I have had a devil of a time getting the correct replacement tension pulley for my Trek 820. The bike now has a Shimano Acera RD and the teeth on the tension pully are worn to nubs. I bought some replacement pulleys from the bike shop and they are smaller than what's on there - they have less teeth. Next I tried ordering a replacement pulley online, and I ordered a Shimano replacement pulley with the same number of teeth. Only problem is the hub (I think that's what it's called. It's the part of the pulley that the shaft goes through.) of the pulley I ordered is much wider than the original, which makes the replacement pulley too wide for the cage.
Can I use one of the smaller, less-teeth pulleys to replace the worn tension pulley? Seems like it wouldn't make much difference.