This is aimed at building a GBC module, not to reproduce an actual engine.
I need a piston type construction with stroke length of 3 units, which means a crankshaft radius of 1 1/2 units. Are there any existing LEGO Technic parts that will support this?
Edit: TechnicTechnician provided the answer - the technic cam (6575) proved to be exactly what I needed. In case my original question wasn't totally clear I've added a picture of the solution below.
The distance between the two crank axles is 1½L, which means that the other end of the connecting rod (point X) will move a total of 3L as the crank rotates.