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I'm trying to set up a bicycle powered machine (grain mill), and I want to drive a 5/8" diameter shaft with a standard bicycle chain. I'm trying to find a sprocket that is 5/8" bore that fits a bicycle chain. I've tried 3 separate ones on Amazon and all of the sprockets are too wide for a standard bicycle chain. Does anyone know where to get a sprocket like this?

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    You may be better off making an adaptor to fit between a bike sprocket and your shaft. If the desired tooth count is high enough, a chain ring might be a good starting point, as they have sensible and specified mounting holes (count and bcd) while most sprockets are part of cassettes and the connection between them is proprietary. Single sprockets for fixed gears, single speeds, or unusual bike uses (sync chain on a tandem, recumbent, trike) are other sources of something adaptable. How do you plan to attach it to the shaft if you get a perfect fit? – Chris H Nov 29 '16 at 19:49
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    One alternative is to grind down the teeth on the sprocket. That's relatively easy (get a grinder, spin the sprocket on its shaft while grinding). Or you could run full-width chain on the bike chainrings. I've done both, both work. The full-width chain won't fit through a rear derailleur, though, so chain tensioning takes a little more effort. – Móż Nov 30 '16 at 2:27
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If your grain mill is a prototype, I would make a sandwich of steel plates (ie really big washers) and clamp it all together.

        | <-- sprocket
      | | |  <-- two steel plates
   ==||=|=|=|======= <-- shaft
      | | |
        |

Getting the alignment perfect might be hard and require tweaking, but this temp solution allows you to undo it and make adjustments.

Consider welding it in place. - this would have to be perfect, and it does make replacement of the cog difficult. But it won't move or slip on you!

enter image description here

Final thought - if you don't want a rigid shaft, consider cannibalising a freewheel or freehub. I'd consider slapping a whole freewheel cassette on, and using an appropriate gear. This way the machinery can move forward on the freewheel but not backward. Clamp it up between two cone nuts on the threadded shaft.

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    I ended up using the rear sprocket of a small bike and welded it to the one that didn't fit. So far so good! – Jonathan Wolfe Dec 13 '16 at 7:31
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    It should be reasonably easy to machine an adaptor that takes 3-notch Sturmey Archer IGH sprockets. – Emyr Feb 28 '17 at 14:28
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You can use more smaller sprockets, it's have pitch 1/2" or 1/4". I think you can buy it from bike shop. If you can't find it, you can ask me. My website is www.besbe.com, we're make sprocket manufacturer,hope help you.

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    Welcome to Bicycles @jack. Thanks for making your commercial connection clear. For more guidance see How to Answer, and also we recommend that new members take the tour to make best use of the site. – andy256 Feb 28 '17 at 20:08

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