NAMBA March 2021 Web

PROPWASH 20 March 2021 Jim’s 90 ran an "I" beam connecting rod, through hardened, and tempered. Both holes were align ground with no bushing in the upper end, allowing the rod to be lengthened to a maximum length of 1.741 center to center. He used S-7 steel hardened and tempered to Rockwell 58C. This gives a 315,00 lbs./in2 tensile strength and a 210,00 lbs./in2 yield strength. The rod has a .250” (6.35 mm) diameter wrist pin with a .360” (9.14 mm) upper outer diameter. The connecting rod is .030” (.760 mm) thick in the middle of the "I" beam with 16 caged rollers; 2 in each window. The maximum clearance in the bottom end is .0003” (.076 mm); top end clearance is .0001” (.025 mm). The unbushed top end runs against a hardened M-2 steel reamer blank wrist pin. The connecting rod runs forever at 32,000 to 35,000 rpm with no wear and 8% oil. The connecting rod is guided in the upper end between the piston bosses with .005” (.13 mm) total clearance. The retainer, with two rollers per window, is made of full hard (Rc-60), C-350 mar aging steel. Its tensile strength is 350,000 psi making it the strongest part in Jim’s motor. Note how thin the steel rod is compared to the MB 40 aluminum rod even though the Allen 90 has more than twice the displacement and runs at a similar piston speed. Jim used a variety of fixtures to machine his rods on a rotary table in a manual milling machine. He also used a fixture to accurately grind the rod bores in a lathe with a tool post grinder. Rod Milling Fixture Rod Grinding Fixture Rod in Place on Grinding Fixture Allen 90 Connecting Rod Front Allen 90 Connecting Rod Side