Putting a 34 tooth chainring on the soul to replace the 30t oval one. Will that be a new chain job or will it be fit it and see??

Fit and see but you should be fine as long as it wasn’t on the limit, if your cage is near horizontal on the biggest gear then it’s a new chain.

Easier with hardtail as you don’t have to cycle the shock to see

Depends, newer mechs are a bit more picky about chain length and how it interacts with the B-tension adjustment.

If you don’t like wasting chain with wear left you could buy the new chain but just use the amount you would have to trim off to lengthen the old one with two quick links. Or if you keep your trimmings you might have enough. Or best of all some part worn chain. You don’t really want to use a new chain on a cassette which is half worn, it might skip if you tried it. A few new links might not cause skipping – this has worked for me in the past.

Only if you need a longer chain obvs.

I did not think of this!!! Great idea. I have got a worn chain lying about too. Cheers mate!

When I went from a 32t to 34t I looked it up and I’m sure the general consensus was that if jumping up or down 2 teeth then you don’t need a new chain. More than 2 teeth (4 in your case) the info wasn’t so clear. Too many variables (is your current chain too long or short, suspension kinematics, hardtail or full sus etc?).

I’d have personally thought that it wouldn’t be much of a problem using your current chain.

How worn out is the chain, ie what’s it measuring at?

If at 0.5 or more I’d replace the chain.

If it is much more than 0.5 you are likely to need a new cassette as well. And if a new chain does run well on the old cassette, you may as well keep the old one with added links and rotate the two once the new one has achieved the same wear.

34t round or oval?

I went from 30 round to 34 oval and didnt need to change the chain (both chain and cassette were reasonably new when I upgraded the CR)