Piston & Valve Damage Diagnosis

Catastrophic engine failure doesn’t occur as often as it did in years gone by. This is due to better lubrication, advanced manufacturing techniques and electronics. Specifically, knock sensors retard engine timing to reduce or eliminate detonation, a major cause of piston damage. Improved lubricants reduce the possibility of bearing and piston scuffing. And advanced techniques on the assembly line result in a final product (the engine) that is just better built than it was 25 years ago.

This is not to say that engines don’t fail anymore. That still happens. Determining the cause of major engine failure can often be determined by examining the damaged engine parts. These include the pistons and the valves. So, let’s take a look at the causes of blown engines and see how we can uncover the underlying cause of failure.

PISTONS

The piston in an internal combustion engine is the component that is responsible for developing power. As the air/fuel ratio is burned, the force from this event pushes the piston down, causing the crankshaft to rotate. A typical piston is shown in Figure 1. Piston damage, in most cases, can be attributed to one of three possible causes: (1) abnormal combustion, (2) dirt and (3) failure of other components.

Abnormal combustion is just that; combustion that does not occur in a normal fashion. Under normal conditions, the air/fuel ratio is ignited by the spark plug and burns in a controlled manner across the combustion chamber. When the fuel mixture is ignited by something besides the spark plug, such as a hot spot caused by carbon build-up, this is called pre-ignition. This is one type of abnormal combustion. The other type is detonation. Detonation occurs when the unburned fuel mixture, remaining after the spark plug is fired, explodes before the flame front arrives. Both types of abnormal combustion can have devastating effects on a piston.

When the pre-ignition occurs, the dome, or top, of the piston may have a melted/distorted, heat-weakened look. Also, sections of the dome can be affected. Detonation can actually cause the head land to break and fracture. When this occurs, the piston lands usually collapse, locking the piston rings in place. The excess heat from abnormal combustion can also scuff piston skirts near the pin boss and discolor the underhead of the piston.

Electronic engine controls and knock sensors have come a long way in preventing abnormal combustion in passenger vehicle engines, but such damage is more common in race engines. In those applications, it is critical that the correct octane fuel is being used, ignition timing is correct and air/fuel ratio is correct.

The second major cause of piston damage is dirt. And this dirt can get into the engine several ways. Probably the most common way for dirt to get into in an engine is during the rebuild process. It is essential all engine components are clean before assembly begins. This is especially true of the engine block. Also, the engine should be assembled under clean room conditions. If these steps are not taken, piston skirt damage (particularly in the center of the skirt) may result. This may lead to damage of the cross-hatch pattern on the cylinder wall (Figure 2) and breakdown of the lubrication film. To further complicate matters, this same dirt will damage the piston ring sealing surfaces, leading to blow-by, more cylinder wall damage and loss of power.

Failure of other engine components can also cause damage to the pistons. For example, let’s say the journal end of the connecting rod breaks (this usually results from a reduction in clamping forces causing the rod cap of the connecting rod to bend and flex). When the rod breaks, the piston will likely slam into the valves and cylinder head. Now the piston is destroyed causing catastrophic failure.

Or, let’s say that a valve breaks (more on that in the coming paragraphs) and the head falls into the combustion chamber. What do you think will happen to the piston? You guessed it. It’s now scrap metal. Now you can see what is meant by catastrophic engine failure.

VALVES

Valves, like pistons, can also be damaged by abnormal combustion, dirt and the failure of other components (refer to our connecting rod failure example). But valves can also be damaged by geometry problems in the cylinder head. Let’s discuss abnormal combustion first.

When pre-ignition occurs, the air/fuel ratio begins burning before the spark plug fires and, in many cases, before the valves are fully seated. The result is burning of the valve as burning fuel mixture escapes past the valve seat and valve face. Quite often this damage looks like someone took a blow torch to a section of the valve and burned it out. Once the valve seal is burned away, the damage propagates regardless if the combustion problem is fixed, causing loss of power in that cylinder.

Dirt affects valves and the valvetrain the same way it does the piston. It causes accelerated and premature wear in the valve guide and on the valve stem. The dirt can be a by-product of a machining operation or dirty engine oil (quite common).

If the dirt is extreme, the lubrication film between the valve guide and valve stem can break down. If this occurs, the valve will stick in valve guide, then be struck by the ascending piston. The valve breaks and damages the piston, too. If the valve is partially stuck open and is not struck by the piston, it can be burned by combustion gases leaking out of the combustion chamber, resulting in a loss of power through the intake manifold (backfire) or exhaust manifold.

As described in our piston discussion, valves can be damaged by other engine components. When the connecting rod breaks, allowing the piston to strike the cylinder head, the valves will be damaged if in the open position. But valve breakage is more often caused by geometry problems in the valvetrain. A typical intake and exhaust valve are shown in Figure 3.

It is crucial that the valve travel in a true up and down motion through the valve guide. This occurs if (1) the rocker arm or valve follower is making contact with the valve tip in the center of the contact area, (2) the valve stem-to-valve guide clearance is correct and (3) the valve face and valve seat are properly ground (this allows the valve to seat properly).

Off center contact on the valve tip can occur if the valve becomes cocked in the valve guide. And this occurs due to either excessive wear between the valve guide and valve stem (possible from dirt in the engine) or excessive valve stem-to-valve guide clearance resulting from an assembly error. Also, if the valve tip height is incorrect, off-center contact can occur. Regardless of the cause, the tip is subjected to side loading (alternating tensile and compressive forces) which can result in the tip fracturing through the lock ring grooves. When this happens, the valve falls into the combustion chamber, damaging the piston, cylinder head and cylinder wall.

Geometry problems can also cause valve seating problems. If the valve seat runout is excessive, the valve stem-to-valve guide is excessive, or the valve stem is out-of-shape from excessive wear, seating problems can occur. When the valve closes, it must bend and flex to properly seat to seal the air/fuel mixture in the combustion chamber. This constant flexing will eventually cause cracks to form on the outside surface of the valve stem just above the radius. Over time, the cracks move inward (this is the same mechanism that broke off the valve tip) and the valve stem breaks. The net result is the valve head falling into the combustion chamber.