The Best Value in Forged Pistons: Pro Tru Street

May 10, 2017 / by Jeff Smith

What if we told you that there was a complete set of pistons available for most popular applications for $410? Yes, even including locks and rings! Learn more here.

In the baseball movie Field of Dreams, Kevin Costner’s character is influenced by a voice that tells him “If you build it, he will come.” At Wiseco, nobody’s hearing voices but the message is still clear. For street engine builders, it often comes down to choosing the best affordable product. That’s what drives Wiseco’s Pro Tru Street line of performance forgings.

The big reason that Wiseco can offer a quality forged piston at such a reasonable price is by designing a piston using what engineers call a near-net-shape forging process. Note that this is a forging process, which is superior in strength in every way over a casting or a hypereutectic. In this approach, a forging die produces a shape that is very close to the piston’s complex final shape. Forged valve pockets can be created because Wiseco has in-house forging capability which allows them to tightly control the valve pocket depth, size and location. The resulting forged valve pocket has zero negative impact in a running engine versus a machined valve pocket. This reduces the time spend making chips in the CNC machine. By reducing the machining time, this substantial cost savings is passed along to the builder.

Center is a standard Wiseco piston forging. As you can see from the outside forgings, the valve pockets are created as part of the forging process. Doing so reduces the amount of time the piston needs to be in a CNC machine, which is where the majority of the cost comes from.

Applications Galore

This near-net shape approach is carefully planned to fit the most popular applications so you might think that this means there may only be a few pistons to choose from. But if you peruse the Pro Tru Street catalog, you’ll quickly realize that besides the multiple small-block Chevy applications, the line includes pistons for the AMC 360, Buick 455, Olds 455, multiple big-block Chevy applications from the 396 to the 502 (including strokers), Chrysler LA’s from 318 to stroker 360’s to the RB 440’s, and a slew of Windsor and Cleveland Ford applications as well.

Construction

A fundamental part of the forging process must also account for the piston alloy. The Pro Tru Street lineup consists of 4032 aluminum alloy forgings. This material is designed with the street engine builder in mind, containing a 12 to 13 percent higher silicon content. This change in the mixture affects the rate at which the piston grows as it achieves its normal operating temperature and improves wear resistance.

All Pro Tru Street pistons start out as 4032 bar stock that is then cut into pucks.

This creates a piston that can be more tightly fitted to the cylinder walls, which creates a very quiet running piston especially during engine warmup. There are some aftermarket forged 2618 pistons that require 0.005-inch of piston-to-wall clearance can be somewhat noisy during warmup. This piston slap can be disconcerting as it can and often does sound like a connecting rod hammering on the crankshaft. The Pro Tru Street line is designed with a tighter 0.003-inch piston-to-wall clearance to minimize the noise during warmup.

The Ring’s The Thing

The Pro Tru Street line also takes advantage of the continual march toward more efficient piston design with regard to ring seal. Often, bargain basement pistons are fitted with old-school 5/64-inch ring packages that were standard production engine fare in the ‘60s. Today, thin ring packages have become standard so the Pro Tru Street line offers the majority of its pistons in the popular 1/16, 1/16, 3/16-inch ring package. Each Pro Tru Street line piston kit comes with a complete ring package along with pins and locks.

The magic to this process is the forging dies. Made in house at Wiseco, the die features built-in valve pockets.

The idea behind a thinner ring package is all about friction. A majority of any engine’s internal friction is generated by the piston and rings. By reducing the thickness of the ring face that contacts the cylinder wall, this demands less force to ensure the ring seals against the cylinder wall. A 1/16-inch ring (0.062-inch) is 20 percent thinner than a 5/64-inch ring.

With a thinner ring face, this allows the ring designer to create less outward pressure while still retaining the critical unit load pressure on the face of the ring. This is called unit loading pressure. As an analogy, think of a normal shoe’s contact patch on soft ground compared to the concentrated point of a stiletto high-heel. The concentration of weight on that small contact point will sink the heel into the ground where a normal shoe will not. In the case of rings, the thinner ring needs much less outward (radial) load to create sealing force on the ring. Therefore, friction is reduced compared to a thicker ring.

The net result is a gain in overall power throughout the entire RPM range compared to a 5/64-inch ring package. The 1/16-inch top ring is a ductile iron, barrel-face ring with a plasma moly sprayed face combined with a cast iron 1/16-inch second ring. The 3/16-inch oil ring package comes with standard tension to ensure proper oil control. All three rings are pre-fitted for the bore size and do not require file fitting.

Where the magic happens. In the Wiseco forging press.

Skirt Coating Comes Standard

These features alone would probably be enough to warrant their selection, but Wiseco also coats each piston with its ArmorGlide, a proprietary “OEM type” skirt coating at no extra cost. This coating uses state-of-the-art bonding technology to reduce friction while improving lubricity. The coating also helps to stabilize the piston in the bore, especially as it moves through bottom-dead-center (BDC) where that rocking motion can cause increased wear.

The performance piston aftermarket is challenging because of the multiple variables. Just within a given engine family the variables that are important to the builder include bore, stroke, and piston crown – whether it be a flat-top, dished, or domed piston. You might expect a bare minimum of options, but just for the 23-degree 350ci small-block Chevy alone Wiseco offers 17 combinations of bore, stroke, and rod lengths.
So let’s take one of these as an example of how a street engine builder could produce a strong short-block without spending a ton of cash.

All Pro Tru Street pistons are fitted with Wiseco’s ArmorGlide skirt coating that offers excellent lubricity and helps stabilize the piston as it rocks across bottom dead center (BDC).

Perhaps the most popular small-block Chevys of all time is the 383, which is a 350 block using a 3.75-inch stroke crank originally designed for the 400 ci small-block Chevy. The Pro Tru Street line offers a flat top piston in 4.030, 4.040, and even 4.060-inch bore diameters using a 6.00-inch connecting rod. In addition, you also have the option of using a reverse dome piston with either a 5.700 or 6.000-inch connecting rods to reduce compression for use with a supercharger. Compression ratios vary depending with combustion chamber size, but just within this one displacement Wiseco offers 13 different options. The choices are even greater when it comes to the big-block Chevy family.

Wiseco offers an incredible breadth of possibilities not just with the popular Chevy engines, but also as far as other popular engine families. As an example, if you want to build a 4.155-inch stroker, 500ci 440 Chrysler, Wiseco has a flat top piston that can make a pump-gas friendly 10.5:1 compression version.

Chances are that if there’s an engine you want to build, Wiseco just might have an affordable piston just sitting on the shelf, waiting for you to give it a new home. Best part of all? The Pro Tru Street line can be purchased for $410 and also includes rings, locks, and pins! Now you no longer have to settle for cheap, cast pistons in your next budget build.

There’s plenty to talk about with the Pro Tru Street piston, starting with its near-net forging technique that minimizes machining operations to keep the cost at a little more than half the cost of a custom piston. And kits include rings, wrist pins, and locks.

Topics: featured, VIDEOS, PISTONS 101, ENGINE TECH

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Jeff Smith

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