Steam Engine Crank

The point of the above is that using a shaft, bearings, and other off-shelf parts - one can put together the crank assembly. The Crank arms go over the shaft and are attached with a set screw (circle in diagram). One can extend this assembly to as many cylinders as needed.

Dimensions here are a 2" shaft - and about 4 inch stroke.

another picture of a crank for a 3 cylinder engine:

Here is another approach: