It seems the more you know, the more there is to learn. For the average guy – stock pistons are cast, race pistons are forged and the rest isn’t really important. In a Peter Pan world that might be enough, but it turns out that technology offers choices and it pays to know the players better by reading the score card. In the case of forged pistons, it comes down to a slightly more complicated choice of the better of two alloys. Too much information might be awkward in certain situations, but when it comes to pistons, more details are always a good thing.

We must start this discussion with a little basic metallurgy. Our approach will be focused entirely on forged pistons. These pieces of art are not pure aluminum. Instead, performance pistons come in two distinctly different alloys.  

First let’s address the 4032 alloy represented by the JE SRP (Sportsman Racing Products) line of pistons. This alloy is created with a high silicon content – a full 12 percent. The addition of silicon greatly reduces aluminum’s expansion rate, which means the piston can run with tighter cold clearances and makes for much quieter piston operation when the engine is cold. Inexperienced enthusiasts often mistake a cold engine piston slap for a deadly rod knock, so a piston with tight clearances is extremely quiet, making it an excellent choice for a street engine.

This additional silicon content also improves this alloy’s durability, again making it a great street engine choice when subjected to thousands of low-load road miles. Besides the typical wear you might see along the piston skirts, the most critical position for ultimate performance is actually in the ring grooves. One major advantage to 4032 is its additional wear resistance, which will maintain the proper ring groove clearance and means a better seal over more miles travelled.

This 4032 alloy can be used in performance and even competition applications with excellent results. However, its reduced ductility does make the alloy less resistant to cracking with extreme impact loads such as detonation or unforeseen physical contact than would be the case with a 2618 alloy. But don’t let that scare you. According to JE’s Director of R&D Dave Fussner, Pro Stock teams have experimented with 4032 pistons because the alloy is slightly lighter.

The main difference with a 2618 alloy, as represented with the JE race piston line, is its very low silicon content. This makes the 2618 piston much more malleable and offers advantages under high load, high stress applications as with power adders like superchargers, turbochargers, or nitrous oxide. However, this lower silicon content also means the piston has a greater linear expansion rate, which must be compensated with greater ambient piston-to-wall clearances. Essentially, a 2618 piston will expand 15 percent more than a 4032 version. This, as we’ve mentioned, is the reason a 2618 piston requires more clearance, and as a result, will be slightly noisier when cold as opposed to a comparable 4032 forging.

This might be a good place to mention that despite the differences in piston-to-wall clearances when cold, once the pistons reach operating temperature, given similar piston structure, both the 2618 and 4032 alloys would be operating with very similar clearances. This further reinforces why the 2618 piston starts with a larger cold clearance.

The 2618’s lower silicon content also makes the piston slightly less wear resistant compared to the 4032. For pistons used in competition applications, this isn’t a major consideration since they will be replaced in the search for ultimate power long before significant wear is introduced.

Part of what makes any metal alloy stronger is the heat treat process the manufacturer uses. Each of these alloys demands a slightly different process to pull maximum performance out of the material. The heat treat is broken down into a four-step process with a preheat, a solution heat treatment in the second step to extract impurities, a water quench, and finally an aging process with 2618 experiencing a more lengthy aging process than 4032.

While these differences may seem to complicate the decision-making process, each of these two alloys really tend to point toward specific benefits for certain applications, which is why the JE family offers both. The 4032 alloy offers less expansion and quieter cold startup operation, but also excellent strength relative to long-term wear characteristics that make it a great street performance piston.

The 2618 alloy is renowned for its excellent high-temperature strength. This makes it the preferred choice for endurance racing – especially in extended, wide-open-throttle competition. It can also be a superb choice for a serious street power application, and if long-term ring groove wear is a consideration, the anodizing option is a great way to extend its performance.

Each alloy offers different characteristics, but at some point the benefits of the stronger, more race-oriented 2618 alloy makes the decision rather simple if your intent is to go racing. If you find yourself straddling the alloy fence, the good news is it’s difficult to make a bad decision.

Piston alloy is really just one of many decisions you will need to make on your way to assembling your next engine. But armed with the right information – that engine has a superb chance to make great power and put a smile on your face all at the same time.

This article was sponsored by JE Pistons. For more piston tech, visit https://blog.jepistons.com today!

Chart 01

Element Breakdown

2618		4032
Aluminum	93.7%	Aluminum	85%
Copper	2.3%	Silicon	12.2%
Magnesium	1.6%	Magnesium	1.0%
Iron	1.1%	Copper	0.9%
Nickel	1.0%	Nickel	0.9%
Silicon	0.18%
Titanium	0.07%

 

 

Chart 02

Alloy Pros and Cons

2618

Pro                                                     Con

Stronger                                             Piston Noise When Cold (wider clearances)

More Ductile                                     No Disadvantage to Being More Ductile

Better Fatigue Life                           Slightly Higher Wear Rate

Excellent High Temp Strength      

 

4032

Pro                                                     Con

Excellent Wear Rate                        Less Ductile

Quieter (tighter clearances)           Lower Strength at High Temperature

Slightly lighter                                    Less Fatigue Resistant than 2618

 

 

Chart  03

Piston Alloy Specs

Material Characteristics	4032	2618
Tensile Strength	55,000 psi	64,000 psi
Yield Strength	46,000 psi	54,000 psi
Fatigue Endurance	16,000 psi	18,000 psi
Modulus of Elasticity	11,400 psi	10,400 psi
Melting Point	990 – 1,060 F	1,020 – 1,180 F