train has less inertia.
        Some ways of decreasing valve train weight are:

Valve Seals

        The valve seals go around the valve stem, and keep oil from going down the valve guides and being burned in the combustion chamber. This is mainly a problem with the intake valves, because of the vacuum created in the intake port.
        If the seals are worn, the engine will smoke on start up for a few seconds, or after a long downhill. On start up, oil has gone into the combustion chamber while the engine has been shut off, and is burned when the engine starts. After a long down hill, the smoke is caused by oil being sucked down the guides because of the high vacuum created by the closed throttle, and high piston speed. There is not enough heat in the combustion chamber to burn the oil though, until the throttle is opened at the bottom of the hill.

        There are three types of valve seals:

        Whatever type of seal is used, some leakage past the seal is necessary to lubricate the guides, and stems.

Retainers, Rotators, and Keys

        The spring pushes against the retainer, or rotator, to close the valve. The retainer, or rotator is locked the to the valve stem by two split keys. A rotator is often used on the exhaust valves to cause the valve to rotate relative to the seat, tripling valve life.

Camshaft

        Converts the rotary motion of the crankshaft into reciprocating (up and down) motion needed to open the valves. The cam opens the valves, and allows the springs to close them at the right time.

        ...more than any single other part, the camshaft determines how much power an engine will put out.

        The cam times the valves and the engine will not run if cam timing is out.

        The width of the cam lobe determines how long the valve stays open, the height of the nose, relative to the base circle, determines how much it opens. The longer the valve is open for, the more the cylinders can be filled, especially at high RPM. Long duration camshafts cause a rough idle because of a loss of vacuum. This loss of vacuum also affects the power brakes.
        One of the most common mistakes is to install a cam with too much duration. For the street, stick to relatively short duration camshafts, of around 260 degrees.
        Another rule of replacing camshafts is to always replace the camshaft and lifters at the same time. Never replace one without the other, or you probably will get to do the same job again, and soon.

Cam Drives

        The cam is driven at one half crankshaft speed. We want each valve event happening only once within a four stroke cycle. If a camshaft sprocket has 44 teeth, then the crankshaft sprocket will have 22 teeth.

        The cam must be timed precisely, and it must not slip. It takes a great deal of power to turn the camshaft. Remember friction between the cam and lifters aside, all the valve springs must be compressed every time the cam goes around, so there is a great deal of tension on the cam drive.

 There are three common ways the cam can be driven:

     Roller chains are the only type of chain used in OHC engines, and because they are so long, stretch more than in OHV engines. For this reason, a chain tensioner is used in Overhead Cam engines. This tensioner can be manually adjustable, or be hydraulically adjusted using engine oil pressure to tension the chain.  
            2. Hy-Vo Chain, or Morse Chain, or Silent Chain - This type of chain has flat links riveted together, and can come in any width. It is used in the transfer cases of many four wheel drive trucks, to power the front driveshaft. It has also been used in the Turbo-hydramatic 425 transmission to power the front wheels of the Oldsmobile Toronado, and Cadillac El Dorado.

    The Hy-Vo chain can also have a nylon toothed cam sprocket to further reduce noise.   This nylon, though, quite often comes off, causing the chain to become slack, and jump. Hy-vo chains are more prone to stretch than roller chains are, and therefore are not used in overhead cam engines.

        You might not think that a rubber band was capable of providing the kind of power needed to drive the camshaft, but it has been used by Kawasaki to drive the rear wheel of their 305 cc motorcycles, and Harley Davidson used it for the same purpose on their Sturgis. In fact a Gilmer belt, (granted a little wider) is used to provide the 350 HP needed to drive the supercharger on a TopFuel car, or Funny Car.

        Gilmer belts do have a definite lifespan. They are designed to last from 50,000 miles to 100,000 miles (80,000 - 160,000 km) and should be changed as a matter of routine maintenance. They are usually relatively simple to change (especially when compared to a timing chain), and severe engine damage can result if they break. The valves stop opening and closing. Those valves that were open when the belt broke, remain open. Meanwhile the pistons still keep going up and down, and if the pistons hit the open valves, goodbye engine! Belt life is drastically reduced by the presence of oil. Oil leaking onto the belt will cause the rubber to break down, the teeth come off, and the cam stops turning. The important thing to remember about timing belts is they do have a definite lifespan, so change them at the manufacturers specified intervals, or it will cost you big money.

Lifters

        The lifters take the motion of the cam, and transfer it to the other parts of the valve train.
Other names for lifters are; cam followers, and tappets.

        There are two types of lifters;

    Roller tappets are nothing new. Engines have used them as far back as 1915, maybe further, but until the mid 1980's they were usually used in race cars, where extreme cam profiles, and high valve spring rates were used. In 1985 Ford started using hydraulic roller tappets in their 5.0 liter V-8, to increase power, and get better fuel economy. Many other car manufacturers now use roller tappets for the same reasons.

Valve Lash

        As we have learned before, when anything gets hot, it expands. That's the whole principle the engine works on. If all the parts of the valve train were butted up against each other, and nothing was done to allow for expansion, then what would happen as soon as the engine warmed up, the pushrods, rocker arms, and valves would all expand. That extra metal has to go somewhere, and it sure isn't going to push the camshaft downward, so what happens is the valve would open up, causing a loss of compression. The engine would no longer run. A small clearance must be left in the valve train to allow for this expansion as the engine warms up. This clearance is called valve lash.

        If there is too much lash, the engine becomes noisy as the rocker arm slams down on the tip of the valve. If there is too little lash, the valve doesn't seat properly, and burns out, or compression is lost. Lash must be adjusted from time to time. This is measured with a feeler gauge, and the lash is adjusted until there is a slight drag on the blade of the feeler gauge. Lash can be adjusted either hot, or cold, depending on the manufacturer, so check the service manual. The exhaust valve will usually have more lash than the intake valve, because it gets hotter. Check the service manual for the specs.

Hydraulic lifters

        Too much valve lash makes the engine noisy. Too little causes the valves to burn out. Hydraulic lifters use engine oil pressure to hydraulically adjust themselves every time the valve is closed. They are much more complicated than solid lifters, but give surprisingly little trouble. They are also heavier than solid lifters, and therefore, are generally reserved for passenger cars, not race cars. the tolerances in the hydraulic lifter, are around .0001", the closest in any part of the engine.

        When the cam lobe pushes on the lifter foot, the valve disc closes, trapping oil under the plunger, and, because a liquid cannot be compressed, the whole lifter goes up as a unit, opening the valve. As the cam lobe moves away from the lifter, engine oil pressure pushes the valve disc off its' seat and allows oil pressure to push the plunger upward, removing any lash out of the valve train.

Pushrods

        An overhead valve engine uses pushrods to extend the motion of the cam and lifter up to the rocker arm. The pushrod is usually a hollow steel tube with a steel ball welded in each of its' ends.
A hole is drilled on both ends to allow oil to move through the pushrod to lubricate the valve and rocker arm.

To check for bent pushrods, simply roll them on a work bench. If they're bent, it will be obvious. Check the ball ends of the rod for galling, and wear, and replace as necessary.

Rocker Arms

        The rocker arms re-direct the up motion of the pushrod, down, to open the valve.

There are two types of rocker arm assembly's;

For extra reading, check out this site.......

http://www.autoshop-online.com/auto101/eng.html

O.K. ...that's all there is to it! Now let's see what you know.
Take the self test, and when you're confident you can pass it, see your instructor for the real thing. ( it's very similar )

Note: There may be more than one correct answer. Always chose the best answer.

Name the parts:

Click on the correct letter by each part's name.
Neck                    A    B    C    D    E    F    G    H    I    
Face                     A    B    C    D    E    F    G    H    I 
Port                      A    B    C    D    E    F    G    H    I 
Guide                  A    B    C    D    E    F    G    H    I 
Stem                    A    B    C    D    E    F    G    H    I 
Seat                     A    B    C    D    E    F    G    H    I 
Head                   A    B    C    D    E    F    G    H    I 
Margin               A    B    C    D    E    F    G    H    I 
Tip                      A    B    C    D    E    F    G    H    I

Click on the correct letter by each type of valve arrangement:
I - Head ( or overhead ) a)    A        b)    B        c)       C        d)    D
L - Head ( or flathead )  a)    A        b)    B        c)       C        d)    D
T - Head                           a)    A        b)    B        c)       C        d)    D
F - Head                            a)    A        b)    B        c)       C        d)    D

What is the most efficient combustion chamber shape?
a)    hemi        b)     wedge        c)     both the same        d)     depends on the engine

Which is larger?
a) Intake valve                            b) Exhaust valve         
c) they're the same size             d) all engines are different

Which valve operates under more extreme conditions?
a)    intake        b)     exhaust    c)    both the same     d)    depends on the engine

What is a Gilmer belt?
a)    a kevlar reinforced fan belt         b)     a type of belt that runs the accessories
c)    the belt in a radial tire                 d)     a belt with teeth

How fast does the cam turn compared to the crankshaft?
a) twice as fast        b) same speed         c) way faster         d) half as fast

What are four ways the cam is commonly driven?
a) shaft, gilmer belt, gears and Hy-Vo chain        
b) gilmer belt, roller chain, gears and Hy-Vo chain
b) gears, shaft, roller chain, and vee-belt               
d) vee-belt, gilmer belt, gears, and shaft

If the crankshaft sprocket had 22 teeth, the camshaft sprocket would have ________ teeth.
a)     11 teeth        b)      22 teeth       c)      44 teeth        d)     88 teeth

What are two advantages of a gilmer belt over chains?  Choose two answers.
a)    resistant to oil        b)     quiet        c)     cheaper       d)      lasts longer

How do you tell if a Gilmer belt needs replacing?
a)    broken        b)     cracked        c)     soaked in oil        d)     all of the above

What is the approximate lifespan of a Gilmer belt?
a)    10,000km.        b)     40,000km.        c)     100,000km        d)200,000km

What opens the valve?
a) the cam        b) the spring         c) the base circle         d) the retainer

What closes the valve?
a) the cam         b) the spring         c) the base circle         d) the rocker arm

What controls when the valve closes?
a) the cam         b) the spring         c) the pushrod             d) the rocker arm

Where is the cam in an overhead valve engine?
a)    in the block        b)     over the head        c)     can be anywhere        d)    on top of the valves

Where is the cam in an overhead cam engine?
a)    in the block        b)     in the head        c)     can be either        d)     above the pushrods

What causes valve float?
a)    weak valve springs        b)     over revving the engine       
c)    heavy valve train            d)    all of the above

What is the advantage of having four valves per cylinder?
a)    better flow for more power     b)     higher RPM    c)     neither     d)    both a & b

When does an exhaust valve cool?
a)     when its' open    b)    when its' closed    c)    neither    d)     depends on the valve

What is valve lash?
a)    clearance in the valve train       b)     when the rocker arm hits the valve hard
c)    when the cam hits the lifter         d)     none of the above

Why is valve lash necessary?
a)    so the valve doesn't wear out                 b)     to allow for expansion
c)    to keep the valve from burning out        d)    so the valve doesn't hit the piston

In an engine with solid lifters, the exhaust valve would have more lash. Why?
a)    because it gets hotter           b)    because its' cooler    
c)    because its' larger                 d)     none of the above

What changes were made to the valve train to allow the engine to run on unleaded gas?
a)    valve rotators added             b)    hardened valve seats
c)    stellite valves                          d)    all of the above

Why was tetraethyl lead added to gasoline?
a)    to increase octane    b)    to protect the exhaust valve    c)    both a & b     d)    neither a or b

Why was lead removed from gas?
a)    because it harms the environment         b)    because it harms the cars emissions systems
c)    because it was no longer needed           d)    all of the above

The timing chain in an OHC engine is very long. What is added to keep it tight?
a)    chain tensioner    b)    tightener         c)    chain adjuster     d) none of the above

What would happen if an engine were put together with the valve timing off?
a)    the engine wouldn't run        b)     the engine would be damaged
c)    the engine would backfire    d)    all of the above

What is the advantage of solid lifters?
a)    higher RPM        b)     less wear        c)     quieter        d)     all of the above

What is the disadvantage of solid lifters?
a)    lower RPM        b)     wears faster       c)        noisier        d)     all of the above

Which lifter is worn? circle one

                A)                                                B)                                                   C)

Why should lifters be changed at the same time as the camshaft, and vice versa?
a)    to get more performance    b)    to get better gas mileage
c)    so the new cam or lifters don't wear prematurely    d)     all of the above

Why are roller tappets better than flat tappets?
a)    they give better gas mileage         b)    they give more power
c)    they last longer                                d)     all of the above

What is the advantage of hydraulic lifters?
a)    no adjustment necessary    b)    higher RPM    c)    less wear    d) all of the above

What does a hydraulic lifter use to maintain zero lash?
a)    oil pressure    b)    hydraulic oil     c)    power steering pump    d)     expansion

In most overhead valve engines, how does the oil get to the rocker arms?
a)    through an oil passage inside the block    b)     through the pushrods
c)    through an oil passage outside the block    d)     thrown off the rod bearings

You have an engine that smokes on startup, and when you step on the gas after a long downhill. What is causing the oil consumption?
a)    rings        b)     bearings        c)     valves        d)     valve seals

What is the advantage of a Hy-Vo chain over a roller chain?
a)    stronger    b)    lasts longer    c)    quieter    d) all of the above

Why does the HY-Vo chain use a nylon toothed cam sprocket?
a)    stronger    b)    lasts longer    c)    quieter    d)    all of the above

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