Most engine builders will tell you, the buck stops at the piston. After all, the piston is what takes the brunt of the fuel, air, and spark that you’ve worked so hard to deliver. So it’s probably no surprise that proper piston choice is critical to both the performance and longevity of your engine, but what may be surprising is that picking the best pistons isn’t as straightforward as it may seem.
To help sort through your modern piston options and applications, we sat down with Trey McFarland from RaceTec Pistons. Trey’s been in the piston game for decades, so when he talks piston tech, we’re giving him our full attention.
You can listen to the entire episode below or in the OnAllCylinders podcast section, and check out 10 snippets from our chat as well, so you can get a good idea of the piston topics we covered.
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10 Questions on Performance Piston Tech with RaceTec Pistons
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1. How Has the Piston Industry Changed in the Past 10 to 20 Years?
“Back in the day, forged pistons were built-on-demand parts—basically all of them were custom pistons. As the industry matured, manufacturers started to identify some custom pistons they were making over and over again. And they decided to put the more common ones on the shelf.
“The market continued to evolve and, today, there’s quite a wide variety of shelf-stock pistons. A lot of times, you can have that piston the next day, without the need for compromises.”
2. What Types of Forged Piston Alloys Are Available Nowadays?
“There are two main alloys that are used for performance forged pistons: 4032 and 2618. Most piston manufacturers use those for different applications, choosing the best alloy for the environment it’s going to be working in.
“For a nice street rod that’s going to be making 400 horsepower for cruise nights—maybe take it to the track every once in a while—just driving around having a good time. The 4032 is a better alloy for that application.
“If you’ve got someone who’s racing and flirting with detonation, the 2618 affords you some insurance. It doesn’t mean that the piston is invincible, but it has a better longevity and durability against detonation.
3. What Kind of Piston Alloy Should I Choose, 4032 or 2618?
“There are some compromises. It all depends on what you’re trying to do. If you’re in a controlled environment, not flirting with detonation on a regular basis, 4032 alloy has got better durability. It runs a tighter clearance, so it’s going to perform a bit better, and its going to have better longevity if you’re not running into aggressive detonation. 4032 is still a forged piston and will handle detonation much better than a cast piston you’ll find in most OE applications.
“Aggressive detonation, like you’ll see in a heavy power-adder application—liked boosted or nitrous—there’s a chance that you’ll surpass what a 4032 alloy will handle. So at that point, you go 2618.
“At RaceTec, in some cases, we’ve got the pistons that are almost identical available in either alloy. Because the same piston might work great for a normally aspirated street application, also could be a great boosted application for an E85 guy.
“So again, it’s not always black and white. The piston manufacturer will ALWAYS help you with a recommendation.”
4. Talk a Bit More About the Distinction Between 4032 & 2618 Piston Applications.
“Both alloys, as they see their first heat cycles, soften a little bit. And then, as they go through subsequent heat cycles, they continue to soften.
“The 4032 alloy softens at a very flat, plateau type of plane, so its graph would be very flat as it goes down in hardness. Whereas a 2618. it’s pretty steady in the way it softens. It’ll soften at a faster rate and more steadily.
“There’s not a problem with that, as long as you understand what you’re working with. You would choose a 4032 alloy if you weren’t building a competition-only engine, where you’re freshening the motor up on a regular cycle.
“If you’re not going to be freshening up the motor on a regular cycle, you don’t want to have to tear into the engine.”
5. What Happens to the Piston Through Those Heat Cycles?
“What a 2618 alloy will do, is it starts moving around a bit. The ring lands move around, the skirts can move in a little bit—it’s not going to blow up on you, but it will not perform at its optimum level, it’s just not the same part anymore.
“And that can be subjective too. If you build a motor and hit it with nitrous four times a month, yeah that’s some work, but you’re not hammering on it either—so it could still have a pretty good life expectancy. Whereas if you put it in a Pro Mod car, depending on how you’re running, it could need to be freshened up once every three events.”
6. Explain Bore Size & the Importance of Piston-to-Wall Clearance.
“The piston is going to expand. One of the differences between the two alloys, the 4032 has a lower expansion coefficient, so it’s going to expand a little less than the 2618. So you have to allow for that.
“What your buddy with a 4032 application can get away with, clearance wise, you with a 2618, you’re going to need to increase your piston-to-bore clearance.
“Good piston manufacturers provide parameters to work within, a sheet that talks about how you’re going to use the piston and they’ll make some adjustments.
“We get people that say ‘looser is safer, so I’ll just run it loose.’ That’s not necessarily safer.
“Yes, you’re not going to scuff the piston, but running a piston loose, once it completely expands, it its still slopping around in the bore, it’s going to beat itself up. And it’s not going to seal as well, the rings won’t seal up, it’ll use some oil, and—obviously—it’ll make some noise.”
7. What are Some Dangers of Improper Piston Size?
“If you’ve got the clearance too tight, and the piston expands more than you allow for, the skirts wipe the oil film off the cylinder bores and you’ll have metal-to-metal contact. Even with a coated piston, you’re going to scuff it and gall. After that, it goes pretty quick—and there’s no easy fix in that. You’ve got to take it all back apart.
“Tighter clearances do perform better, as long as you don’t get into a situation where the pistons outgrow the cylinder. At the end of the day, a little bit more clearance is always safer than not quite enough.”
8. What Happens if You Need to Modify the Piston Heads for Valve Clearance?
“There are a number of reasons why the valve pocket in the piston you’ve purchased may not be exactly what you need—you should always clay-up your pistons. Put some clay in the valve pocket, put the cylinder head on with an old, compressed gasket, and turn the engine over slowly, by hand. It’ll compress the clay and you can look at the thickness. Typically if you can manage 0.060” then you’re about safe for anything going on out there.
“If you’ve got an area that needs to be modified on the piston, it’s not the end of the world, but you’ve GOT TO CALL the manufacturer and tell them—because you don’t know what that cross-section is.
“In fact, if we get a call like that, we’ll go pull the drawing. You wouldn’t believe how many times we get someone who’s already modified a part before they call, and they’ve modified it into something that’s not going to work for what they’re doing.”
9. Is Bigger Always Better When it Comes to Wrist Pin Size?
“A lot of people lose sight of the fact that the piston, the pin, and the connecting rod are a working system—three different components that have contact with each other. Those three pieces have got to work together. If you make an aggressive move on one, it doesn’t necessarily benefit the whole combination.
“The piston moves under load, the small end of the rod moves under load, and the pin moves under load. If you make that pin stronger and more rigid, the load gets redistributed. It puts the load onto the small end of the rod and the pin bores on the piston—sometimes that’s OK and sometimes its not. If you’re putting enough load into that piston to deform the pin, then you can have some problems.
“The pin spins, and you want it to spin. It helps with the oiling and it helps with heat.”
10. Do You Have Any Tips or Advice on Piston Ring Selection?
“You don’t need to buy the most expensive coated rings for every application, but those options are there because they do make a difference. Choosing the right ring for your application is key.
“The difference between the bigger ring versus the smaller ring is that, the smaller the ring gets, the more conformable the ring is. The pistons move around a little bit, the ring lands move around a little bit.
“Most people thing of ring seal as the face of the ring making contact with the bore. That’s important, but just as important is the flank area of the ring, the bottom part of the ring, making contact with the ring groove in the piston.
“And if the ring groove is moving around and distorting under load, than if the ring is less conformable, it’ll move away from the piston and the cylinder pressure goes around under the ring, and you lose some cylinder pressure.”
“There are a lot of different materials out there, they act a little differently—some are more friendly than others. So when it comes time to select your ring, I’d always suggest that you use a reputable ring manufacturer. And again, ASK QUESTIONS—call the ring manufacturer, tell them what you’re doing and what piston you’re using. They’ll help you understand what you need to buy.”
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You can hear more from this interview in the OnAllCylinders Podcasts section.
If you’re building a new engine or freshening up the rotating assembly in your current one, Trey and his pals over at RaceTec Pistons can help you make the best piston choice for your specific application. Contact the RaceTec experts at via the RaceTec Pistons website to begin the conversation.
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