NAMBA March 2021 Web

PROPWASH 17 The Nitro Engine – Part 6 By Lohring Miller NAMBA Safety Chairman Mechanical design and construction methods are the final, critical part in building nitro engines. I’m dedicating this article to the late Jim Allen. He was one of the last students of high power, nitro engines. These include Dick McCoy, the Dooling brothers, Duke Fox, Henry Nelson, Bill Wisniewski, Dubb Jett, and many others. I will be comparing Jim Allen’s highly refined but older design with the most modern engine I know of, the MB 40. It was designed by Rob Metmemeijer and Frits Overmars. Rob built the prototypes but the Ukrainian firm Profi handles production. There are two issues with high power engines. The parts must be strong enough to stand the loads from high pressures at high temperatures and high rpm. The parts must also be stiff enough to maintain alignment of the moving parts. The crank- case is the center of this. Its main job is to maintain alignment of the piston and crankshaft at minimum weight. It has been evolving toward a one-piece unit for stiffness and thermal control. Earlier designs had a central section with front and back covers. This allowed ease of manufacture with an adaptable design. The front plate holds the crankshaft in position with ball bearings. Problems arise with aluminum for this. Aluminum is considerably less stiff than steel and has a higher thermal expansion. Bearings need a serious shrink fit, .003” (.080 mm) in aluminum, but can get by with a .0005” (.013 mm) to .008” (.020 mm) in steel. In both cases the bearings need to have a large, class C3 clearance. Jim used O-1 tool steel hardened to Rockwell 63C for his experimental front plates. However, aluminum is used on production engines. The inner races of the front and rear bearings can be clamped together by controlling the length between the front and rear shoulders on the crankshaft. This avoids spinning the crankshaft in the bearing race. Crankshafts need to be strong and stiff. We can get away with overhung designs in our small engines. Jim liked a separate crank pin to allow different materials for the crankshaft and crank pin. His crank pin was made from M-2 tool steel reamer blanks, hardened to Rockwell 63 to 65C for the inner race of the rod needle bearing. The rest of the crankshaft was made from S-7 steel hardened to Rockwell 53C. The pin was a .0012” to .0014” (.030 to .035 mm) press fit in the crank- shaft. He had problems with pin or crankshaft failures with other combinations. Most manufacturers make one-piece crankshafts. They sometimes have pin failures and/or fractures through the intake window with improper heat treatment. They also run connecting rod bushings that don’t need as hard a crank pin. Allen 90 Jim’s crankcase was made on manual machines with the help of accurate fixtures for alignment of the bore and crankshaft center lines. Crankcase Fixture March 2021 Various Allen Crankshafts Allen Steel Front Plate with Bearing Inner Race Seal and Spacer