I know roadies always swear by this formula but surely mtb chains take a lot of stick and can be renewed without a new block? My mate has just broke a relatively recent chain that was fitted with a new block but the bike shop are now trying to sell him both. Can he get away with just a chain?
Incidentally, how can you tell when a cassette is worn out, it doesn't look as obvious as the hooky teeth on chain rings?
If it's relatively new, like only a few hundred miles, then you just need a new chain.
Although why can't you just repair the chain..?
If it's been ridden too much, you will know about it, then you buy a new cassette.
if the chain hadn't "stretched" too far, then cassette won't be worn out
use a chain checker - some suggest they're inaccurate but it doesn't matter as long as they're consistent; when they say 0.75%, I change the chain, if they get to 1% before you notice I'd change the lot
Get a chain stretch measuring thing. If it's not too stretched then the cassette should be ok. If it looks ok and the shifting is ok then the cassette is probably ok. Tend to find I get through 2 or 3 chains to a cassette.
If its a recent set up, a new chain only may be fine. I would just get the chain first. If the cassette and front ring or rings need changing as well it will soon become evident.
I use a chain stretch tool. Even when I replace the chain at equal to or less than .75 my cassette is knackered usally on the the 3/4 smallest rear cogs, my most used gears. However the .75 switch seems to save my front rings. So far anyway but still testing.
if you go for a new chain only- keep the old chain! If the new chain skips I would be tempted to repair old chain and run it on old cassette until you get obvious shifting problems and or hit .75 wear than change both at same time.
Just my opinion though, I have looked into this a fair bit (from a novice viewpoint) and it seems to just come down to how hard you personally wear this stuff out.