I have a Schwinn hybrid that is a 21 speed and I want to put a new chain on it. 7 gear cassette and 3 rings up front. Can anyone point me in the right direction to the proper chain size to buy.??

I just thought of this.... Do I need to know how many teeth are on the gears.??

KMC 7/8 speed chain will work well and last a long time. It may or may not have to be shortened. Roger

For chain length, just count the links on the existing chain. You can find chains that are sold with specific numbers of links, or the shop can shorten one for you. I suggest putting a "missing link" in there, if it doesn't already have one, as it makes future service easier.

Do check the existing chain for "stretch." There are guides online for checking it with a ruler. The reason is, if you're trying to solve a problem such as poor shifting, it's good to know if the chain is the root cause or not before spending excess money.

If the current chain and cassette have significant mileage, a new chain may skip badly on the old cassette, particularly on the cogs you use the most. Plan on replacing the cassette too.

This very thing happened to a 1991 bike I am rebuilding. I asked on BikeForums what could be causing the skipping and this is the same answer I got. Changed the cassette and all was good.

You can still find Shimano HG-71 chains here and there. I use them. I replace my chains relatively often, as a worn chain contributes to cog wear.

Thanks all. OP here. The cassette was changed about 200 miles ago. A week ago I cleaned the chain and there is a lot of rust. The chain has well over 1,000 miles on it. I just think it's time. Am I correct in my thoughts.??

1000 miles is not a lot of miles for a chain. I do that much in a couple of months and I don't ride nearly as much as some people do. 5-6k is more like it.

Is it rust or dirt? Do you keep the chain lubed? Do you ride a lot in the rain or dirt?

A rusty chain with 1000 miles, that sounds more like you ride it once or twice a month and leave it sit, or store it outside.

Replace this one if you think it necessary, but take care of the new one with a little lube every now and then. Wipe it down if it gets dirty. Put a removable link on it and put it in some cleaner occasionally and relube thoroughly.

Chains are cheap, so I replace rusty ones. I've tried a couple times to just oil over it, but the rust seems to soak up the oil and make a remarkable mess of things. Also, iron oxide (aka rust) is used as jewelers rouge, a fine polishing grit. Better to start with a new chain.