NAMBA March 2021 Web

PROPWASH March 2021 23 The head is another element. We covered the combustion chamber shape previously. Most engines use head buttons with a finned or water-cooled cover. Jim tried all of these with a large diameter button over the liner lip in the final version. The cover held the cooling water. MB 40 Piston and Rod ECM Machining MB 40 Piston Insides Allen 90 Head Buttons and Cooling Jackets Rob used a one-piece head with a snug fit into the upper cylinder. It was air cooled as is his cylinder. The head uses a “turbo” style plug with its tapered seat. MB 40 Head Design Jim was a believer in the bell valve as the best intake system for small engines. His precision machining allowed a very small clearance between the bell and the backplate. The valve runs in a ball bearing with a bushing for a seal. This is a difficult to make but low friction intake system. Jim also tested a more conventional rear drum valve. (Continued on page 24) MB 40 Piston Design