Re: Sprocket adaptor
Contrary to popular belief, the center hole in the sprocket is not critical. It can be oval, egg shaped or square. The important things are if it clears the hub and the teeth of the sprocket are concentric with the hub. You don't need a machinist or a lathe or a mill to open up the sprocket hole. A rotary file in a drill, a Dremel tool, even a rat tail file will work. Just as long as the teeth of the sprocket run true when the wheel is spinning is all that is important. The center hole is just a hole. Not an alignment issue.
As for the coaster brake arm clearing, you need to put a "Z' bend in it to clear the sprocket bolts. It usually doesn't take much bending to get the required clearance.
Warped sprockets aren't that uncommon. They can be straightened with a flat surface and a mallet. Just don't damage the teeth. I've used a large Crescent wrench to bend a warped sprocket back into shape as well as a hammer. Just keep checking it against a known flat surface as you work. However warped sprockets are far less common that inconsistancies in rubber thickness. If they aren't the same thickness across their surface they can make it appear the sprocket is warped. Check the rubber parts.
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