Most rods purpose-built for racing utilize a fully machined con-struction with cap-screw s
Engine failures that many racers blame on the connecting rods are often actually installation mistakes. When their characteristics are correctly understood, connecting rods can be one of the strongest parts of a modern racing engine.
Let's do a quick mental exercise: How many times-either at the track or while watching television-have you heard a driver or crewchief explain away a blown engine with the phrase, "we broke a rod"?
Plenty of times, probably. But what does that mean? Connecting rods built for racing are manufactured with as much or more precision (and expense) as any other piece that goes in the engine. Rods are often the most stressed pieces in an engine, yet many Winston Cup engine builders will tell you they have rarely seen a rod failure that is truly the fault of the rod.
The fault can come from any number of places, but the failure often occurs at the rod because of the stresses involved. Often, the fault from an early rod failure comes from sheer human error. Simple things such as improper rod bolt installation, rough handling, or over-cycling the rods can lead to expensive rebuilds. Fortunately, these mistakes are easy to prevent if you have a solid understanding of what types of forces you expect your connecting rods to live through (horsepower, reciprocating weight, expected life cycle, etc.) and how to install them properly.
Although it can be done with different styles of calipers, a dedicated bolt stretch gauge
Since almost all forms of stock car racing limit rods to stainless steel in the interest of cost containment, this article will concentrate only on processes involving that material. For stainless rods, there are several options for construction, namely forged, powder forged, billet, and fully machined.
Powder-forged rods are used almost exclusively in OEM applications. Powder forging allows for a cheap, fairly rugged rod, but it isn't the quality of a standard forging. You will be hard-pressed to find a purpose-built racing rod that has been powder forged. However, if you are racing a strictly stock class that doesn't allow aftermarket rods, this might be what you are stuck with. As long as they are balanced, you aren't likely to exceed a powder-forged rod's horsepower limitations in a stock engine.
Next up the durability scale are forged rods. "The advantage of forging is that steel, just like wood, has a grain direction," explains Jack Sparks of Carrillo Industries. "When put in the proper position, it enhances the fatigue properties of the part. Forging a rod allows you to manipulate the grain direction in the steel. The optimum is to have longitudinal grain flow (up and down) in the blade area and cylindrical grain flow around the big end."
The problem with a forged rod also comes from the act of forging. The forging process leaves an inconsistent, rough surface on the rod. This roughness creates a multitude of stress points on the rod. "If you disregard the fasteners, most failures in a connecting rod are generated from some sort of inclusion or stress riser on the surface," Sparks explains. "It's like putting a notch in a coat hanger. When you bend it, it is going to break at the notch."
Ken Troutman of KT Engine Development demonstrated the process of checking rod bolt stretc
A billet rod is created by literally machining the rod out of a solid chunk of metal. The machining process creates a smooth surface that is (unless there is a problem in the manufacturing process or simply a bad design) free of surface inclusions. By avoiding the forging process altogether you have avoided a weakness, but you haven't been able to take advantage of strength by influencing the grain flow.
Enter what is commonly known as the "fully machined" rod. This process produces the strongest rod (gauging material strength only) by taking the best traits of the forging and billet processes and combining them. A fully machined rod starts out as a large forging (over six pounds in the case of most of Carrillo's rods) and then is machined into its final form much like a billet rod. This produces a rod that has the optimum grain structure of a forging along with the smooth exterior that you find in billet pieces. It's the best of both worlds.
Fasteners, no matter what type, are the greatest weakness in a connecting rod. However, if installed correctly, most fasteners from quality manufacturers are capable of handling the stresses they are designed for. Before we talk about correct installation, we first need to have an understanding of what's out there.