Here's a simple "wire crank" engine that uses just one small piece of aluminum plate. Crank and rods are built from .093 coat hanger wire. The base, block spacer and main bearing are built from wood. Valve is parallel to the piston and operates 90 degrees out-of-phase to it. "Flywheel" (not shown in sketch or photo) is simply two steel strips bolted together (with crank pinched between them). Valve contains a through hole and ground "flat". Both must be positioned carefully. "Kink" in valve pushrod allows slight bending, to adjust overall length (for tuning purposes).
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