Use a cut soda can to cut a circular piece from the 1" thick piece of floral foam to form a piston.
Use your hand to reduce the diameter of the foam piston so that is falls freely through the pressure vessel.
Push the wire from a marking flag through the center of the foam piston. Connect this piston rod to the piston with JB Kwik.
Top Can Assembly
Cut the top out of another soda can.
Use a marker to make a circle that is 3/8" from the top ridge of this can. Also make a circle 1/8" from the bottom ridge and a third circle 1/2" from the bottom ridge. Use a small nail to make 2 holes on the top circle that are 180 degrees apart. Also use the small nail to make a small hole in the bottom of this can for the piston rod.
Crankshaft and Flywheel
Make a crankshaft by bending the wire from a 2nd marking flag. The crankshaft should have one crankshaft throw that is 3/8" from the crankshaft axis and another crankshaft throw that is 3/16" from the crankshaft axis. These crankshaft throws should be 90 degrees out of phase with each other.
Connect 2 CD's to the crankshaft using superglue to create a flywheel. The crankshaft and the flywheel can be connected either by putting a piece of plastic between the CD's or by using a large wire nut.
Clamp 3 paper binder clips around the circumference of the CD's. These can be used to balance the engine.
Assembly
Thread the piston rod through the small nail hole in the bottom of the Top Can Assembly.
Put the piston inside of the Pressure Vessel and press the Top Can Assembly onto the top of the pressure vessel. Make sure that one of the crankshaft holes is vertically above the hole in the pressure vessel. The aluminum will allow the pressure vessel to expand at the top so that the top can assembly will slide in 1/8" up to the marked circle at the bottom of the top can assembly.
Cut a rectangular opening in side of the Top Can Assembly. The opening will be a curved rectangle extending around the can starting at vertical lines that are 1/2" from the crankshaft holes. This curved rectangle also extends vertically from 3/8" below the top ridge of the can to 1/2" above the bottom ridge.
Cut off the excess wire on the piston rod so that it extends about 1.5" into the Top Can Assembly when the piston is sitting at the bottom of the pressure vessel.
Use superglue to attach the PVC elbow to the pressure vessel so that it covers the hole in the pressure vessel. Then use JB Kwik to make this connection airtight.
Put a 1" piece of a soda straw over the piston rod and use JB Kwik to connect this straw to the inside-bottom of the Top Can Assembly. Be careful not to get any JB Kwik on the piston rod.
Use a paperclip to make a foot that will connect the top of the balloon diaphragm to the crankshaft. A square piece of balloon and superglue can be used to connect the foot to the. The top of the paper clip can be wrapped around the crankshaft a few times and then loosened to allow the crankshaft to rotate inside of this small coil.
Thread the crankshaft through the handles of a paper binder clip so that the binder clip hangs from the 3/8" crankshaft throw. Then clamp the clip onto the piston rod.
Troubleshooting
Add ice inside the Top Can Assembly to create a greater temperature difference.
Use the three paper binder clips on the flywheel to balance the engine before heat is applied.
Reduce friction by putting a drop of oil on the piston rod and on the crankshaft contact points.
Make sure that the pressure vessel is air tight.
The balloon should not be pulled too tight. The flywheel should turn freely without heat being applied. You can feel confident that your engine will run if the flywheel is more difficult to rotate in one direction than in the other.