Jun. 16, 2025
One could say that pistons are the heart of an engine. Without pistons there is no compression, without compression there is no power. Pistons have the simple yet very important task of compressing the air – fuel mixture inside your engine so it can be ignited by your spark plug, in case of a gasoline engine, or ignited by the very act of compression, in case of a diesel engine. Pistons are then exposed to the great pressures and temperature of combustion which forces them down the bore and creates the power stroke. Finally they have the very important task of pushing out exhaust gasses out of your combustion chamber when they come back up the bore. As you can see pistons are a key component of an engine and they are absolutely crucial for its operation.
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The manufacturing process of cast pistons starts with a molten alloy which is composed mostly of aluminum and a few other metals. Once the alloy is completely melted it is poured or injected into a mould. It then cures and solidifies. After this the mould retracts and the basic piston shape is complete. Cast pistons require very little machining before they can be installed into an engine block. Cast pistons are extremely widespread and can be found in many vehicles. They have the advantage of having low thermal expansion which means they allow tight piston to bore clearances which minimizes blowby, emissions and bore wear.
Forged pistons
The forged piston manufacturing process starts with an aluminum alloy rod that is cut up into piston sized billets. The billets are then heated up and put in a forging press that applies extremely high pressures onto the billet shaping it into a basic piston shape. After this the forged piston shape needs a lot more machining compared to the cast pistons. Forged piston production requires a lot more time which explains why forged pistons are more expensive than cast ones. But forged pistons have many benefits. The forging process pushes the metallic particles of the piston into a very uniform shape, giving forged pistons a superior grain flow compared to cast ones. This makes forged pistons significantly more ductile than cast ones, giving them greater strength. This is why forged pistons are better suited to the high combustion pressures and temperatures in forced induction and high performance engines, but are also better at surviving the high loads found in high revving engines. The only downside of forged pistons is increased thermal expansion, which means that typically forged pistons require a bit more clearance to account for this expansion.
Hypereutectic pistons
Hypereutectic pistons are made using a casting process and are ultimately less ductile and don’t offer the same strength as forged pistons, but are a step up from typical cast pistons. The word hypereutectic denotes the silicon content of the piston. Any piston with more than 12.5% of silicon in it is going to be called hypereutectic. Silicon is important because it improves the wear characteristics of pistons, making them harder and thus lighter, but most importantly hypereutectic pistons expand even less than typical cast pistons which have on average 8-10% of silicon in them. This means that engine designers can specify extremely tight engine clearances when hypereutectic pistons are used, this in turn improves performance, efficiency, fuel economy and reduces emissions.
When forged pistons are in question, one of two alloys is most often used, and the main difference between the two alloys is the silicon content. forged pistons have around 11% silicon in them, while forged pistons have 1-2% silicon in them (some as low as 0.2%). This means that is the best alloy for extreme racing applications while pistons offer both the benefit of high ductility, but with reduced thermal expansion when compared to alloyed pistons.
There are a few questions you must ask yourself before deciding on a piston. The first question is the condition of the bores inside your cylinder, because this determines whether you will be buying standard size or oversize pistons. Oversize pistons have a slightly larger diameter than standard size pistons, usually between 0.5 – 2.00 mm larger. This oversize is used to account for the over-boring process that is done to the bores in your engine block to renew their condition and make them perfectly round once again. Once you have settled on size you have to decide whether you will be sticking with OEM piston specifications or whether you will upgrade your pistons. If you are rebuilding or repairing an engine that will retain OEM specs than the best option is to keep the same piston specs. But if you are modifying or tuning your engine you have the option to buy domed or dished pistons to increase or decrease your compression ratio, or you can upgrade from cast to forged pistons to give your engine internals more strength.
Any engine failure associated with pistons is always going to be a critical engine failure and will require an engine rebuild or overhaul. Because pistons sit deep inside your engine replacing them almost always requires complete engine removal and machining of various components to bring the engine back to proper working order.
Loss of power / blue-ish smoke from the exhaust / engine burns a lot of oil
The most common piston associated failures come from your piston rings and your bores and results in the engine loosing its ability to compress the air – fuel mixture and make power. The above symptoms usually occur late in the engine’s life, after a lot of miles. At this stage the piston rings have been worn to the point where oil escapes past them into the combustion chamber causing the engine to burn a lot of oil and generate excessive smoke from the exhaust. The rings are also worn to the point where they can’t compress the air – fuel mixture properly which leads to a loss of power and is often accompanied by a blue-ish smoke from your exhaust tailpipe. An engine like this has come to the end of its life and will require a total overhaul to restore its power. In most cases this means an overbore, with new rings and new oversized pistons.
Loss of compression in one cylinder
If your engine isn’t making the power it used to you can test its compression. If only one cylinder fails to meet the minimum required compression and gives significantly different readings, a possible culprit is a broken ringland. Ringlands are the parts of pistons on which piston rings sit. They can break when exposed to various abnormal conditions such as over-revving, lack of maintenance, engine knock, etc. When a ringland breaks it will prevent the piston ring from properly sealing against the cylinder bore and will result in a loss of compression and power. When this condition gets serious the cylinder might not even fire and your engine will be running with one cylinder less. If the issue is ignored the cylinder might experience hydro-lock due to too much fuel washing down the oil from its bore leading to catastrophic engine failure. The only way to fix a broken ringland is to replace the pistons
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If asked what you wanted for piston rings in your engine build, would you know how to answer? Piston rings have the most important job in your engine. We will get arguments on this one because there really are no unimportant parts in your engine. However, what makes piston rings so important is what they do.
The humble, hard-working piston ring dates back to when a man named John Ramsbottom demonstrated the friction-reducing value of piston rings along with the sealing and cooling benefits. Piston rings improved efficiency. In those days, it was more about steam engines and less about internal combustion.
Piston rings provide cylinder and combustion chamber sealing, which keeps heat energy contained where it belongs, above the piston. Any heat energy that escapes past the piston rings is lost power—period. Heat energy contained above the piston goes to work making power at the crankshaft. Piston rings also carry destructive heat into the water jacket via the cylinder wall to control heat and prevent piston meltdown.
What people want most from piston rings is cylinder sealing along with low tension to achieve less friction and better efficiency. It is challenging to get both. We live in an age of skinny, low-tension compression rings—sometimes as narrow as 0.023-inch, or 0.6mm. This works if you have perfectly honed cylinder walls. If you don’t, rings tend to distort and you’re not going to get optimum cylinder sealing.
Proper ring selection means understanding ring function, material, piston design, and bore dynamics. Pistons, rings, and cylinder bores must have a perfect marriage to function properly. Proper engine break-in is critical to endurance and reliable ring function. The type of piston ring you choose depends on how you intend to use your engine. Mild street performance engines call for a more “vanilla” ring package than supercharged, turbocharged, or nitrous-fed engines. Racing engines demand a much tougher ring package on par with what’s used for supercharged, turbocharged, or nitrous engines.
Which ring you choose boils down to how much heat and force you intend to impose on them. If your engine is bone stock as delivered from the factory, you’re probably not going to want to hear this. A box-stock engine is equipped with ductile iron and cast-iron piston rings. This means your rings are not going to like a supercharger or that occasional nitrous blast because stock ductile and cast-iron rings can’t always stand the heat and pressure associated with forced induction or squeeze.
If you’re opting for nitrous or forced induction, you’re going to need a top compression ring capable of withstanding the heat and pressure associated with these elements. This calls for high-end materials according to Ed Law at Total Seal. Ed suggests an AP Stainless top ring with PVD (Physical Vapor Deposition) for forced-induction and nitrous applications.
Total Seal’s high-performance piston ring sets include an AP Steel top ring that has been coated using PVD-applied C-33 chromium nitride anti-friction coating for greater efficiency. The C-33 coating is easy on cylinder walls while the steel top ring still has the ability to handle extreme pressures. Napier secondary rings and three-piece stainless oil control rings come standard with the AP Stainless Steel Ring Set.
As a rule, pistons and rings are generally sold in sets unless you’re reusing old pistons or are choosing a different type of ring than the manufacturer provides. Manufacturers such as Federal-Mogul Speed Pro from Summit Racing Equipment sell pistons and rings as sets for your convenience. This makes piston and ring selection a no-brainer for the average enthusiast. Just look at what the manufacturer suggests for the type of driving you intend to do and refine your decision from there.
An important consideration as to how well the piston rings seal is the hone of the engine block. Your machine shop should have a PAT gauge to accurately measure the final hone’s surface roughness. Total Seal says typical values (measured in microinches) for general performance applications should be around RPK 8-12, RK 20-30, and RVK 30-50. Is your local machine shop capable of this caliber of work? Not all of them are. If a machine shop can finish late-model Ford or GM stockers with their thin rings to maintain original emissions compliance and factory tolerances, the answer is likely affirmative. Confirm this when you drop the block and pistons off. CHP
Photos by Jim Smart
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