Piston selection boils down to what you want your Cleveland to do. When you’re planning an engine, you tend to go overboard and build more engine than you actually need. It is best to watch your money and build an engine as conservatively as possible. If you’re building a Cleveland for a daily driver, weekend cruiser, or tow vehicle, you don’t need a steel crank, H-beam rods, or forged pistons. Even if you intend to spin your Cleveland to 6,500 rpm on occasion, you can get away with a cast crank, stock rods, and cast or hypereutectic pistons. Choosing a piston evolves from material to dimensions, which can get tricky if you don’t know what you’re doing. This is why you want to be knowledgeable about crank throw, block deck height, compression height, and more. It is so easy to get this wrong and wind up with pistons that don’t fi t. This is why you must first select a crank and rod before settling on a piston. Manufacturers make this easy because engine kits typically include pistons. Because Cleveland engines aren’t as common as Windsors and 385-series bigblocks, it’s tougher to find a stroker kit with everything you need.
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Marvin McAfee of MCE Engines calculates compression by measuring volume on everything between piston dome and chamber. Here, he measures the piston’s negative volume, or dish, which takes away from compression. This piston isn’t dished; however, it has valve reliefs.
Compression comes not only from piston profile, but from everything in between BDC and TDC. Valve reliefs, no matter how small, constitute negative dish, taking away from compression. Any dome adds to compression. Then, add head gasket thickness and the area above rings and you lose even more compression.
This is a Speed Pro 4.030-inch forged piston for the 351C-2V open chamber head. All you have here are two valve reliefs in a flattop piston. The 351C-4V and 302C wedge head piston is a different experience entirely.
You have a number of choices when it comes to piston rings, conventional, gapless, file-to-fit, ready to install. When it comes to any piston ring with an end gap, check the end gap—no exceptions. Cleveland piston ring end gaps should be .010 to .020 inch. On the wider side for high-performance applications.
Cast
Cast pistons are the most basic type you can stuff into an engine. The Mahle ECOFORM cast piston is designed for modern engine building because it is a lighter piston thanks to fresh casting technology and it is more durable than your average cast piece. The Mahle cast ECOFORM pistons are 20 to 25 percent lighter than cast pistons the company was making 16 years ago. This feature enables you to get good throttle response from a Cleveland because there’s less reciprocating weight to sling around.
All cast pistons have a certain amount of silicon (sand) in them for strength and hardness. The thing is, cast pistons are also brittle and can’t take the kind of extreme shock loads and heat that forged pistons can. This is where you need to know up front how you’re going to use your Cleveland. What you get from a cast piston is stability, with predictable expansion properties and quiet cold operation.
Ring end gapping can be performed a couple of different ways. To get the ring perfectly square in the bore, you may use Summit Racing’s #SME-906002 piston ring squaring tool. There’s also a complete kit from Summit, the #SUM-CSUM2505 piston ring filer combo that enables you to square the ring, gap it, and file to fit. There’s also Total Seal for billet piston ring squaring tools.
Although every engine builder has their own technique for piston ring installation, the best approach is a ring expander installation tool over rolling rings on. Rolling rings on can cause distortion.
Piston ring end gaps are ground to width with a manual or motorized grinder. Soften any sharp edges that can cause a ring to hang up or damage piston and cylinder wall.
Piston ring gaps should be positioned at 90-degree increments around the piston to get them off to a good start though you can expect them to move. What you hope for is that they move uniformly.
When you stroke the 351C to 408 ci with a Speed-O-Motive kit, it becomes necessary to dish pistons to keep compression conservative. Note the valve relief at the deck to clear huge 2.190-inch intake valves. Piston also has a large valve relief for the same reason.
You get two elements from increased stroke—a greater air/fuel charge and the mechanical advantage of additional stroke. This is why you use a dished piston to keep compression in check.
The 351C-4V piston has a dome engineered to fit the closed 64-cc wedge chamber yielding more than 11.0:1 compression. Boss 302 piston is quite similar for use with the same chamber.
Forged
Forged pistons cost considerably more because they call for many more manufacturing steps to get a finished product. Once molten piston forgings are slammed (forged) into shape under very high pressure, they have to be machined through a series of complex steps. What makes forged pistons more challenging is what the machinist has to think about during the block machining process. Because forged pistons possess greater expansion properties, the machinist has to allow for this in the way cylinders are bored and honed to size. There has to be sufficient piston to cylinder wall clearances.
The first company to develop forged pistons was Federal-Mogul’s Sealed Power division in the 1960s. There were learning curves, as you saw with the Boss 302 engine in 1969–1970 with cracked piston skirts and other failure issues causing a lot of warranty claims and engine replacements. Sealed Power developed forged pistons using the VMS75 aluminum alloy, which has been a factory piston alloy for many years. The aftermarket industry utilizes the 2618 alloy for racing pistons with great success because it can withstand up to 575 degrees F. One shortcoming of 2618 is hardness. It isn’t as hard as another widely used alloy known as 4032, which has higher amounts of silicon, which makes aluminum allows very hard. The 4032 piston makes more sense for street and racing use. As always, chat with your favorite piston manufacturer for best results before making a decision. Make sure you speak with an engineer or sales person qualified to help you make a decision.
Hypereutetic
Hypereutectic pistons are a nice compromise between cast and forged. Though “hypereutectic” sounds high tech, the process has been around since 1902. Hypereutectic indicates a highsilicon cast piston, which is made of a harder material, yet without the expansion issues you see with forged pistons. Hypereutectic pistons are more durable than cast without the high price tag and those expansion issues just mentioned.
Though cast pistons have their place, hypereutectic pistons make sense as a base piston selection for any Cleveland build because they are more durable than cast yet don’t have the issues you see with forged. Hypereutectic is a mixture of alloys melted together at just above the point where they become liquid. This is an oversimplification, but suffice it to say it defines how alloys are melted together temperaturewise. In other words, hypereutectic is a process of the way alloys are blended together. As I understand it from those who design pistons, cast pistons have roughly 10-percent silicon in the aluminum alloy. The hypereutectic piston has higher amounts of silicon, which calls for heat-treating until the silicon blends into the aluminum creating a harder surface. Make no mistake, a hypereutectic piston is not a forged piston nor does it have the same strength.
Written by George Reid and Republished with Permission of CarTech Inc
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