If you’re like many of us, you don’t spend a lot of time thinking about the pistons in your engine. Most of the time you don’t need to, but there are times when burnt, scuffed, or cracked pistons need to be replaced or upgraded. If you’re rebuilding your engine, chances are you’re probably upgrading to a set of forged aluminum pistons. But which alloy should you go with? And should you buy into new technologies? Here are some tips on choosing the proper pistons for your engine.
2618 vs. 4032 Aluminum Alloy Piston Comparisons:
No matter which type or brand of piston you decide to use, they are all made from a combination of both aluminum and silicon, and almost all forged aftermarket pistons are manufactured from either 4032 or 2618 aluminum alloy. It is the amount of silicon, though, which determines the piston’s overall strength versus wear resistance properties.
2618 Aluminum Alloy
A 2618 aluminum alloy has a higher overall strength and can ultimately take a bit more abuse than 4032 alloys. A 2618 piston is ideal for forced-induction engines that experience higher temperatures and/or any racing application. In comparison to the 4032 alloys, 2618 with its low-silicon alloy requires larger piston-to-wall clearances due to its higher coefficient of thermal expansion, which causes the piston to grow more when exposed to heat.
- Fatigue life
- High-temperature strength
- Conductivity (heat transfer)
- High strength
- High expansion rate (more clearance required)
- Poor wear index (lower silicon content)
- Piston rattle or slap due to larger piston-to-wall clearance
4032 Aluminum Alloy
A 4032 aluminum alloy is designed for high-performance applications where a strong and quiet piston is required. A 4032-alloy piston requires less initial piston-to-wall clearance than 2618 due to its higher silicon content, and is ideal for street/strip applications.
- Wear index (good at resisting wear due to higher silicon)
- Low expansion rate (less clearance required)
- Low density (lightweight)
- Limited temperature strength (lower strength at high temperatures)
- Notch sensitivity (brittle)
Gas Ports are small holes drilled into the top of the piston crown or on the sides of the top ring land that lead to the back of the top ring groove. When gas ports are used, pressure is directed to build up behind the compression ring and seal it against the cylinder wall. As a result, less pressure can leak past the top ring and more horsepower is generated. Generally, lateral gas ports (right) are drilled through the bottom side of the top land and extend to the back wall of the ring groove and are used for endurance or road racing. Vertical gas ports (left) have the holes drilled from the deck of the piston into the top ring groove and behind the ring, and are mainly used for drag race applications, which accelerates ring wear from the increased pressure on the ring face. Gas ports work best for race engines that are torn down and rebuilt often, and are not recommended for daily street cars.
The piston’s wristpin is arguably the most highly stressed part in an engine, as shown by the red areas in the FEA (finite element analysis) image shown here. The expansion of burning gases in the combustion chambers applies tremendous force on the piston top. That force is transferred to the connecting rod via the wristpin. To give you an example of how much force is applied to a wristpin in race applications, consider the inside of an 850hp NASCAR Sprint Cup engine running at 9,000 to 9,500 rpm: A great deal of stress from both cylinder pressure and inertia loads applies crushing force equivalent to six tons, hammering each wristpin about 77 times each second. This punishing, cyclical load can last up to 600 miles in some races.
The FEA image below shows the pin under firing pressure (power stroke) on the left, and inertia loads on the right (just after TDC on the intake stroke, where the pin is being yanked downward). The bending is exaggerated, but shows the type of stress on the pins. If too thin of a wristpin is used, the pin will bend. If the wrong material is used, it may not be able to handle a high-stress environment for extended periods and could fail. It is critical to select the proper wristpin thickness for your application, to prevent excessive bending to support the piston.
A basic guideline for selecting the proper wall thickness for your wristpins:
Up to 150 hp per cylinder
Up to 250 hp per cylinder
It’s important that a thin oil film remains between the wristpin and piston pin bore. There are several methods of distributing oil to the wristpin. One that piston manufacturers use is called forced-pin oiling, which utilizes one or two small holes in the oil ring groove that connect to the pin bore itself and “force” pressurized oil to the wristpin. The second method is the use of “broaches”, or small rounded notches that run parallel to the wristpin. These small reliefs allow the oil splashing in the crankcase to reach the wristpin from the inside of the wristpin boss. Forced-pin oiling is more commonly used with over-the-shelf pistons.
Because the wristpin is essentially a bearing, a polished finish on the OD is critical to preventing wear and failures. One area often overlooked is the ID finish. A large amount of bending stress is concentrated within the wristpin, therefore a smooth finish on the inside is also critical.
Even if the bores in the piston’s pin towers measure out right, never assume they are straight. The test for this is relatively straightforward: Insert the wristpin in the bore in one pin tower and see how easily it spins. Now slide the pin so that it extends into both pin towers. Sometimes, you can feel the wristpin catch as it enters the second pin tower. Also, if it’s more difficult to spin when it’s in both towers versus a single tower, that’s a good sign the pin bores aren’t correctly aligned and additional honing or replacement is required.
Valve reliefs are the notches on the crown of a piston that provide clearance for the intake and exhaust valves. Valve relief diameter, location, and depth vary for every application. Engines with oversized valves and racing camshafts may require a larger diameter and deeper valve relief in the piston crown to provide the necessary clearance. Fortunately, many aftermarket piston manufacturers design their off-the-shelf pistons to accommodate these modifications.
Oil jets are used on many modern high-performance engines such as the 4B11, 2JZGTE, SR20DET, 4G63, and others. In Formula 1 engines, as many as six oil jets per cylinder are used. Although oil jets are not a piston feature, they do play an important role in keeping the piston cool and well lubricated. If the piston alloy can’t stand up to the rigors of the application, the piston can soften and lose strength at high temperatures. This process is called annealing, and can significantly reduce the strength of the piston and eventually lead to failure. Oil jets spray a stream of cooler oil to the piston under-crown that reduces the overall piston temperature during operation.
Ring Land Thickness
Ring lands are the “bands” above each piston ring. The thickness of the lands depends on the piston’s intended purpose. Higher-horsepower applications require thicker lands to provide the proper strength. The top land on the Subaru EJ257 turbo piston (right) is thicker and designed to handle up to 300 hp per cylinder compared to the normally aspirated piston on the left with the thinner land, designed for up to 65 hp per cylinder. But don’t assume the thicker the ring land the better. If it is too thick (over .300-inch top land), the engine can lose its efficiency and power output. A higher ring location is more efficient and will typically make more horsepower on NA applications, but in turbo applications, using a piston with a higher ring location (and small top land) is a recipe for piston failure. A turbo piston needs the extra strength from a thick top land.
Piston ring endgap provides the necessary room for rings to grow when exposed to heat. Because every engine is different, the proper ring endgap varies for each application. Variables, such as power output, bore size, ring material, and more, can change the required gap. Insufficient ring endgap will lead to contact between the ends of the rings causing damage to the rings, cylinder walls, and pistons, and can potentially destroy your engine. If the ring gap is too large, the ring will not effectively control the combustion chamber’s blow-by gasses and cause a loss in power. Remember, a few thousandths too big is much more desirable than a few too small. Some people gap rings by hand, using a file and a vice. However, for greatest accuracy using a ring filer is the correct way, as the grinding wheel will evenly grind material away from both sides of the ring. Here is a recommended guideline for ring endgap:
|Top Ring||Second Ring||Oil Ring Rail|
|Vehicle Application||Bore x||Bore x||Min. Gap|
|High-Performance NA Street/Strip||.0045 inch||.0050 inch||.0015 inch|
|Moderate Turbo/Nitrous||.0050 inch||.0055 inch||.0015 inch|
|High-Power Street Turbo/Nitrous||.0055 inch||.0057 inch||.0015 inch|
|Turbo/Nitrous Race Only||.0060 inch||.0063 inch||.0015 inch|
Piston-To-Cylinder Wall Clearance
Piston-to-cylinder wall clearance, like ring endgap, is dependent on the environment to which the piston will be exposed. Engines that generate higher cylinder pressures will typically transfer more heat into the piston, therefore requiring more clearance to the cylinder wall. Normally aspirated engines will see significantly lower cylinder pressure/heat and require less initial clearance. Another factor that effects piston-to-cylinder wall clearance is the actual piston design. Depending on the location of the aluminum within the piston, it will expand differently in each area. This factor is typically taken into consideration by the piston manufacturer’s clearance recommendations.
Pistons can be coated with three different systems: dry film lubricants, thermal barriers, and oil shedding coatings. All these coatings can be beneficial as they protect the piston against damaging heat transfer. By retaining minimal heat on the surface of the piston, less heat is transferred to the incoming fuel mixture, leading to a reduction in preignition, or detonation. The coatings can also allow heat at the surface to move more evenly over the surface, reducing hot spots or evens reflecting heat into the chamber for more efficient combustion of the fuel and less thermal expansion due to a reduction in the heat absorbed. Many piston manufacturers offer various types of coatings, including this JE Subaru WRX FSR piston with a thermal barrier coating applied to the crown and skirt.
Asymmetrical vs. Round Design Pistons
JE Pistons recently released an asymmetrical forged piston that utilizes two different-sized piston skirts, giving it the “asymmetrical” piston design. Asymmetrical piston designs can be found in many professional-level racing engines competing in Formula 1, ALMS, and NASCAR, and have been kept a secret until JE began designing the first aftermarket asymmetrical forging in 2010.
Both the asymmetrical piston and “full round”—otherwise known as standard piston designs—are available for most popular applications. In general, both pistons will perform their function in any sport compact application if designed properly. However, the asymmetrical piston design has a significant advantage over traditional round designs, including reduced skirt width on the minor thrust side to help minimize piston contact with the cylinder wall, which causes friction and power loss; a shorter wristpin that reduces overall weight; internal/external bracing to provide more rigid construction; and most importantly, a significant weight reduction due to a combination of all three previously mentioned. A quick weight comparison between a Mitsubishi 4G63 traditional round piston and asymmetrical (both weighed with wristpins) showed a difference of 15 grams per piston, which is a huge deal in the performance world.
To date, the asymmetrical-designed piston has been successfully run in applications ranging from daily driven vehicles to the infamous 800-whp AMS Performance time-attack EVO X and Titan Motorsport’s 2,000-plus horsepower Supra drag cars.