A mountain bike chain is the most important component of the drive train. It takes the bulk of the stress and should be changed regularly. As it’s used, it stretches. A chain wear indicator can show when a chain is stretched enough to be in need of replacement. However, it can be a good idea to replace the chain before it gets to this point.
There are 3 major brands of chains: SRAM, KMC, and Shimano. SRAM and KMC chains are assembled using a master link, while Shimano uses a standard pin system. Chains come in 8,9, & 10 speed. It is important the chain is the same speed as the cassette.
Start by removing the old chain. Open the new chain and remove the two halves of the master link. Now the chain needs to be measured for length. Do this by feeding the chain over the largest chainring, through the front derailleur, and over the largest sprocket on the cassette. Do not feed it through the rear derailleur. The correct length of the chain is the distance around the largest chainring and largest sprocket plus two links plus the master link.
When using a master link, both ends of the cut chain need to be inner links. It is always better to add a link to break at an inner link than to take it away. Then, use a chain break to break the chain. Now run the chain through the rear derailleur. Shift the derailleur into the position of the smallest sprocket.
Slide half of the master link into each end of the chain. Pull the chain together and assemble the master link. Pinch the middle and pull until it snaps into place. Test to make sure the chain is running smoothly.