Tiny 4 cylinder engine, fully functional, rotating the gear moves the pistons up and down. 

For best results print the camshaft with support and the main engine body with a brim. Assemble the engine by first adding the pistons then camshaft with the gear attached, and finally add the bottom (this may require some glue). Due to tight tolerances the engine may require some breaking in before it performs optimally. A lubricant can be used to make the camshaft spin smoothly, but do NOT add anything to the pistons as the will seize.

For a splash of color print the pistons, camshaft, and/or gear in a contrast color, or paint them after printing. I added disks of red electrical tape to my piston-heads to this end.