I made myself a toy over Christmas, finished it this weekend.
It is of course a steam engine. It is designed to run on extremely low pressure. All the rotating bearings are ball bearings. The piston is silicon bronze running in an aluminum cylinder. The valve system is balanced so that the valve does not work against air pressure. It runs either direction by moving the air hose from one nipple to the other. It is a very long stroke engine to maximize the use of low pressure. Stroke 1", bore 1/2". The connecting rod is very long to minimize side force on the piston thereby reducing friction.
In the photo above it is running around 200 rpm on 0.6 psi. I measured the running air pressure from the aquarium pump with a highly accurate medical mercury manometer, accurate to 1 mm pressure over a range of zero to 300mm.
This is the first engine I have built, I think it won't be the last.