The rods have "H" profile beams. The "H" profile puts the rod's stiffness axis in line with the bending stresses it sees under use, making the rod strong for its weight at 100 grams per rod lighter than stock. If you look in the machined valley of the "H," you'll notice a rib at the base of the valley; this rib increases the fatigue strength of the rod's beam. The oiling holes for the piston pin are drilled upward at an angle toward the pin. Putting two smaller holes at the bottom makes a much stronger part.
The entire surface of the rod is machine finished. This ensures consistency in dimensions and weight. It also creates a smooth finish with no irregularities where a crack-inducing stress riser can form. After machining, the rod is subjected to shot-peening. This is where the rod is bombarded by hard steel shot at high velocities. Shot-peening helps refine the grain over the surface of the rod, creating a finely grained compressed layer of steel where cracks have a hard time propagating. Shotpeening causes the fatigue strength of most steel parts to rise by over 100 percent.
The piston pin bushing is a tough silicon bronze, which is one of the most wear resistant bushing materials available. Since the rod bolts are the most critical part on the connecting rod, no expense was spared here. Eagle uses genuine ARP 8740 7.16-inch rod bolts on all of their sport compact series rods. These rod bolts have a tensile strength of over 230,000 psi, many times stronger than the factory bolts.
In our next installment of the low-buck SR20 we'll get more into what we're doing with the engine's bottom end, and discuss what can be done and what is optional for a really low-dollar build.