Generator Valve Tappet Guide: Types, Wear and Adjustment

The valve tappet, also called the valve lifter or the cam follower, is the component that transfers the motion of the camshaft lobe to the pushrod or directly to the valve. It is the interface between the rotating cam and the reciprocating valve train, and it carries the highest contact stress of any component in the engine: the load of the valve spring, concentrated on a contact patch only a few square millimetres in area, repeated millions of times in the life of the engine. On a generator running at 1500 rpm, each tappet cycles 750 times a minute, which makes the tappet one of the most highly loaded parts in the machine.

Tappet wear and failure is a common cause of top-end noise, loss of valve lift, camshaft lobe destruction, and, in the worst case, a broken valve train. This guide covers the tappet function, the flat and roller designs, the materials and coatings, the valve clearance adjustment, the wear and failure modes, the inspection and replacement procedures, and the OEM part numbers for common generator engine families.

What Is a Valve Tappet and What Does It Do?

The tappet is a cylindrical follower that slides in a bore in the cylinder block or the cylinder head, depending on the engine design, and contacts the camshaft lobe with its face or its roller. In a pushrod engine, the tappet transmits the lift to the pushrod, which operates the rocker arm, which opens the valve. In an overhead cam engine, the tappet acts directly on the valve stem or on a bucket, or it operates through a rocker. The tappet also provides the adjustment point for the valve clearance, either through an adjustable screw in the pushrod end, a shim on top of the tappet, or a hydraulic element inside the tappet.

Three functions define the tappet. First, it converts the rotating motion of the cam into linear motion. Second, it provides a wear surface that is harder and more durable than the cam lobe, or, in some designs, a sacrificial surface that wears in preference to the cam. Third, it maintains the valve clearance, either as a fixed clearance adjusted periodically or as a zero-clearance hydraulic device. The camshaft guide covers the lobe that drives the tappet, the pushrod guide covers the link to the rocker, and the rocker arm guide covers the component that opens the valve.

The tappet bore in the block or the head must be reamed to a precise clearance, typically 0.02 to 0.06 mm, so that the tappet can slide freely but cannot rock. The tappet is usually positioned to rotate slightly as it moves, which distributes the wear across the face and prevents the formation of a single wear groove. On some engines the tappet bore is machined directly in the block, and on others the tappet runs in a removable bushing.

Flat Tappet and Roller Tappet Designs

The flat tappet has a slightly crowned face that bears against the tapered cam lobe. The geometry between the crown and the taper causes the tappet to rotate as it is lifted, which spreads the wear and keeps the contact surface polished. The flat tappet requires an oil film between the face and the lobe, and it is sensitive to oil quality and to the correct break-in procedure; a flat tappet that is run with the wrong oil or without the correct cam break-in lubricant can scuff and destroy the lobe within minutes.

The roller tappet replaces the sliding contact with a rolling contact. The hardened roller follows the lobe with much lower friction and much higher allowable contact stress, which allows a more aggressive cam profile, higher valve lift, and higher spring loads. The roller tappet is not sensitive to the oil additive package in the same way as the flat tappet, but it is sensitive to the condition of the roller and the pin: a roller that has seized or developed flat spots will destroy the cam lobe. Roller tappets are standard on modern high-output diesel generator engines.

Valve Clearance and Adjustment

The valve clearance, or lash, is the small gap in the valve train that ensures the valve closes fully when the engine is at operating temperature. If the clearance is too small, the valve may not close completely when the engine is hot, which causes a loss of compression, a burnt valve, and a rough run. If the clearance is too large, the valve opens late and closes early, which reduces the engine’s breathing and power, increases the noise, and hammers the valve train, which accelerates the wear of the tappet, the cam lobe, and the valve seat.

The clearance is set with the engine cold for most industrial diesels, using the specified clearance for the intake and the exhaust valves, which usually differ because the exhaust valve runs hotter. The adjustment is made at the specified point in the firing order, with the valve fully closed, usually by the rule of nine, the rule of five, or a specific sequence given in the service manual. The clearance is measured with a feeler gauge between the rocker arm and the valve stem tip, or between the cam base circle and the tappet, depending on the design.

Hydraulic tappets require no periodic adjustment, but they must be checked for correct function: a hydraulic tappet that has bled down, that is clogged with sludge, or that has a failed internal check valve will produce a persistent noise and a loss of lift. Hydraulic tappets also require the correct oil viscosity and clean oil, because the internal clearances are very small. The maintenance schedule guide covers the adjustment interval, and the valve guide guide covers the guide clearance that also affects the valve train noise.

Tappet Face Wear and Cam Lobe Damage

The classic flat tappet failure is face wear, in which the contact surface becomes concave or develops a groove, and the cam lobe wears correspondingly. The wear begins when the oil film between the tappet face and the lobe breaks down, which causes metal-to-metal contact and a rapid, self-accelerating wear process. Once the surface is roughened, it abrades the lobe, the lobe becomes rounded, the valve lift is reduced, and the wear metal circulates through the engine.

The causes of tappet face wear are: incorrect oil with an inadequate anti-wear additive package, particularly in engines designed for a flat tappet; a cam break-in without the correct lubricant; a tappet that has been reused in a different bore or against a different lobe; a tappet that does not rotate because the bore is worn or the face crown has been lost; excessive valve spring load; and dirt or wear metal in the oil. Once the tappet face and the lobe have worn, both must be replaced, because a new tappet on a worn lobe will wear rapidly, and a new lobe on a worn tappet will do the same.

The symptoms of tappet and cam wear are a metallic tapping or a heavier knocking noise from the top of the engine, a reduction in valve lift which shows as a loss of compression and power, a valve clearance that opens up because the wear reduces the effective length of the valve train, and wear metal in the oil analysis report. On a generator running at constant load, the noise and the loss of compression are often the first signs. The camshaft guide covers the lobe measurement and replacement, and the valve spring guide covers the spring load that affects the contact stress.

Roller Tappet Wear and Seizure

The roller tappet fails in a different way. Because the contact is rolling rather than sliding, the face does not scuff, but the roller can develop flat spots if the needle bearing or the pin seizes, and the roller surface can spall if the contact stress exceeds the material strength. A seized roller slides along the cam lobe and destroys it, and a spalled roller produces a rough surface that does the same. The tappet bore can also wear, allowing the tappet to rock, which loads the roller unevenly and accelerates the wear.

Roller tappet inspection requires the tappet to be removed and the roller checked for freedom of rotation, for flat spots, for spalling, and for axial play. The roller pin and the needle bearing can often be renewed separately in tappet designs that are rebuildable, which reduces the cost of a valve train overhaul. The tappet body and the bore must also be checked for wear, because a worn bore allows the tappet to tilt and the roller to misalign with the lobe.

Inspecting and Replacing Valve Tappets

1. Remove the valve cover, the rocker assembly, and, where required, the pushrods and the intake manifold, to gain access to the tappets.
2. Measure and record the valve clearances before disassembly, as the pattern of clearances gives information about the wear.
3. Withdraw each tappet with a magnet or a tappet removal tool, keeping each tappet identified with its bore and its lobe. Never mix tappets between bores.
4. Inspect the tappet face for wear, scoring, pitting, and loss of the crown; measure the face for concavity with a straightedge. Inspect the roller for freedom of rotation, flat spots, spalling, and axial play.
5. Inspect the cam lobe for wear, scoring, and the loss of the nose profile; measure the lift with a dial gauge if the equipment is available.
6. Check the tappet bore for wear and scoring, and measure the clearance with the tappet fitted. A worn bore must be reamed and an oversize tappet fitted, or the bore must be bushed.
7. Check the pushrods for straightness and for wear at the ends, and check the rocker arm pads and the adjuster screws.
8. Fit the new tappets with the correct assembly lubricant, particularly on a flat tappet engine, where a cam break-in lubricant must be used on the face and the lobe.
9. Reassemble the valve train, set the valve clearances to the cold specification, and run the engine through the specified break-in procedure before returning it to service.

The most common installation faults are reusing a tappet in a different bore, fitting a new tappet against a worn lobe, and running a flat tappet engine without the correct break-in lubricant. All three cause rapid wear and can destroy the camshaft within minutes of the first start. The installation guide covers the general assembly practice, and the camshaft guide covers the cam replacement that accompanies a tappet replacement.

Tappet Noise and Diagnosis

Tappet noise is a sharp, regular tapping that occurs once per valve event, at half engine speed on a four-stroke engine, and it is loudest when the engine is cold and the oil is thick. The noise can come from the tappet itself, from excessive valve clearance, from a worn rocker arm or pushrod, from a worn valve guide, or from an exhaust leak at the manifold. The diagnosis is made by isolating the cylinder and the valve: a stethoscope or a long screwdriver against the valve cover with each cylinder cut in turn identifies the offending cylinder, and a check of the valve clearance identifies whether the noise is a clearance problem or a component problem.

An excessively tight clearance can be more damaging than a loose one, because it holds the valve off its seat, and the valve overheats and burns or the valve contacts the piston. A clearance that has closed up between services indicates valve seat recession; a clearance that has opened up indicates wear in the tappet, the cam lobe, the pushrod, or the rocker arm. Recording the clearances at each service and watching the trend is the most effective way to catch a valve train problem early. The valve seat guide covers the seat recession, and the pushrod guide covers the pushrod wear.

Selecting Valve Tappets and Ordering

When ordering, specify the engine model and serial number, the tappet type and diameter, and whether the engine uses hydraulic or solid tappets. Order the camshaft, the pushrods, the rocker arms, and the valve springs in the same order when a valve train overhaul is planned, because the components wear together and must be matched. The sourcing guide explains the verification process, and the Cummins and Perkins cross-reference guide covers the interchange data.

Extending Valve Train Life

The valve train is the most highly loaded part of the engine and it depends on the oil for its life. The key practices are: use the oil specified by the manufacturer, with the correct viscosity and the correct anti-wear additive level for the tappet design; change the oil and the filter on schedule, because contaminated oil carries the wear particles that destroy the tappet face and the cam lobe; use the correct cam break-in lubricant on a rebuilt flat tappet engine; check and record the valve clearances at every service; and keep the cooling system and the oil temperature within specification, because excessive temperature reduces the oil film strength at the tappet contact.

On a generator that stands idle for long periods, the valve train is also at risk from corrosion: condensation forms on the tappets and the cam lobes and pits the surfaces, which shortens their fatigue life. Running the engine to operating temperature under a reasonable load at regular intervals is the best protection. The maintenance schedule guide covers the service intervals, the oil filter guide covers the filtration that protects the valve train, and the after-sales guide covers the parts support available.

Hydraulic Tappets and Lubrication Quality

Many modern generator engines use hydraulic tappets, which maintain the valve clearance automatically by means of oil pressure inside the tappet body. The hydraulic tappet removes the need for periodic clearance adjustment, but it introduces a dependence on the oil quality and the oil pressure. A hydraulic tappet that receives dirty oil or oil at the wrong viscosity can pump up or collapse, which changes the valve timing, causes a misfire, and can lead to a valve that does not close fully. The tappet is therefore a component whose condition reflects the condition of the lubrication system.

The hydraulic tappet must be filled with oil before it is fitted, and the engine should be cranked or run briefly before the covers are refitted, so that the oil pressure builds and the tappets fill. A tappet that is fitted dry will run with a clearance until it fills, which can damage the camshaft. The oil specification and the change interval must be followed strictly on an engine with hydraulic tappets, and the correct filter must be used so that the oil gallery remains clean.

The mechanical tappet, by contrast, has no dependence on oil pressure for the clearance, and it is adjusted with a feeler gauge or a screw and a locknut. The mechanical tappet is simpler and is often used on industrial and older engines. Both types perform the same function, and both require the camshaft lobe and the contact surface to be in good condition. The camshaft guide covers the camshaft, the pushrod guide covers the pushrods that transmit the motion, and the maintenance schedule guide covers the oil service intervals.

Frequently Asked Questions