What Causes A Dropped Valve?

There are several primary causes that can lead to catastrophic valve failure in the form of a dropped valve in internal combustion engines. The main factors are;

• Valve Spring Failure
• Worn Valve Guides
• Valve Seat Recession
• Improper Valve Lash

Additional contributing factors include;

• Over-Revving the Engine
• Detonation
• Inferior Valve Materials
• Improper Valve Installation

This article explores each of these failure modes in detail, analyzing the specific mechanical, thermal, and wear phenomena that ultimately result in valve drop events. It also provides best practices for inspection, maintenance, component selection and installation that can mitigate the risk of this severe engine damage mode.

1. Valve Spring Failure

Valve springs play a vital role in controlling valve movement and ensuring the valves close completely after opening. The springs exert upward pressure on the valves to quickly snap them shut after the camshaft opens them. However, springs are susceptible to failure after prolonged use.

Fatigue is the primary cause of valve spring failure. The constant cyclic loading and unloading placed on the springs during engine operation leads to metal fatigue. Cracks eventually form in the coils and cause them to break. Improper installation techniques can also lead to uneven spring stresses and premature failure. Moreover, excessive engine speeds and revving can over-stress the springs beyond their fatigue limit.

When a valve spring breaks, the valve can flutter wildly or float at high engine speeds. This allows the valve to bounce around and smash into the piston, resulting in catastrophic engine damage. At low speeds, the valve can also drop completely into the combustion chamber and collide with the rising piston.

2. Worn Valve Guides

The valve guides are cylindrical sleeves pressed into the cylinder head that maintain proper valve alignment as the valves move up and down. However, guides are subject to wear over time, especially in high-performance engines.

Frictional wear between the valve stems and guides gradually enlarges the guide holes during normal operation. High engine temperatures accelerate guide wear, as does the presence of abrasive particles in the oil. Insufficient lubrication also contributes to accelerated guide wear.

Excessively worn guides allow the valves to move around in an uncontrolled manner. This lateral valve wobble can permit exhaust gases to leak past the valve seal during compression strokes. It also increases the risk of the valve dropping into the cylinder at high engine speeds.

3. Valve Seat Recession

The valve seat is the machined area in the cylinder head where the valve rim makes contact. This mating surface must form an airtight seal to contain combustion pressures. However, factors like wear, corrosion, and heat damage can cause the seats to recede.

Excessive valve lash is a leading cause of seat recession. Too much lash allows the valve to pound against the seat violently. This distorts the seat surface over time. Moreover, overheating exhaust valves above 1600°F can initiate seat burnout and thermal erosion. Electrochemical corrosion between dissimilar seat and valve materials also degrades the seat surface.

A receded seat changes the valve’s sealing angle and allows compression gases to leak past the valve. This loss of cylinder pressure causes reduced power output. Moreover, the altered valve geometry increases the risk of the valve slipping off the seat and dropping at high RPMs.

4. Improper Valve Lash

Valve lash, or clearance, is the small gap between the rocker arm and valve stem with the camshaft at rest. This lash allows the valve to close completely before the camshaft reopens it. Incorrect lash leads to premature valve train failure.

Too little lash prevents the valve from fully closing and maintains continual pressure on the components. This strains the springs and accelerates valve seat and guide wear. Excessive lash also allows the unsupported valve to violently slam into the seat up to 20 times per second. This pounding damages the seats and stresses the springs.

Improper lash settings often result in bent valves, broken springs, excessive wear, and potential catastrophic valve drop. Setting and maintaining proper lash as specified by the manufacturer is critical for engine longevity.

Secondary Contributing Factors to Dropped Valves

Over-Revving the Engine

Exceeding the engine’s maximum design speed (redline RPM) dramatically increases stresses on valve train components. The springs cannot control the rapidly oscillating valves properly at such high speeds. This allows valve float to occur, where the valve does not follow its normal motion. Floating valves can collide with pistons and bend or break. The risk of valve drop escalates exponentially at over-rev speeds.

Detonation

Detonation, or engine knock, can cause high localized heating loads around the exhaust valves. The sudden pressure spikes impart shockwave impacts that overheat and fatigue the valves. Detonation often leads to burned, cracked, or even holed valves that later drop. Avoiding knock events by ensuring proper fuel octane prevents such valve damage.

Inferior Valve Materials

Using cheap low-quality alloys for valves instead of premium alloys can have dire consequences for valve longevity and likelihood of drop. Inferior alloys lack corrosion resistance and are more prone to fatigue, wear, and stem breakage. Always consult the OEM manufacturer for proper replacement valve material specifications.

Improper Valve Installation

In addition to proper valve lash settings, valves must be installed correctly in the cylinder head. Valves that are not seated squarely can bind against the guide or seat. This leads to uneven loading and wear. Moreover, improper valve stem protrusion affects spring tension. Incorrect installation procedures often accelerate valve wear and potential failure.