To reverse engine's direction, use ice water.
Rotate the flywheel by hand. 
Avoid rough handling.  
Made of durable stainless steel, this stylish and affordable low-temperature differential unit demonstrates how heat engines convert heat energy into mechanical work. 
A classic demonstration of thermodynamics for middle school. Overall height, about 5-3/4". 
Includes instructions; hot and cold source and cup are needed but not supplied.
Heat source should not exceed 100° C and cold source should not be below −20° C.
This is a delicate instrument and the connections between the flywheel and pistons can be damaged. 

To ensure that the engine works well, perform the following 2 steps before each experiment:

• The flywheel, connecting rods, and pistons should move freely and without resistance.
• Check that the connecting rods are properly seated. Linkages to the piston should move freely.

Flywheel diameter: 3-1/2" (9 cm)
Unit height: 5-3/4" (14.5 cm)
Unit weight: 7.6 oz (216 g)
Displacement cylinder diameter: 3-1/2" (9 cm)