My students named this one (as they often do). It really looks like it's built from coat-hanger wire (actually, it's built from 1/8" steel gas welding rod. Brass piston and steel block. The main bearings and crank have sort of a "saw-horse" appearance. After crank is placed through bearings, it's bent 90 degrees to about 4 inches and doubled back 8 inches (to form two "blades" for a flywheel of sorts). Rubber tubing slipped over the "blades".....when running, it has sort of a "helicopter" appearance. Pivoting arm acts as valve and is controlled by pushrod (which operates 90 degrees out of phase with con rod). Air supply hose slips over hollow bolt (8-32 screw drilled .050) and alternately lines up with or exposes intake/exhaust port. Hose flops back and forth in a goofy manner as engine runs. Surprisingly, it runs very smooth.