| Watts Shop Performance |
| Aluminum Rods |
| Aluminum rods are manufactured by the forging process or they can be cut from a sheet of aluminum plate, billet style. Aluminum rods are generally 25% lighter than steel rods and for this reason they're very popular with racers looking to shed mass from the reciprocating assembly. Lighter reciprocating parts demand less energy to set into motion, allowing more of the force of combustion to be applied to the wheels. Lower reciprocating mass also allows the engine to gain crank speed faster for quicker rpm rise after each upshift, to keep the engine near the peak of the power curve. That's the good news. The downside is that aluminum has a much shorter fatigue life than steel, perhaps one- tenth as long in a racing environment. This means you'll have to measure for stretch and replace suspect rods at regular intervals to stay ahead of possible catastrophic failure. How long will they go? That depends on how hard they're loaded and if they're abused. We've all heard stories about hot rodders getting 100,000 street miles out of a set of aluminum rods. Could be. But the fact remains that aluminum has a tendency to work harden with use. Using the analogy of a coat hanger, if you keep twisting it, it'll break. That's work harding and an aluminum coat hanger can't handle the same strain for nearly as long as a hypothetical steel coat hanger. Another hassle is the fact that aluminum rods must be made physically larger because the ultimate tensile strength is about half that of a good steel rod. The added bulk often causes clearance problems inside the crankcase, especially when they're swinging from a stroker crank. Some aluminum rod users abuse them without even knowing it. A cold motor must be warmed thoroughly because the expansion rate of aluminum is twice that of steel. The difference in expansion between steel crankpin and aluminum big end can restrict the oil film clearance until the temperature of the parts stabilizes. Wing the throttle on an ice cold motor and you might be looking at spun rod bearings or worse. Aluminum rods can handle plenty of horsepower. You'll want to check with the manufacturer for specifics, but it is safe to say that 2 horsepower per cubic inch is just the beginning. We'll err on the side of caution and say that aluminum rods are best suited to race only engines where regular inspection can ward off potential trouble. |