I have a bike with a very rusty and worn freewheel. How do I tell what replacement type to get? This is an old made in Taiwan bike with a 6 cog rear (12-speed bike). The rear deraileur is Shimano Tourney. Also the round plastic thingy which goes alongside the large cog is chewed up, is that a standard replacement part. I noticed some bikes have a metal ring instead of plastic.
Does the fact that this is a Shimao derailleur mean the freewheel is Shimano? Could I replace a 6 cog freewheel with a 5 or 7 cog one? Is the width of the hub a factor? Short of going into a bike shop, is there any way to figure out what I need?