|Watts Shop Performance
| 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.