Deciphering piston issues and decoding failure clues.
Nobody wants to deal with catastrophic engine failure. It will never happen at an opportune moment, and it will always cost thousands of dollars to fix. The only good thing about it is that it presents a learning opportunity for you to determine what went wrong so you can take the correct steps to ensure it doesn’t happen again.
However, trying to pinpoint the root cause of engine failure can be easier said than done when you have a pile of broken parts lying before you. It’s a lot to take in, and the answer isn’t going to jump right out at you. You’ll need to put on your detective cap and start looking for clues.
In the case of piston failure, details can be exceedingly difficult to decipher at times. That’s not just because the piston itself will be mangled, but also because the damage can start a chain of events that takes out more parts, leaving you with a lot to decode. That’s not to say it’s impossible, though. There are some tell-tale signs that can provide insight into what went wrong.
What Exactly Is A Piston?
The piston is one of three major components that make up a combustion chamber, the other two being the cylinder itself and the cylinder head. It’s a small cylindrical device with a diameter that closely matches the bore of the cylinder, and piston rings sealing the small gap between them.
The piston travels up and down in the chamber, with a connecting rod linking it to the crankshaft. As the piston begins its cycle, it travels down in the cylinder, creating a vacuum that pulls air and fuel through the intake valve and into the chamber.
Once the piston reaches the bottom of the chamber, it begins to travel back to the top, compressing the mixture and priming it for ignition. At just the right moment, the fuel mixture ignites, pushing the piston to the bottom, and transferring the energy to the crankshaft. The piston then travels back to the top of the cylinder, pushing out spent exhaust fumes, and then the process begins again. (At least, this is how it all occurs in a four-stroke engine cycle, which is found in all modern passenger vehicles.)
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The wild part? All of this happens thousands of times per minute, putting serious stress on the piston. (Check out the video above to see just how fast we’re talking.) That stress becomes greater and greater as elements like compression, boost levels, ignition timing, and engine speed are altered to create more power.
Types of Piston Failures
As complex of a role as pistons have, only so many things can go wrong. That’s not to say pinpointing the moment of failure will always be easy. Sometimes, the damage that ensues can obscure things. So, you’ll need to look really closely at the parts for clues indicating which of the following failures occurred.
Mechanical Damage
Mechanical damage to the piston is what we’ll classify as a foreign object falling into the chamber and destroying the piston, such as a bolt or a valve stem. It’s not technically a form of piston failure, but it’s important to understand nonetheless.
Todd Lappin | Flickr
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If you drop something down the intake and it takes out the piston, then it’s a simple ‘shame on you’ moment that tells you to work more carefully. However, if a valve causes the damage, it can indicate two things.
One reason why a piston would potentially contact the valve is that not enough clearance was built into the assembly. This happens when the valve is too far open relative to the piston’s travel. It’s important to know that, even if the two don’t touch during assembly, that doesn’t mean they’ll fit together when the engine heats up and parts expand.
The other reason a piston would whack the valve is because the valvetrain failed for one reason or another. This can happen if parts aren’t properly assembled or if the valvetrain is overstressed.
Hole in Piston
A hole in a piston shouldn’t be immediately linked to a mechanical failure. In fact, it may be something else altogether, and where it’s located can tell you why. If the hole is located below the spark plug, then it’s likely you’ve got a pre-detonation issue.
Pre-ignition or pre-detonation can occur for a number of reasons, but it is generally on account of cylinder temperatures being too great, causing the fuel mixture to ignite before it reaches the top dead center. This can do a lot of damage if left unchecked – including burning holes in pistons. The excess heat builds up in parts, especially the spark plug ground strap. This increased focal point of heat can be enough to melt through the piston.
Piston Skirt Scuffing
The piston skirt is the portion of the piston that extends past the ring lands, extending past either side of the connecting rod. It’s generally an inconsequential area, but damage can and will occur here.
Wear on the skirt occurs when it comes into contact with the cylinder wall during operation, which, in theory, should never happen. When it does, it usually means one of three things: an overheating or lubrication issue, or exercise wear in the piston assembly.
For the most part, this is considered normal engine wear, as it will happen as parts age and wear down. However, if you’re having issues well before the end of the engine’s service life, it can be a useful tool in determining what the problem is.
Piston Cracks
Pistons can crack in multiple places. Where they crack tells you a lot about what went wrong. Keep in mind that cracks do take time to form, and you may catch a problem early enough for them not to occur. Still, there will be signs of damage, warping, or other irregularities in the following areas that indicate the problem you’re looking for:
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Cracked Piston Crown
The crown of the piston is the top portion facing combustion. Being on the front line exposes it to all sorts of forces and stresses, and it’s only natural for cracks to form here if those forces are too great. A cracked crown usually indicates an issue of excessive cylinder pressure. This generally occurs when there’s a detonation issue. It can also occur due to thermal fatigue. Fatigue occurs when an engine is repeatedly placed under full load and then light load, which is common for diesel applications.
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Cracked Piston Skirt
A crack in the skirt of a piston can form on high mileage engines, due to excessive wear. However, it can also be associated with over-revving of the engine. The excessive load creates forces that a piston may not be designed to operate with, leading to cracks. Understanding the engine’s operation and service life will help determine the root cause of these cracks.
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Cracked Piston Pin Bore
The piston pin bore is where the wrist pin attaches the connecting rod to the piston. It’s faced with the most shear forces of the entire assembly. Cracks can form here for a few reasons. Poor fitment or improper installation are obvious ones. Otherwise, the likely cause for cracks to form is due to extreme, abnormal combustion events. They’re most likely to form due to severe detonation.
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Cracked Ring Lands
If you’re following, you’ll see that the most common reason for piston failure is detonation. And that’s certainly true. Another place for it to leave traces is by cracking the ring lands. This is the area on the outside diameter of the piston where the rings are set. Detonation is really the only reason cracks will form here. Other issues can technically place stresses on the ring lands, but it’s rare for them to create a crack.
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Cracked Ring Groove
Cracks in the piston ring grooves, despite what you may be inclined to believe after every other type we’ve discussed, don’t generally occur on account of detonation issues. The most likely reason for this to happen is that the ring groove wasn’t set wide enough for the application.
All piston rings need to be opened up so a light gap is present when they’re installed in the cylinder bore. The reason for this gap is to accommodate for expansion when a piston reaches operating temperatures. Excessive heat causes the ring to butt up against itself, creating stress in the ring gap of the piston, and eventually cracking it.
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Stay Ahead of the Curve
While deciphering the cause of piston failure may seem daunting, it’s a challenge worth tackling. These hard-working parts have a big job and maintaining their integrity is crucial to keeping your engine running. By recognizing the various types of piston failures mentioned above (and the clues they leave behind), you can empower yourself to diagnose any potential issues early on and take proactive measures.
And remember– vigilance is key. Keep an eye out for symptoms such as knocking sounds, loss of power, increased oil consumption, excessive smoke, poor acceleration, and, of course, visible damage. If you notice any of these indicators, don’t hesitate to address the issue.