Hi,
About to fit a one up 42t expander cog and would like some advice on if I should just remove the 17t cog or the 17 and 15 and put the 16t in. It’s a xt cassette.
Cheers
Shaun

You will feel less of a noticeable jump in ratios if you use the 16t.
Not a major issue either way.

Depends on whether it’s a Shimano 16t cog and on whether you’ve got a One Up cage or a new hanger

If you’re relying totally on the b-tension screw and have a non-ramped 16t cog then there are times (in my experience anyway) that I would have preferred to keep the 17t cog (as I had problems shifting down to the 16t)

Once I had the One Up cage, it became less of an issue – but could still be problematic. So if you can get a ramped 16t cog then go for that

I have built a few of these set ups now using One Up’s 42t and rear mech plate. I have used the One Up 16t cog each time (as it comes with the big ring anyway). I have not had an issue with set up of performance. Only one build was for one of my bikes but it all worked very well. Not had any of my mates come back to complain about performance on theirs either.

Cheers I’ll put it in then.
Yes I have a radr cage to go on too. I just remembered reading something awhile ago about not putting it in.

I added a shimano 16 (from Bird Cycles) after a few weeks last time… The original 17 setup wasn’t terrible but the 16 is just nicer- OEM-feeling.

I did that ^ but found the shifting was always a bit shit on to and off 16t one proper Shimano one). I wouldn’t bother again.