I just stumbled upon this video and was amazed that this insanely small V-12 engine was handmade using traditional methods. From YouTube:

v-12 engine with compressed air injection, hand made using taditional methods: 12 cm3 of displacement, the diameter of the cyllinder is 11,3 mm, the stroke of the pistons 10mm, it works with only 0,1kg/cm2. Is constructed with stainless steel, aluminum and bronze. Patelo dedicate this engine to his grandchildren: Sara, Carmen, Jose and Pablo. This engine is for education, exhibitons, etc., that is the reason to work with compressed air, to avoid contamination. He has made the plans and all parts, except the screws. The engine is not for sale. Thanks to everybody for the comments and sorry for my bad English.

[Via Reddit]