PERFORMANCE CONNECTING RODS
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PERFORMANCE CONNECTING RODS |
High Performance Racing Parts and Engine Components! Including Custom Engine Kits and Crate Motors! |
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Performance Connecting Rods
Cast-Steel
We won’t waste much time discussing cast-steel rods because they’re poorly suited to any type of serious performance use. Though the casting process is very inexpensive and results in “near net” shapes that require minimal machining, the lack of a cohesive grain pattern and compromised molecular binding yields brittle parts. Trust us, brittle connecting rods are the last thing you want in a performance engine.
In the ’60s and ’70s, American Motors, Cadillac, Buick, and Pontiac all used cast rods in a wide variety of engine designs. In an effort to improve molecular binding and strength, the molten metal was injected into the mold cavity under high pressure. The resulting castings may have been good enough for use in everything from GTOs to Jeeps, but they have no place in anything other than the most fanatical numbers-matching restoration effort. Worst of all, these cast parts had to be made heavier than comparable forged rods to maintain strength. When you consider that a cast “Arma-Steel” Pontiac 455 rod weighs 31.7 ounces and a stock Chevy 454 forged rod weighs 27.4 ounces, you’ll agree they’re the automotive equivalent of recycled cardboard.
Stock Forged Steel
Original-equipment forged steel rods are the next step up the strength and reliability ladder. Detroit-sourced OE-forged rods begin life as bars of carbon steel that are passed through a rolling die. The rolling process compacts the molecular structure and establishes a uniform, longitudinal grain flow. The bars are then heated to a plasticized state, inserted into a female die, and pressed into the near-final shape while a punch locates and knocks out the big end bore. In doing this, the grain flow at the big end is redirected in a circular pattern, like wood fibers surrounding a knot, and excellent compressive/tensile strength results. Finally the rod is put through a trimmer (that leaves the characteristic thick parting line on the beam), the big end is severed and machined to create the cap, bolt surfaces are spot-faced, then final machining and sizing take place.
But there are some drawbacks. When the forging hammer hits the hot bar, heat transfers from the bar to the hammer causing a phenomenon called de-carb (decarburization). Here, trace amounts of the carbon in the steel migrate to the surface resulting in a rough finish full of what metalurgists call “inclusions.” An inclusion is described as anything that interrupts the surface of the metal, or a lack of cleanliness (impurities) in the material. The effect of a surface inclusion can be likened to a nick in a coat hanger. Bend it enough times and the wire will fail, usually right at the nick. The rough surface caused by de-carb affects the surface to a depth of 0.005 to 0.030 inch and is packed with inclusions that are a breeding ground for cracks. The old hot rodder’s trick of grinding and polishing the beams is a valid solution to this problem, though far too labor-intensive to ever be considered by Detroit.
When it comes to inclusions caused by impurities, Detroit’s need to control costs can result in purchases of bulk steel that may (or may not) contain contaminants such as silicon that are not detected during manufacture. Such impurities can interrupt the grain boundaries between the parent molecules and lead to a fracture minutes or years after the rod is first installed in an engine. It’s a matter of luck and what kind of abuse the flawed rod is subjected to.
With very few exceptions, the weakest link in a stock forged rod is the fastener
system. The rod bolt is usually the most marginal component. Simply upgrading
from stock bolts to quality aftermarket replacements can improve durability
by 50 percent. Just be sure to have the big end re-sized to restore concentricity
any time the bolts are removed. Stock forged steel rods are an economical choice
that should be able to handle one horsepower per cubic inch with quality fasteners,
and as much as twice the factory-rated output if the beams are polished.
Call 1-877-354-3812 to order your Connecting Rods Today!!