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2618, 4032, and 390.0 (OEM) Aluminum Alloy Piston Primer:

**Please Note** Although these conclusions are based on extensive research, communication with industry professionals, and advisement from an expert in metallurgy, I am not a piston designer or engineer, and thus everything below should be taken as opinion only.

Aftermarket piston manufacturers proudly boast of the alloys used in their products, typically AA designation 2618 and 4032 alloys. Honda, however, seems to have left us in the dark… For the most part. After consulting with several industry professionals, referring to a few Honda patents, and personally testing/examining an OEM k20a2 piston donated for such purposes (“Thanks!” to Boom07ZX6R) The census concludes that Honda utilizes a 390.0 (or similar) cast alloy.

Given this, I've performed an evaluation of these three alloys for use specifically in K-series engines, based upon principle Physical, Mechanical, and Thermal properties of each alloy. The evaluation was based upon 3 types of performance builds: Normal to mild Naturally Aspirated, High Horsepower Naturally Aspirated, and Forced Induction.

Each alloy has its strengths and weaknesses, and some of those strengths (and compromises) have been found to be more or less suitable to each of the applications listed above. Below are the conclusions deducted from the research performed. (If you would like a copy of the full research paper which goes into more specifics, PM me with your email address, and I’ll gladly send it your way.)

390.0 Alloy:
390.0 Alloy cast aluminum pistons provide excellent characteristics under normal conditions. Low thermal expansion, a high modulus of elasticity, and a high specific heat capacity help insure predictable longevity and performance characteristics, with a low requirement for maintenance throughout the cycle of the engine’s lifespan. In high performance engines, the characteristics of 2618 and 4032 alloys may provide considerable advantages when High RPM and High piston temperatures are taken into consideration.

4032 Alloy:
Naturally aspirated high performance engines may find 4032 alloy pistons most beneficial, as these engines typically see extremely high rpms, demanding better thermal conductivity then 390.0 alloys can provide, and better sealing characteristics (a lower rate of thermal expansion) then 2618 alloys allow. Attention should be made in ensuring adequate valve reliefs when using this alloy since the elongation properties are a bit higher then the other alloys mentioned.

2618 Alloy:
2618 alloys may be best suited for forced induction applications, as their ability to resist pre-ignition is critically important under high boost. Since forced induction applications typically see higher Intake air temperatures (due to the nature of super & turbo chargers,) the higher thermal conductivity of 2618 alloy pistons may aid in transferring additional heat away from the combustion chambers. While extended warm-up times (due to the expansion characteristics of this alloy,) may deter use under normal and high performance naturally aspirated engines, forced induction components also require additional warm-up time, mitigating the disadvantage of this characteristic. Forced-induction applications are typically designed with additional positive crankcase ventilation systems due to the potential for “blow-by”, allowing for drivable characteristics on a conservative cold-start tune.
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