Selecting "Slugs"

Slug PistonMay - June 1998 Lane Racing And Rodding Article By Jim Kaekel, Jr.

Many pistons used to be made of cast iron, but were discontinued about 45 years ago as far as passenger cars are concerned, when Pontiac shelved it's straight six and eight engines in favor of a modern V-8 with cast aluminum slugs. Cast iron pistons are still used today, but only in low RPM industrial engines. Aluminum pistons, in one form or another, are the mainstay of today's passenger car and truck engines, but even they have come a long way since replacing their cast iron predecessors. Today, three different types of aluminum pistons are manufactured: cast, hypereutectic and forged. Each has its own benefits and drawbacks, and each is designed for a specific purpose.

CAST PISTONS have been the backbone of OEM engines for several decades due to their low manufacturing cost and relative durability for their intended usage. Cast pistons are designed to last tens of thousands of miles under normal, every day use. Most cast pistons are manufactured with steel struts inserted in the piston pin bosses to hold the piston skirt in a permanent position and prevent thermal expansion. This, in turn, allows piston-to-bore clearance to be reduced to as little as .0015". Cast pistons also have large slots located directly behind the oil rings, which weaken the piston somewhat, but do prevent heat transfer to the piston skirt. The cast piston contributes to a trouble-free, long lasting, reliable and quiet engine, but are not recommended for use in any high performance application because of its lack of overall strength and limited resistance to extreme heat and detonation.

Introduced fairly recently, HYPEREUTECTIC PISTONS are gaining popularity for street performance use, and even for some limited circle track claimer applications. Hypereutectic pistons are cast pistons manufactured with over 2-1/2 times more silicon than a standard cast aluminum pistons. The stepped up silicon content provides hypereutectic pistons with increased strength, and more resistance to high cylinder pressures and temperatures than standard cast pistons. Hypereutectic pistons also require very little skirt clearance, resulting in long piston ring life and reduced amounts of blowby. Although they are stronger than standard cast pistons, hypereutectic pistons are not recommended for use in all-out race applications. They do not have the resistance to extreme temperature, detonation or high cylinder pressures, especially in engines using nitrous-oxide injection, turbocharger or supercharger systems.

For durability and dependability in a racing environment, FORGED PISTONS are the racer's best friend. Manufactured using a forging die, from a solid slug of heat treated aluminum alloy, the end product is a piston capable of withstanding extreme temperatures, high cylinder loads and even some detonation. Forged pistons require much more piston-to-bore clearance than cast pistons - as much as .004" to .015" depending upon the piston type application and manufacturer - because of their construction and lack of dimensional stability. Always follow the piston manufacturer's recommendations for preparation and clearance specifications. Remember, because forged pistons are the result of a more involved manufacturing process, they will cost more than cast pistons.

Piston selection can be somewhat confusing. Budget restraints, intended use - race or street?, - and availability all come into play. Condition of the engine being modified is a determining factor also. An engine block with over 100,000 miles of street use may have cylinders with .005" of taper and out-of-round. Before installation of a new set of oversize pistons, the block will have to be bored at least .020" and honed to the piston manufacturer's piston-to-bore specifications. A block with relatively low miles may just need to be honed. During boring, honing, or when checking blocks for bore taper and out-of-round conditions, always be sure that the block has a DECK PLATE affixed and properly torqued to attain ACCURATE readings. Deck plates simulate the cylinder head being torqued to the engine block and ensure that cylinders are as round as possible.

Another chapter in the piston selection process involves the type of ring groove needed for a particular application. Street engines should retain popular OEM compression ring sizes of either 5/64" or 1/16" and oil ring sizes of 3/16" or 1/8". Under no circumstances should you purchase a set of pistons with compression rings less than 1/16" thick for a street engine. Thin compression rings, such as .031" or .043", help reduce rotational drag and increase horsepower in a race only engine, but not without sacrificing ring seal and life. These types of rings are designed for maximum output race engines which are rebuilt frequently and commonly equipped with a crankcase evacuation system to control blowby.

Use caution when selecting pistons with higher compression ratios for a street engine. A street engine that has a set of 11.5:1 pistons installed, running on 93 octane fuel, will live a very short life and wind up detonating itself to death. Some street car owners have experienced success running engines with close to 10:1 compression without having to add octane boosters or racing gasoline. (By the way, what the heck happened to that good ole Sunoco 240 or 260 we used to get at the pump? It probably just got shelved...just like those old cast iron pistons!)

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