How it Works: The Tappets

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How It Works: Tappets

Most of us call it the tappet, but it is also known as the cam follower or valve lifter. Whatever you call it, it is a small member interposed between each camshaft lobe and the valve lifting mechanism, or the valve itself, depending on engine design. 'Valve lifting' means raising the valve off its seat. Only on side valve engines is the valve actually lifted upwards; overhead valves are actually depressed in order to 'lift' or open them. As a camshaft rotates, each lobe produces both a vertical and a side thrust.

The side thrust does no useful work and the purpose of the tappet is to eliminate it so that only vertical motion is transmitted to the push rod or valve. The centre of the tappet is usually offset in relation to the cam - this is done so that the tappet is rotated slightly on each contact with the lobe, thus spreading tappet wear over a large area.

Tappets for conventional-camshaft engines are usually of cylindrical or mushroom shape, about one inch long, and are located in guides machined in the engine block. Sometimes, the inside of the tappet is partially hollowed out so that the push rod fits down inside it; in others, an adjustment screw and locknut is fitted to one end, the pushrod resting in a depression in the screw head. Lubrication is sometimes provided by spiral oil-retaining grooves, machines on the cylindrical face, but more often by oil mist or splashes, or by oil running down the push rod.

The Automatically Adjusting Hydraulic Tappet

The automatically adjusting hydraulic tappet incorporates a spring-loaded plunger inside the main body. A cavity in the plunger is charged with oil through a ball valve. Initially, the spring expands the tappet to take up all the clearance in the valve train. As the cam lifts the valve, the ball valve closes, retaining the oil in the plunger. When the tip of the lobe has passed the tappet, the spring expands slightly, so that oil is drawn into the plunger cavity from that collected at the base of the push rod, the cavity thus being constantly replenished. When valve stem lengthens with normal heat expansion, the cavity is compressed slightly and surplus oil leaks out through the slight clearance around the circumference of the plunger.

Tappets for overhead camshafts take other forms: some are caps resting directly over the tops of the valve stems (with direct acting camshafts), while others are levers or 'fingers' (with indirect acting camshafts), hinged either at the centre or at one end, sometimes with a roller to actuate the valve. All tappets are 'case hardened'; that is to say, a very hard surface layer is produced on them by heating in a carbonaceous medium. Case hardening enables the tappet to withstand the constant battering it receives, without risk of shattering. When the engine is running, the valves become heated and the valve stems expand. A certain gap, or clearance, must therefore be provided in the valve operating train to accommodate this expansion, otherwise the valve will not close properly.

On the other hand, excessive clearance reduces the valve lift and hence the period during which it is fully open. Valve clearances grew smaller as engine manufacture enabled lower tolerances. On older vehicles valve clearances are about 0.003 in (0.1 mm) for the inlet valves and 0.005 in (0.15 mm) for the exhaust . Frequent adjustment was necessary. Car of the 1960s and 1970s have cearances generally between 0.010 and 0.015 in (0.25 and 0.40 mm) for inlet valves and 0.012 arid 0.018 in (0.30 and 0.45 mm) for the exhaust , although some manufacturers specified the same clearance for both inlet and exhaust valve mechanisms.

Tappet Clearance

The correct clearance for each valve can be set only when the relative cam is turned directly away from the tappet affected. If the camshaft is not visible, the correct cam positions can be found by taking note of the opening and-closing of the valves; appropriate information will be found in the manufacturer's handbook. Measurement of the clearance is made with a feeler gauge, inserted between the cam and tappet or the valve stem and its operating member. The clearance is correct when it is just possible to enter a gauge blade of the designated thickness between the gap. Some manufacturers provide a simple go/no go gauge in the tool kit-the 'go' gauge should pass easily between the gap, but the 'no go' gauge must not enter.

A common method of adjustment is by set screw and locknut; with a conventional camshaft and overhead valves, the screw is usually located in one end of each rocker arm. Side valve engines have either adjustable tappets or adjustable push rods. Always re-check the clearance after tightening the locknut, in case the set screw has moved. Another method is by a self-locking nut holding down the rocker arm at its centre against a spring; some manufacturers require the engine to be running when this type of operating mechanism is adjusted. Sometimes shims or pads (known also as capsules) of varying thicknesses are incorporated into the valve operating train.

Jaguars from the 1970s use a series of pads, lettered A to Z, rising in steps of 0.001 in from 0.85 in to 0.110 in. If an inlet valve clearance measures 0.007 in, the correct clearance being 0.004 in, and the pad in place is lettered D, the clearance must be reduced by 0.003 in, therefore a 0.003 in thicker pad is required, lettered G. With this system, the pad is usually located in a shallow recess in the tappet and it may be necessary to compress the valve spring in order to remove and replace it. Similar pad or shim systems are used by other manufacturers.

Tappets with adjusters built in
Tappets with adjusters built in.

This tappet has a roller to reduce friction between itself and the cam
This tappet has a roller to reduce friction between itself and the cam.
On some overhead camshaft engines, the camshaft has to be removed before the pads can be changed. This necessitates the measuring and noting down of all of the clearances and the correct replacement pads before the camshaft is removed. On some early Holden overhead camshaft engines, the clearance is adjusted by a small socket head screw set in the side of the tappet-one complete turn varies the clearance by 0.003 in (0.08 mm). When the engine is dismantled, the cam lobes and the cam face of each tappet should be carefully checked for wear. These parts, ideally, should be smooth and free from scores; the circumference of the tappets and the bore of their guides or housings should be perfectly round and an easy fit without side-play.

The cam face of each tappet will show whether the normal rotating or non-rotating wear pattern is present. The tappet may be excessively worn, but still in the case-hardened layer; or it may be 'soft worn', when this layer has been eroded away. If this softer metal is exposed, wear increases at a much faster rate and will already have become apparent by the necessity for frequent adjustment of the valve clearance. With hydraulic tappets, the walls as well as the ends should be checked for wear and blowholes (which will result in oil leakage). If more than 0.003 in (0.08 mm) side play in the tappet guides is present, the guide should be reamed out and an oversize tappet fitted.

Remember that if any part of the engine that affects the valve clearance is removed and subsequently replaced, 'bedding down' usually occurs necessitating a further tightening of securing nuts or screws. Such items include the cylinder head, rocker arm supports and overhead camshafts. It is essential to check, and adjust if necessary, the valve operating mechanism clearances, after this second tightening of these major components.
Of course these days modern cars have done away with the tappet, and instead use variable valve timing systems.

Silencing Tappet Noise

"Tappet noise" is a term used to describe the sound created by faults in any part of the valve gear, which consists of; the valve stems, rocker arms, rocker shaft, the pushrods, and the tappets (cam followers). The tappets are usually contained within the engine block and require an engine strip to reach. This is covered in a later chapter. This chapter deals with the rest of the valve gear in an OHV engine and the work should only be carried out if valve adjustment fails to eliminate the noise.

Fault finding and remedial work on overhead valve (OHV) engines is quite straight-forward, though often laborious. However, care and patience will reward you with a smooth running and quiet engine. Although there are three components transferring the action from cam to valve, this system is often simpler to deal with than many overhead cam engines with far fewer parts. Although faults in this system are often interrelated and in most cases an overall examination is necessary, a few running tests can be made to narrow down the causes.

Incorrect Tappet Adjustment

The most common cause of general tappet noise persisting after adjustment is that the adjustment is simply not accurate. A wrong setting can easily be made if you fail to establish whether the adjustment should be done with the engine hot or cold. Not all manufacturers make this clear, so verify the setting temperature with your dealer service department. One thing is common to all cars; the temperature must be either hot or cold, never merely warm. A setting made with the engine just warm is never satisfactory because the valve clearance gap may be at its tightest: if the tappets are set under these conditions the gap may become just large enough to make a noise.

Locating The Source Of Noise

Before starting to strip and check components, it is possible to save a great deal of work and time by first pinpointing the area generating the most noise. A general tappet noise coming from near the top while the engine is running, providing the valve clearance is known to be correct, is an indication of general wear in the valve train. In this case a complete strip down of the train will be required. A few noisy tappets, or even one tappet click, indicates a more specific fault. It can be directly traced using an old set of feeler gauges. The check must be made with the engine running, and with the rocker cover off, but must not be made on OHC engines where the clearance is between a rotating cam lobe and the valve stem. Before starting work, take precautions to avoid catching clothing in moving parts. You will also need a temporary shield if your engine squirts oil with the cover off, in particular on engines with an external oil supply.

First, select a feeler gauge blade that is about half the thickness of the normal valve clearance. Insert the blade between the end of the rocker arm and the valve stem. Do this for each valve. Where there is only one noisy tappet, the noise will disappear when the blade is placed upon the offending valve. In the case of several noisy tappets, one click will suddenly disappear from the general sound when a particularly noisy valve is found. Note each valve on which the insertion of the feeler blade has this effect. These are the faulty valves to be worked on.

Another useful check is to place the end of a long screwdriver against the rocker studs or shaft, and against the engine block in the area of the cam followers. Carefully place an ear against the screwdriver handle; the screwdriver will function as a sound transmitter. Although the noise will probably be heard in most places around the engine, the volume will increase significantly when the screwdriver is placed near the source of the noise. At least, you should be able to tell whether the noise is coming from the top end of the engine or from near the camshaft.

Rocker Pad Wear

After the car engine has been used for a fairly high mileage, wear may occur on the face of the rocker pad where it contacts the top of the valve stem. A wear pattern will form on the face of the pad, especially if the tappets have been allowed to build up an excessive clearance before adjustment is attempted. If the wear is only light, it may still be possible to set a correct clearance by making sure that the feeler gauge blade measurement is made between the valve stem and the worn surface of the rocker pad. If a wide feeler blade is inserted in the gap, especially if inserted at an angle, it will bridge the worn section and give a false reading.

There are two ways of avoiding this and obtaining a correct reading. The cheaper but less accurate method involves using a feeler with a narrow tip. You can make one of these by carefully grinding the edges of a standard feeler gauge blade and stoning off any burred edges. When finished, the blade should be narrow enough to rest between the valve stem and the worn face of the rocker pad. The other, more accurate method, involves buying and using a tappet-adjusting tool with a built-in micrometer. The principle behind this tool is that you wind down the adjuster so that the rocker just makes contact with the valve, then wind the micrometer adjuster back off the screw the appropriate number of turns to achieve a correct gap reading. The tool compensates automatically for wear in the valve train. It is readily available from most good auto accessory shops. The above methods should be used only where the wear is in the form of a slight indentation.

Excessive Rocker Wear

If the above methods do not cure the fault and the noise persists, or if the rocker face appears to have more than a slight indentation, the following check should be made, dealing with each rocker in turn. Rotate the engine to bring one rocker to the fully closed position on its valve. With a small mirror inspect the rocker face with the rocker in place on the shaft. If this proves too difficult, it may be possible to slide the rockers along the shaft for inspection, providing your engine is the type that has the inner rockers held in place by springs and the end rockers held by clips. To inspect the mid-positioned rockers release fully the rocker adjuster and slide the rocker along the shaft. The rocker face can then easily be checked for wear. To inspect the end rockers, again release the rocker adjuster, remove the retaining clip, and slide the rocker off the shaft. If your engine does not use this system of retaining the rockers you will have to remove the rocker shaft.

Rocker Shaft Removal

There are three main rocker mounting systems. The easiest to deal with is that which has the rockers on a shaft with its own separate fixings. To remove these shafts slacken each adjuster with its valve in the open position. Next slacken the bolts holding the shaft in place. These must be released evenly, working backwards and forwards along the shaft until any remaining valve spring tension is released. The shaft can then be lifted out and the rocker gear inspected for wear. After renewing any worn parts, refit the shaft and tighten the retaining bolts.

The second main type of rocker shaft mounting uses rocker shaft posts held down by the cylinder head nuts or bolts. With this type of mounting, you must make sure that you do not rotate the engine while carrying out the work. Carefully remove the bolts, again releasing them evenly. Take great care not to break the cylinder block seal. You may then lift out the rocker shaft and inspect the rocker faces. When replacing the shaft you must carefully retighten the head according to the manufacturer's recommended torque settings, especially if your car has an aluminium block or head.

The third system of rocker mounting has a different method of attachment for the rocker. It consists of an individually pressed steel rocker mounted on a stud in the head, working on the ball-and-socket principle. The rocker face can be inspected by removing the central adjusting nut and sliding the rocker off the stud with the other components attached. If your checks reveal any heavy pitting or scoring, or some evidence that the surface of the metal is actually flaking, the rocker should be replaced. While checking the rocker faces, it is also advisable to inspect the valve stem and the rocker pivoting surface for evidence of heavy irregular wear.

Stud type rockers should be checked for scoring on the internal rubbing surfaces and the pushrod dimple. Shaft types should be checked for excessive up-and-down play on the shaft and damage to the pushrod adjusting ball or face. If noticeable play is found on the shaft, it will probably be accompanied by a wear ridge on the underside surface where the pressure is applied. If the shaft shows signs of excessive wear, it will need to be replaced with a new shaft. The shaft often wears much faster than the rocker bushes but, although it is possible to renew the shaft only, the old bushes may leave minimal rocker play with the new shaft. If this is the case, it is advisable to fit a complete set of new rockers, avoiding the cost and delay of having new bushes fitted and reamed. If general wear and incorrect adjustment are not causing the noise, it may be due to a variety of other causes, some of them not directly related to the valve actuating mechanism.

Checking Oil Supply To Valve Gear

When you lift the rocker cover, everything underneath should be liberally coated with oil. If there is a lack of oil or, worse, if there is no oil at all, check the oil supply immediately. In order to do this you should first put the rocker cover to one side. Next remove the spark plugs in order to prevent the engine firing and running. Then turn the engine using only the starter. Oil should be seen to spurt from the valve gear, when supplied internally, or from all the oil spray holes on engines with an external supply. Any shortage of oil accompanying tappet noise will usually mean that severe wear has already taken place. All oil-ways in the head and valve gear will have to be thoroughly washed out with any proprietary flushing oil and blown through with an air-line. If this fails, the engine will have to be stripped down and cleaned.

Partial Valve Seizure

There are several faults in the valve train that cause a noise in the top end. One fault that produces a noise detectable in the top end of the engine is a partial seizing valve. This usually produces a tappet noise which recurs after adjustment when the engine has been run up to full working temperature. The valve partially seizes when the engine is hot and bends the pushrod. This, in turn, creates an increase in the valve clearance. This can often be seen when the engine is running with the rocker cover off. Many pushrods have a tendency to spin while the engine is running, and bent rods can easily be spotted by their distinct wobble as they revolve. If your pushrods do not spin, turn off the engine and remove the pushrods. These can sometimes be removed quickly by first moving the rocker to one side. To do this you may have to remove the spark plugs and make sure that the piston is a little way down the cylinder.

Slacken off the adjuster to allow a ring spanner between the valve stem and the rocker pad, then insert the spanner and lever the rocker against the spring. Once you have levered down the rocker, you can remove the pushrod and check it. Rolling each pushrod in turn on a flat surface will show up any that are bent. Take care to keep the components in order so that each one can be refitted in its original place. Mixing the components may cause accelerated wear due to mismatching of established wear patterns. If a valve has proved to be seizing, owing either to incorrect valve guide fitment or to lack of lubrication, you will have to remove the cylinder head and attend to the fault. The bent pushrods will have to be replaced with new rods and the valve guides either refitted correctly or they will have to be replaced with new guides.

Faulty Rocker Mounting Studs

Another contributory cause of increasing valve clearance and, consequently, noise in the top end of the engine is a fault in the rocker stud mounting system. In these systems the stud, which is often a press-fit, may start to lift out of its mounting hole in the head. To check for this, look at the base of the stud. If there is a band of differently shaded or bright metal, this indicates an area of recently exposed metal where the stud may be pulling out of the hole. It may be easier to inspect these components if you remove the rocker assembly.

Even though the stud may be loose enough to lift under valve spring pressure, you may not be able to remove it entirely. If this is the case, the easiest cure is to take the car to a garage and have the stud tapped down and a small blob of weld applied to it. It is a good idea to clean the area surrounding the stud thoroughly before you take the car in as the garage may omit to do this. If the stud is easily removed, then you can easily fit a new stud with a smear of Loctite.

Bearing Fit

There are various cures for loose fitting studs, including reaming or threading the hole and fitting an oversize or threaded stud. These jobs are best left to a specialist. The time, money and specialized equipment necessary make home repairs uneconomical.

Worn or Damaged Camshaft

If the noise appears to be coming from the camshaft area of the engine, it is likely that the camshaft is worn or damaged. Serious damage to the cam lobes will be obvious if the engine is slowly rotated and the lift height of each valve is noted. A worn cam lobe will not lift the valve as high as a good cam lobe. If the engine noticeably lacks power, you may also note that the rockers are not travelling downwards against the valve as would those on a good cam lobe. In extreme cases one cam lobe may be virtually worn away, which results in the valve not lifting at all. If you suspect worn cam lobes, check the height of each valve at full lift with a ruler. They should all lift to the same height and to the manufacturer's specifications.

Any discrepancies indicate cam lobe wear. Next, remove the pushrods and followers and shine a light down the hole. It may be possible to see the camshaft lobes. If you can see down the pushrod holes, have a friend turn the engine slowly by hand and look at the shape of the cam lobe as it turns. If there are any irregularities in what should be the smooth and gentle curve of the cam lobe on the shaft, or any pitting of its surface, cam wear is likely. If you cannot see the cam lobes in this way, look for an inspection cover over the camshaft. If you are not sure whether your car has one, ask your dealer. Removing these covers is described below.

If you cannot see the cam lobes by either of these methods, you will have to remove the camshaft. Camshaft removal is usually a tricky job, involving removing major components and should be done only if you are fairly certain that cam lobe wear is the cause of the noise. There are a number of cars on which the camshaft can be removed without lifting off the cylinder head; some of them have the camshaft floating in an oil bath at the side of the engine block. Your main dealer should be able to tell you whether your car is one of them. If cam lobe wear is the problem, you will have to replace or repair the camshaft. Removing the camshaft is described in later chapters on stripping down various makes and types of engine.

Worn or Damaged Tappets

Worn or damaged tappets are another cause of noise in the camshaft area. To check for wear, the engine may have to be removed, but a number of pushrod engines have facilities for checking them in the car. Notable examples are some of the Leyland and Chrysler models. These have tappet chest covers, which are easy to spot. They consist of a pressed steel cover held by a centre bolt, or small bolts around the edge. On Leyland cars these covers will be under the manifold and this will have to be removed to gain access to the tappet cover. Tappet covers are fitted with a cork gasket which must be renewed when refitting the cover. The tappets are exposed for easy checking when the cover has been removed. Rover V8 models have tappet chest covers, but they also have hydraulic non-adjustable tappets. Despite the additional complications, the tappets can be checked for wear as described below for other cars. Any repairs that are necessary will have to be carried out by a specialist garage.

Typical cars having tappet chest covers are Chrysler Hillmans and Sunbeams, the Leyland Mini 850 and all A40 and A35 models. To carry out an inspection on these cars, first move the rockers away from the pushrods, using the method described above. Next remove the pushrods and lift out the tappets. Removing the tappets is made easier if you rotate the engine to lift each tappet in turn. Any signs of scuffing, pitting or general wear will mean replacement is necessary. Tappets may appear to have a smooth surface while still having concave wear. This can be easily checked by holding a straight edge across the face. Any concave wear found on the tappets also indicates worn cam lobes and these should be checked. On some models with tappet covers, not only can the tappets be removed, but the camshaft can be removed without taking out the engine. To do this, you must first remove the crank pulley, timing cover and cam drive sprockets. On the Mini models you will also have to remove the radiator and cut a hole in the inner wing grille to allow removal.

On most cars without tappet chest covers, you may have to remove the engine to reach the tappets and cams. Once the engine is removed it is a simple matter to extract the camshaft. On many models, the tappets can only be removed in one direction because of their 'mushroom' design or lack of space above them. Camshaft removal is described in a later article.

Tappet and Cam Repairs and Replacement

Tappets can be refurbished, but the expense usually makes this less practicable than fitting replacement parts. Cam lobes can be reground and hardened, but this requires special equipment not available to the DIY mechanic and is best carried out by a specialist. It is also possible to exchange your camshaft for a reconditioned one, but make sure you buy only from a reliable dealer. If you managed to remove the camshaft without full instructions, there are several rules that apply when you replace it:

  • First, make sure that there is no play in the cam bearings. New bearings will have to be fitted and reamed by a specialist if they are scored or worn. Cams running direct into the block will need boring and fitting with new bearings if you find they are worn.
  • Make sure that you check the end float limits on the cam. They should be kept within the manufacturer's limits as given in the car manual.
  • It is most important that the tappets have no side play in the block, so it is necessary that new tappets are also fitted if the cam is renewed.
  • On re-assembly, you should coat the cam and tappets liberally with a friction-proofing additive, such as STP or Wynns. If these are not obtainable, you could use engine oil. New components should always be carefully run-in.

An overhead valve engine, with the camshaft in a mid-engine position. The rocker shaft lies in a central position above the engine. As the cam lobes revolve, they act on the cam-followers causing them to rise and fall. Each cam follower links up with a rocker arm by means of a pushrod. As the cam follower lifts, it pushes up the pushrod. This in turn lifts the stud on the rocker arm. As the rocker arm pivots about the rocker shaft the other side of the arm, which has a pad, moves downwards. The rocker pad acts on the valve stem and pushes the valve down. When the cam follower falls, the rocker stud also falls and the rocker pad rises. The action of the spring on the valve stem assembly against the cylinder head forces the valve back up.
Tappet Feeler Gauge
Using a feeler gauge with a conventional wide blade will only measure the gap across the flat surface of the rocker pad. By using a narrower feeler gauge blade, you can insert the blade between the valve stem and the worn area of the rocker pad.
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