Which 40, 41 or 42 rear sprocket are you using ( should I use) on my 29 Enduro, currently it’s a Sram PG-1030 11-36, SRAM XO type 2 with a 33/22 up front. If it was on a 26 I’d say leave the 33 on.
Is a 10-42 possible & Is there a N/W chaining I should avoid or are they all basically the same or are some harder wearing than others?

Not sure what you are asking? All SRAM 11s cassettes are 10-42. I use a 32 xx1 ring on my 27.5 bike and it’s fine, can’t speak for other brands though

I demo’d a Turner Czar yesterday set up with XX1 10-42 with a 30T chainring & used the full range around Dalby Forest.

I use a 32 on my Sync, and would be fine with a 34 most of the time (it’s a light hardtail and I live in Norfolk). The beauty of x11 is that you can swap the chainring a over in a couple of minutes. If you’ve got a 33/22 on at the moment, I’d probably go for a 30t to start with.

Pretty sure you’ll have issues finding a 33 tooth NW ring but definitely give it a go.

Had you thought of working out the ratios on a spreadsheet?

You can’t have an odd numbered narrow wide ring – think about it 🙂

What I meant by the 33 ring is that I would of kept the 33 with a bash on a 26 inch as I previously had a 32 on my HT.

Ok then I’ll re-phrase my q.

What chainring on a 29. Will give me the same roll-out as 34 on a 26 ( on any given same rear cog) ?

Most questions on bikes can be sorted by Sheldon Brown 😀

30T

34×559/622 = 30.56