Ask Away! with Jeff Smith: How 2618 vs. 4032 Alloys Affect Forged Piston Noise

I am building an LSA supercharged 6.2L motor. I would prefer to run a 4032 forged piston with a 12cc dish to create the proper compression ratio. I recently watched Summit’s video of the new line of Pro LS pistons. In the middle of the video I noticed a 12cc dished piston for the 6.2L engine that is exactly what I’m looking for.

Here’s my question: The video says this is a 2618 alloy piston but it also mentions tight clearances for this piston. What is the minimum skirt clearance spec for this 4.070-inch diameter piston? Is there any evaluation on how quiet these are? I’m going to use this engine in a summer hot rod. I’d prefer a 4032 alloy piston because I don’t want to listen to piston slap when the engine is cold. I own a small-block Chevy with 2618 pistons and it sounds like the pistons are trying to swap holes when the engine is cold. What would you suggest? — B.I.

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Jeff Smith: The question of piston noise when cold is very much a subjective situation.

We have built a few small-block Chevys with 2618 pistons. One was ridiculously noisy while a later engine using a different piston manufacturer was not nearly as obnoxious. Much of this has to do with the specific forging’s skirt design. This has an effect on piston wall clearance.

Before we get too deep into this, we should probably review the details of these two different alloys to bring everybody up to speed. Forged aluminum automotive pistons are almost exclusively created from two different alloys — 4032 and 2618.

The 2618 alloy expands more than the 4032 piston, which is why the 2618 alloy calls for a wider piston-to-wall clearance. The 2618 is generally used in higher performance and race applications because it offers a greater ability to absorb abuse. It tends to bend while retaining its original shape, making it a perfect alloy for performance use. This is why Summit chose to use that alloy for the Pro LS piston line.

The 4032 alloy includes a small percentage of silicon that inhibits growth. This alloy accommodates a tighter piston-to-wall clearance. The advantage of this is that the piston is tighter in the bore when the engine is cold, which drastically reduces the noise factor during warm-up.

The difference in alloy however makes the 4032 piston less malleable which means it tends to be slightly more brittle compared to a 2618 piston. It’s important to note that a 4032 forging is still far superior in strength compared to either a cast aluminum or cast hypereutectic piston.

This Summit Pro LS piston would be a good choice for your supercharged application. One aspect that you may have already picked up on is the skirt coating that is included on all these pistons. This does take up a small amount of the true piston-to-wall clearance.

According to Summit Racing’s Brain Nutter, the minimum piston-to-wall clearance is 0.004-inch measured metal-to-metal.

This is done by measuring the piston at the small hole in the skirt coating. This allows a true metal-to-metal clearance to be obtained. Nutter says the skirt coating measures a nominal 0.0006-inch per side. This means that the 0.004-inch clearance when cold is really somewhere around 0.0028 to 0.003-inch. This is the cold measuring clearance. When the piston expands to its dynamic shape, it will reduce the clearance down to roughly 0.001- to 0.0015-inch.

Beyond just the alloy makeup of this piston, there are other specs worth mentioning.

These pistons are machined to accept a 1.2/1.2/3.0mm ring package although they are not supplied with the pistons. The pistons do come with a 0.927-inch diameter wrist pin that is smaller than the stock LS 0.945-inch pin, so that is also important.

The smaller wrist pin diameter is intended to be used with much stronger aftermarket LS connecting rods such as Summit’s H-beam Pro LS rods.