Is a 5 speed chain the same size as a 7 speed chain? I converted a 15 speed into a 21 speed and the chain does not work right on the small cog on the cassette. Thanks, Rich

Yeah it should be. Is your derailer properly adjusted?

I think the chain should not the problem - even old 5-speed drivetraisn used 3/32" chains, which are the same as used on 7/8 sp. Do you have friction or indexed shifters?

If I read your post right..."converted a 15 speed into a 21 speed", why not just buy a new chain? The old 5-speed chain must be stretched by now and a new chain doesn't cost that much. Would check the wear on the chainrings too. Actually, there isn't enough info on your upgrade and parts (new or used).

Well it turned out to be the chain. I bought a new one and it worked fine. Thanks for you comments.

Is the new chain narrower?

I suspect the problem was a worn chain on a new cassette.

Yes the new chain was narrower according to my LBS. The cassette was not new but in very good condition, the main issue was because of the 5 speed chain. Live and learn.

Older (5&6 spd chains) were a lot burlier than current ones; they don't like the more severe angles of a modern drivetrain.

Older 5 and standard spaced 6-speed chains are wider than almost any chain currently available. 7 and 8-speed (and Ultra 6-speed) chains are narrower. 9-speed chains are still narrower and 10-speed chains even narrower yet. It's got to end sometime.

However, the "pitch" (length between pins) is exactly the same at 1/2" for all of them.

^^^It would help if you could define what you mean by width.

It's the outside width that's a bit wider. A modern 8 speed chain will work fine on an old 5 speed freewheel, for example, but an old chain that came on a bike with a five speed freewheel would be a bit too wide for a seven or eight speed cassette or freewheel-