You might as well just get a smaller chainring.

But why would you do this? Surely the same result would be had by putting on a smaller chainring?

I think you’re missing the point of a wider range cassette, as 13-40 would give you a very similar range to a standard 11-36 if used with a smaller chainring.

It doesn’t make any sense. Also I don’t think there’s any reason to change lockring, I used an 11T one no probs on a 12T cassette.

Let me clarify my post above, Yes it’s possible, however you may need to change the chain if it’s been sized to suit a 36t cassette. (Is that a sensible enough answer?) However it makes no financial sense whatsoever! The cost of a range extender is double the cost of a smaller chainring, plus the cost of a chain, plus the doobry above. Then when the cassette & extender are worn out you have to replace both parts, again increasing the cost of ownership.

I struggle with long sentences. Just get a smaller chainring.

OK then I’ll answer the original question.

You will need one of those things from SJS, you will probably need a new chain and if your existing chain is worn then your cassette may well be worn as well so you might need a new cassette. If your existing rear mech isn’t a long cage one then you’ll need one of those as well.

So the disadvantages of what you are suggesting:

Lots of money
More weight
Worse shifting
More chin slap (as you have to have a longer chain)

Advantages, compared with just getting a smaller chainring):

Ermmm, none.

You see, you get get grumpy about typical STW not answering the question etc, but if I’d just listed the bits you might need to make it work properly, you’d probably have come back with “Why didn’t you just tell me to get a smaller chainring, you are all idiots”.

Anyway, I’ve now answered your question and told you why you shouldn’t do it.

You’d need a proper 12T end sprocket aswell rather than one that sits second in line, as that one wont have the knurled surface for the lockring to tighten against.

Yes, it’s possible but only really worth it if you’re already using the smallest possible chainring that your chainset will take.