NAMBA March 2021 Web

PROPWASH 18 March 2021 Later crankcase designs are one piece. Even though steel parts can overcome some of the stiffness issues, a one-piece aluminum crankcase is stiff enough, and modern versions of older two-piece crankcase designs are more powerful. Note also how widely spaced are the mount holes on the MB 40 crankcase compared to other designs. This helps stabilize the engine under the vibration from single piston designs. Allen 90 Steel Front Plate More modern designs incorporate the rear bearing race into the crankshaft. This allows a larger intake valve area in front intake engines. MB 40 Crankshaft The MB 40 crankshaft uses a shimmed front bearing with a retainer threaded into the crankcase to control end play. MB 40 Front Bearing Old and New K&B 7.5 crankcases MB 40 Crankcase Design MB 40 Crankcase The connecting rod and its big end bearing are the most highly stressed parts in a high rpm engine. However, most rod failures are really big end bearing failures. A bushed rod end must have at least .003” (.080 mm) clearance for an adequate oil film. Even so, a lean mixture or low oil percent in the fuel will allow the bushing to seize and spin; destroying the rod. On the following page is the MB 40 rod; a high-quality aluminum rod with bushings.