The faster and farther we want to go, the harder diesels get pushed, creating a greater demand for engine components to handle the abuse. As such, custom piston options continue to rise as we continue to innovate within the diesel industry to find out what works and what doesn’t. It has often been the school of hard knocks, but those lessons allow piston manufacturers to design custom options based on what they’ve learned and what they already know about the durability and function of original equipment (OE).
First and foremost, the common sentiment among piston manufacturers is they want to be contacted when you’re selecting pistons. There are so many different options available that it’s a huge advantage to leverage their knowledge.
“Since we’ve been through all the hard knocks of diesels, we know what works, what doesn’t work, and what you should do on every application,” says Rick Canning, technical specialist, CP Carrillo.
Most stock diesel pistons are cast, and cast, traditionally, is at the bottom of the performance food chain. Cast OE diesel pistons, however, have proven durability and can run, at least to a certain extent, with the performance crowd. One advantage is they have a steel insert for the upper ring. In addition, they utilize a keystone type of upper ring, creating radial motion that cleans carbon buildup from the groove – an important benefit to diesels.
Cast pistons also feature oil-cooling galleries behind the rings to help control heat, and it’s the combination of these design aspects that allows them to handle mild power increases. In the case of Mahle Motorsports, their cast pistons are among the company’s best-selling diesel pistons, according to Joe Maylish.
Cast pistons have many advantages, but when you’re pushing performance way beyond stock levels and looking for more than the occasional drag race, you may be pushing more than cast can handle, which means it’s time for an upgrade to forged pistons.
“Forging produces a tighter grain structure, crushing the molecules and making them strong,” Canning says. High-horsepower pulling or drag racing engines are a common application for forged pistons, and while trucks are the obvious home for most diesel engines, more and more are showing up between the fenders of cars, which has helped push the envelope of power even further, proving their place in performance.
“People that are introducing very high amounts of boost and trying to make outrageous amounts of horsepower, choose to go with forged,” Maylish agrees.
However, forged isn’t without drawbacks, and one of them is the expansion rate. “The heat from combustion dissipates into the top of the piston, and because it’s a forged piston, the land diameter has to be reduced, so when it gets a lot of heat, it won’t grow and hit the cylinder wall,” Canning says.
Many forged pistons use low-expansion alloys in order to keep the piston fitting as tight as possible, without creating clearance issues, and because of the extreme heat, clearance is of critical importance. The average cast aluminum piston will need a cylinder wall clearance of about .005”-.006”. A low-expansion forged piston requires .007”-.008”, and a high-expansion alloy can require .010”-.012” or more. Due to these varying expansion rates, piston manufacturers always stress the importance of measurements during assembly.
“With a diesel, even the best components can be destroyed if they are not put together correctly,” Canning points out.
Forged pistons pose another problem. While they are stronger than cast, they don’t have the long-term durability, because you’re not able to forge a piston around a steel insert for the top ring, nor are you able to create an oil cooling gallery behind the rings. The benefits of a keystone ring in a diesel are great for longevity, but the drawback to this style of ring is that it will cause accelerated wear without a hard material to protect against it, which is the purpose of the steel insert. This is the one true advantage to stock pistons, and a reason why many people stick with them as long as they can.
The movement of a keystone ring in its groove becomes more pronounced when additional cylinder wall clearance is required to compensate for piston expansion.
“Diesels are very hard on ring grooves,” Canning says. “Forged diesel pistons are hard anodized to provide additional protection. We also don’t use keystone ring grooves. We use a conventional rectangle ring groove because when the land diameter is brought in from the bore, what you have for vertical clearance and backspace stays the same.”
Another aspect of custom diesel pistons is relocation of the top ring groove because of the extreme heat and pressure loads on the top of the piston.
“To make these pistons live in high-horsepower and high-heat applications, on certain applications, we bring the top ring down .600”,” he says.
Expansion can compound as boost levels and performance increase, and it’s not the only heat-related factor to consider either. Most stock pistons have a protrusion or peak in the center of the bowl that helps disperse the fuel sprayed by the injector into the surrounding bowl. As the bowl curves upward, there’s usually a lip or overhang that redirects the air/fuel mixture back toward the center of the bowl. This is specific by design for a cleaner burn to reduce emissions, but heat tends to concentrate in the upper lip area of the bowl, and on an engine running high boost, the heat can build in the lip or overhang and lead to detonation or cracking in the piston.
“Hot spots are a big problem on diesels,” Canning says. “Sharp edges cause problems and the heat will penetrate where it can burn holes in the pistons. We made non-nitrous piston dishes with generous radiuses going from the dish to the valve pockets, and around the valve pockets. Everything has a generous radius around it to prevent any kind of heat sink. The whole top is completely smooth, machined and radiused.”
Custom work is often done to OE-style pistons, by machining off the lip designed around the bowl, along with beveling or blending the top edge of the bowl to eliminate the possibility of hot spots. This is proven to be effective, but doesn’t hold the same advantages as custom pistons, which are designed without hot spots from the start.
Many factors are considered when designing a diesel piston, including the type of injector and spray pattern, as well as the fuel and ignition strategies that will be utilized in the tuning. This can affect the design or inclusion of a peak, and a common question asked by consumers is related to the injector angle of the dish.
“We made a wide bowl dish that covers all the different angles,” Canning says, reinforcing again the idea that it’s critical for piston manufacturers to know everything about the application to provide the correct piston. “Small dishes make less power without nitrous, but with nitrous, a small dish is better.”
Another aspect of a custom piston that can improve on strength is the pin boss. The pin end of a diesel connecting rod is tapered toward the top in order to increase the bearing surface and spread the load across a wider area on the underside of the wrist pin at the rod on the downstroke, and also the top side between the pin and piston. As opposed to machining the taper into a forging, it’s an advantage to duplicate the rod shape in the forging.
“Since we have Carrillo rods and CP pistons, we duplicated the taper of the Carrillo rod into the CP piston forging,” Canning points out.
Another piston option is steel, which offers the performance of forged aluminum, but the added advantage of durability. Steel pistons are incredibly strong and they’re able to handle the heat. Based on those factors alone, they may seem like the ideal choice, but they also have drawbacks. If any scuffing occurs between the cylinder wall and piston skirt, steel is most likely going to do damage to the cylinder wall, whereas aluminum is easier on the cylinder walls and any damage will only happen to the piston itself.
Coatings are a technology that add multiple options and can vary between custom pistons. Top thermal coatings are not intended as a substitute for quality, but when you’re pushing the limits, they just might be that little extra advantage that gets you through without piston damage. They can also help prevent hot spots and give you a slightly larger window for tuning.
“We offer a thermal barrier coating to the crown of the piston,” Mahle’s Maylish told us. “It’s an added layer of protection against thermal shock, to prevent excess heat from penetrating the crown of the piston.”
Hard anodizing is a common process to protect the ring grooves, but some companies offer a nickel-based anti-wear coating. Although it does add thickness to material, anodizing is a process that increases the thickness of the natural oxide layer in metal, as opposed to a coating that is applied, requiring a different type of machine finish.
Skirt coatings are designed for friction reduction, with the primary intent to reduce scuffing and reduce piston skirt fatigue.
“The material we use on Mahle piston skirts is Grafal,” Maylish says. “It has a cushioning property, and where there’s potential for contact with the cylinder wall, the Grafal will prevent metal-to-metal contact.”
Oil cooing is another important aspect of diesel pistons. They are cooled by spraying the underside of the piston and directing some of the oil into cavities behind the rings. Since the same galleries cannot be cast into a forged piston, many custom pistons are designed to direct the oil spray in order to maximize the cooling.
Ultimately, one of the most important factors when ordering diesel pistons is to provide your piston manufacturer of choice with all the information about your build and the intended use. Piston manufacturers encourage their involvement to make sure you get the pistons you need.
“If you have any questions of any sort, call us,” Maylish reiterates. “We have people who have been on our sales staff for over 20 years and people who have been on our engineering staff for over 30 years. We can help you.” EB