Generator Valve Spring Guide: Free Length, Testing and Failure

The valve spring is one of the few components in a diesel engine that is loaded continuously, at every revolution, for the entire life of the engine. A generator engine running at 1500 rpm opens and closes each valve 750 times per minute, which is 45,000 times per hour and more than a million times in a single day of continuous operation. Every one of those cycles compresses and releases the spring, and the spring must return the valve to its seat at exactly the right moment, at every speed and every load, without losing its force.

Valve spring failure is a classic generator failure mode because of the constant-speed duty. A spring that has lost a small amount of its free length or its load may still close the valve at low speed, but at rated speed the valve begins to float, and a floating valve can strike the piston. A broken spring, on the other hand, allows the valve to drop into the cylinder immediately. This guide covers the function of the valve spring, the spring types and materials, the free length, squareness, and load testing procedures, the installed height, the failure modes, the replacement and installation practice, and the OEM part numbers for common generator engines.

What Is a Valve Spring and What Does It Do?

The valve spring holds the valve closed against the valve seat and keeps the entire valve train in contact throughout the valve event. It must generate enough force to seal the valve against the compression and combustion pressure, to accelerate the valve and the valve train components from their closed position to full lift and back again, to hold the valve on its seat during the combustion event when the cylinder pressure is high, and to prevent the valve from bouncing on its seat when it closes.

The spring works against the camshaft. The cam lobe pushes the valve open through the tappet, the pushrod, and the rocker arm, and the spring closes it as the lobe rotates away. The spring force must be high enough to keep the follower in contact with the cam at all speeds, but not so high that it overloads the cam, the tappet, and the rocker arm. The spring is therefore matched to the valve train mass and the engine speed by the manufacturer. The camshaft guide covers the lobe and the valve timing, and the valve tappet guide covers the follower that tracks the lobe.

The spring also works with the valve guide, the stem seal, and the retainer. The valve guide guide covers the guide and the stem seal, the valve seat guide covers the seat that the spring holds the valve against, and the cylinder head guide covers the head that carries the spring seat.

Valve Spring Types and Materials

Valve springs are made from high-carbon or alloy steel wire, usually chrome-silicon or chrome-vanadium, which retains its elasticity at high temperature and resists fatigue. The wire is wound, stress-relieved, shot-peened to induce compressive stress at the surface, and pre-set or scragged to remove the initial loss of length. Shot-peening is critical to fatigue life: it creates a surface layer in compression that inhibits crack initiation. The spring is then finished with a protective coating, often phosphate and oil or a paint, to resist corrosion.

The double spring arrangement is used on heavy duty engines for two reasons. The inner spring provides additional force without increasing the stress in a single wire, and the inner spring’s natural frequency differs from the outer spring’s, which suppresses the surge that can cause spring failure. Surge is a wave that travels up and down the spring at high speed; if the wave frequency matches the engine speed, the spring loses control of the valve. The damper or the second spring breaks up the surge.

Free Length, Spring Rate and Installed Height

Three dimensions define a valve spring: the free length, the spring rate, and the installed height.

The free length is the simplest check and the one most often used in the workshop: a spring that has shortened by more than the specified limit has lost its preload and must be replaced. The spring rate and the load at the installed height are more accurate measures of the spring’s condition, because a spring can lose load without losing much free length. The installed height is set by the cylinder head machining and, where necessary, by shims under the spring seat; it determines the load that the spring applies with the valve closed.

Testing Valve Springs

Valve springs are tested in four ways:

1. Free length: Measure the spring with a caliper or on a flat surface. Compare with the specification and the wear limit. This is the minimum check and can be done in the field.
2. Squareness: Stand the spring on a flat surface and place a square against it, or measure the offset of the top coil from the bottom coil. A spring that is not square loads the valve stem sideways and wears the guide.
3. Load test: Compress the spring to the installed height and to the full-lift height on a spring tester and record the loads. Compare with the specification. This is the definitive test and should be used for every spring in a top-end overhaul.
4. Crack and damage inspection: Inspect the spring for cracks, particularly at the ends, for corrosion pitting, and for wear marks where the coils or the damper touch. Use a magnifier or a dye penetrant on critical springs.

Springs are usually replaced as a set even if only one is out of specification, because the remaining springs are of the same age and have suffered the same fatigue. Mixing a new spring with old springs gives uneven valve loading and uneven valve seat wear. The springs must also be replaced as a matched pair in a double-spring arrangement.

Valve Spring Failure Modes

Fatigue breakage is the most damaging failure mode. It usually begins at a corrosion pit or a surface defect on the inside of a coil, where the stress is highest and where water can collect. In a generator application, the engine often stands idle for long periods, and condensation forms on the springs; the pitting that follows reduces the fatigue strength and the spring breaks after a period in service. A broken outer spring allows the valve to drop onto the piston, with serious damage to the piston, the cylinder head, and the liner. A broken inner spring with an intact outer spring may continue to run but with reduced control.

Loss of free length is the more common and more insidious failure. The spring slowly relaxes under the sustained load at operating temperature, and the installed load falls. At some point the spring can no longer control the valve at rated speed, the valve floats, and the valve and the piston can contact. Even before contact, a floating or bouncing valve does not seal properly, so the compression falls and the valve overheats and burns. On a generator engine running at constant rated speed, this occurs at a predictable number of operating hours, which is why the springs are inspected at every top-end overhaul. The valve seat guide covers the seat damage that follows a burnt valve, and the piston guide covers the piston damage from valve contact.

Valve Float and Valve Bounce

Valve float occurs when the spring cannot accelerate the valve train fast enough to follow the cam profile at high engine speed. The follower loses contact with the cam, the valve motion becomes uncontrolled, and the valve may not close fully before the piston reaches top dead centre. Valve bounce occurs when the valve closes on its seat at high speed and rebounds instead of staying closed. Both are caused by insufficient spring load, excessive valve train mass, or a spring that has lost its rate.

On a diesel generator the speed is fixed, which means that the spring does not have to control the valve over a range of speeds; but it also means that if the spring load falls below the critical value for that speed, the valve floats continuously. The symptoms of float are a loss of compression, a rough run, a burnt valve, and, in the worst case, valve-to-piston contact. The remedy is to restore the correct spring load, either by fitting new springs or, where the design permits, by correcting the installed height with shims.

Installed Height and Shimming

The installed height is the length of the spring with the valve closed and the retainer fitted. It is determined by the height of the spring seat in the cylinder head and by the height of the retainer, and it is adjusted with shims under the spring seat. The installed height sets the spring load at the closed position; increasing the installed height reduces the load, and reducing it increases the load.

When the cylinder head is resurfaced or the valve seat is cut, the valve sits deeper in the head, the retainer moves closer to the spring seat, and the installed height increases, which reduces the spring load. This is why a valve job must always include a check of the installed height and, where necessary, the fitting of shims to restore the correct load. Leaving the installed height too long after a valve seat cut is a common cause of premature spring relaxation and valve float. The valve seat guide covers the seat cutting and the valve tip height, and the cylinder head guide covers the resurfacing limits.

Replacing Valve Springs

1. Remove the valve cover and the rocker assembly, and, where required, the injector or the fuel lines to gain access.
2. If the head is to remain on the engine, use a cylinder pressure adapter to hold the valves closed with compressed air, and use a valve spring compressor to compress each spring and remove the retainers and the keepers.
3. Remove the spring, the retainer, and the spring seat, and keep the components for each valve together.
4. Clean the spring seat area and inspect the valve stem and the retainer for wear. Check the valve stem tip and the rocker arm pad.
5. Test each spring for free length, squareness, and load, and replace any that are out of specification. Replace the springs as a set.
6. Check the installed height with the valve closed and the retainer fitted, and fit shims if required to restore the specified height.
7. Fit the new spring, the retainer, and the keepers, and release the compressor. Check that the keepers are seated correctly in the retainer groove.
8. Refit the rocker assembly and set the valve clearances, then run the engine and check for noise and correct operation.

The most common installation faults are a spring that is fitted upside down (some springs are progressive and must be fitted with the closed coils towards the head), keepers that are not seated and release the valve on the first start, and an incorrect installed height that changes the spring load. Always fit new keepers and retainers if they show any wear, because a worn retainer can release the valve. The installation guide covers the general assembly practice, and the valve tappet guide covers the clearance setting that follows the spring replacement.

Selecting Valve Springs and Ordering

When ordering, specify the engine model and serial number, the valve position, and whether the engine uses a single or a double spring. Order the valve stem seals, the retainers, the keepers, and the valve guides in the same order, because they are all renewed in a top-end overhaul. The sourcing guide explains the verification process, and the Cummins and Perkins cross-reference guide covers the interchange data.

Preventing Valve Spring Failure

Valve spring life is influenced by the operating temperature, the lubrication, the corrosion environment, and the number of cycles. The preventive measures are: keep the valve cover gasket and the head sealing in good condition so that water does not collect on the springs; use the correct coolant and change it on schedule so that the head runs at the design temperature; keep the engine oil clean so that the valve train is lubricated; do not overspeed the engine, because the spring load is critical at rated speed; and inspect the springs at every top-end overhaul, replacing them as a set when any is out of specification.

On a generator set, the springs are often neglected because the engine runs well and the springs are hidden under the valve cover. A spring that has lost 10 percent of its load may produce no symptom at all until the valve burns or the piston is damaged. Recording the free length and the test load at each overhaul is the only reliable way to monitor the condition and to plan the replacement before a failure. The maintenance schedule guide covers the overhaul intervals, the valve cover gasket guide covers the cover sealing, and the after-sales guide covers the parts support available.

Spring Selection and Matched Sets

Valve springs are selected for the load they must control and for the engine speed at which they operate. The spring rate, the installed height, and the seat pressure are specified by the engine manufacturer, and a spring that is outside the specification changes the valve dynamics: a spring with too little pressure allows the valve to bounce at high speed, and a spring with too much pressure increases the camshaft and the tappet wear and can cause the camshaft lobe to fail. On some engines the intake and the exhaust springs are different, and on others they are the same, so the parts must be checked against the engine data.

The springs should be replaced as a set. A new spring fitted beside a worn spring gives an uneven load across the valve train, which causes a variation between the cylinders and accelerates the wear of the new spring. The matched set, supplied to the same specification, keeps the valve dynamics consistent. Where the camshaft is also replaced, the springs must match the camshaft profile, because a performance camshaft needs a higher seat pressure and a different spring rate. The camshaft guide covers the camshaft, the tappet guide covers the followers, and the cylinder head guide covers the assembly that contains the springs.

Frequently Asked Questions