i just bought a 21 gear mtb and the chain is quite rusty so i need a new one. but when i go to ebay im just getting confused because it says things like shimano 5,6,7,9 or 10 speed chain. so i dont know which one i need

This is where bypassing a bike shop fails you.. they will sell you the right chain and dissolve
your ignorance of common terminology, and for a modest cost even put it on for you.


but the basics, math, 21 is what? 7x3, right? do you count 7 cogs
stacked up on the back wheel..?
your first hint...

While chains can be had quite cheaply, it might be possible to resurrect the one you have through liberal application of a steel wire brush and the oil of your choice. Just make sure there are no frozen links.

Count the sprockets at the rear, that's your answer.

But this decides chain width, in one way or another, and will do nothing to sort out the issue with chain length, which you mention in the headline. Chains are generally sold a few links long, and will need to be trimmed to fit your particular bike setup. Get a chain breaker tool, and preferably not the cheapest versions. It'll make the job a lot easier.

You need a 3/32" wide chain. Wmart has a chain by bell called speedy which will work. Pick up a chain breaker at Wmart so that you can break the chain to make it the correct length.

To determine the chain length run the chain around the large front ring and the large rear cog without going through the rear derailuer or the front derailuer if possible. Pull the chain tight and then break the chain so that it is at least two links longer. Sometimes you will have to go 3 links so that the chain will mate back together using the quick connect. This one inch increase in length will allow the chain to shift into the large front and large back cog without damaging anything.

Now route the chain through the derailuers and snap it together and go ride.

I believe that's Bell multi-speed, as in 10-24 speed (or 5-8 cogs). I know nothing of the quality, but I would not use it beyond a 7 cog.

JimP is right about getting the chain length correct.

If you compare length to the old chain, note that it may have stretched, and it's the # of links and not the measured length that is more important in that case.

Those Bell/TaYa "Speedy" (WalMart, Drugstore) chains are the only correctly-installed modern chain that I have seen break under moderate use. Low quality here.

Rusty chains also are very likely to break at the worst time, as they are seriously weakened by weathering!

KMC sells what I believe is the best "very low cost" chain for 7-8 speed freewheels, the Z-model.

A shimano HG70 is much better yet, but costs about $10 more. It will easily outlast the cheap chains but requires somewhat careful installation using only the enclosed "special pin".

You could also install the HG70 chain using a ~$3 KMC Missing Link (NOT the Missing Link II for narrower chain!!!).

Wrong. Youtube how to size a chain by mock fitting it, then buy a cheap chain breaker and a universal chain. No math needed, no LBS upselling, you get the chance to learn a fundamental skill, and you can show up the forum know-it-all who thinks that using the word 'terminology' against someone is a put-down.

Even after you edited it, it's still rambling!
I have to give you a D on that one!