Here an oscillating engine with a neat CNC flywheel and frame. This engine (which has many hours of operation in our shop display) has a loose main bearing...as a result, the flywheel lightly taps the frame as it runs, making a "jingle bell" ringing sound (hence the name). Hole on lower left is exhaust port. A hole on opposite side (not seen in this photo) is intake port. Crank is pressed together in vise using 1/8 dowel pins (flywheel is also press fit). We built the piston from brass. Hinge bolt is 8-32 ......goes thru cylinder and bottoms out in blind hole (in frame). Spring around hinge bolt keeps cylinder against frame. Lower machine screw in frame is merely used for holding frame to a short length of aluminum (so it will stand up). Note:In plans below, small hole in cylinder is .09 Dia. and breaks thru into cylinder. Top hole in frame(for crank main bearing) is .126 Dia. Engine runs about 500 RPM on 7 psi.