Mine was a 6 speed freewheel as opposed to cassette. If it is a cassette the hub will have a 'swollen ' bit at the sprocket side to accommodate the freewheel mechanism. I would guess a mountain bike would be a freewheel type. There are a number of videos on you tube re chain length. I had to remove 2 links which is easy enough but you will need a rivet extractor (quite cheap). Also if your 6 speed assembly is worn, this will affect gear changing. I had to replace the 6 speed freewheel which is also fairly cheap. The removal tool will be essential and makes life much easier. All available from wooley hat shop who provide an excellent service. I struggled without the removal tool. I am assuming your bike will have Shimano gears or the above may not fully apply or mislead you a bit.