Fidget model of one cylinder with piston and crank shaft. This is a print in place model and requires removal of a lot of support material.

I made this as a proof of concept to see if I could create something like this this no assembly required. It works pretty well but you do need to be careful when removing support material, especially around the gears (a pair of needle nose pliers is helpful). Recommend pushing the gears along the axis of their shaft to free them up instead of rotating.