How to build a flywheel crankshaft piston pattern with technic?
Here's one simple way to accomplish this using just basic parts:



This isn't quite a pure Technic example, but that construction is almost exactly what was used on the steam cylinders on Emerald Night:



Depending on exactly what you are trying to do, there are also some specialized parts that make building decorative pistons simple. For example, here's the V8 from the 8880 Super Car:



There's also the much earlier piston design on 853: