Bottom crank jack shaft.
I am making a recumbent, so the I have the engine mounted forward of the my rear bottom crank. (I left it intact on the donor bike for the rear half of the build). So I am able to use the crank as a jackshaft. Here is how I am doing it:
1. From a donor single crank remove the chain ring and weld the key way spacer from the left side into the cent of the chain ring. On single cranks the left side thread is smaller than the right side thread. The by welding the key way spacer into the chain ring it gives it the right diameter and spins the shaft via the key way.
2. I chopped off the pedal arm so I had a straight shaft. I had to weld the right side sprocket to the shaft, because the peddle arm is what used to spin the sprocket.
3. I installed the sprocket from step one on the left side, just pulled off the big nut and put it on and put the nut back on.
4. I discovered that the crank covers from a three piece crank that I had lying around was the same thread as a shimano free wheel, so I welded the crank cover with threads facing out to the right side of my shaft so I my pedal chain will now hook directly to the jack shaft on a free wheel.
I will post some pics, but it is a cheap easy why to do a jack shaft. It is also super easy to change the left side gear, so I could make different sized sprockets to change gearing ratios.