What size chain???

[bikester]

Can anyone tell me If the chain on a 21 speed mountain bike is a SIZE 40?

 

[qwerty1]

21 speed is usually a three speed front chainring hooked to a seven speed cassette or freewheel
A seven speed dérailleur chain is 1/2 X 3/32.
A 40 chain is 1/2 X 5/16

Not the same.

 

[bikester]

So does the chain size 1/2 X 3/32 have a number to go with it ie: 35,40,41 and so on?

 

[The_Aleman]

No. The chain is commonly called "multispeed chain", it is application-specific. You must use that size chain if you want to keep your gears working properly.

So does the chain size 1/2 X 3/32 have a number to go with it ie: 35,40,41 and so on?