[Biff]

I have a pair of castle nuts on my bench that I got to fill out a min order with an online supplier of boat hardware. They do fit together as you would hope, but these are only 3/8" and don't fit in all rotational alignments possible. A little work with a small file and I'm sure they would. I have looked at them and wondered the same thing. There are most likley better choices, ie real shift dogs or ball bearings and sliding hollow shafts. The ones I have I wouldn't try and put real torque on, say near the final output, they are also stainless and were like 3 dollars each (filled min order) May be next year in a much larger version, Say 5/8, have a key way cut and the threads cleaned up, real steel, attached or welded to a hubless sprocket, may have the seeds for a chain based two speed. I will have to sleep on it.