Changing 52/36t Dura ace 9000 into 52/42t

By the way, whilst we're on the subject of Shimano chainrings, are dura ace 9000 and Ultegra 6800 chainrings interchangeable? They look the same.

I ask, mainly because no one seems to stock Ultegra chainrings

I think the max difference is 16 teeth

This might be a daft comment but if the biggest difference is 6 teeth then how can you have a 53/39? Is that not 14 teeth?

The gap is up to six teeth (see above).

You know that's a fair point, I don't think they do a 42. Might need to be an alternative supplier. Google is your friend here.

No you shouldn't have to adjust the position of your mech as you're making the gap smaller so no changes to the outer tolerances of the range. That said, you should check it as a small tweak might get the shift in a better yaw position for both directions. Fettling. That's one of the joys of having a bike.

Assuming you've got a compact chainset, can you get a 42 tooth inner for a 110bcd crank?

I have a DA9000 53/39 and swapped the inner ring for a 36 for a trip the Alps. This is not recommended by Shimano as they say the gap between teeth numbers should not exceed six - primarily for the purposes of facilitating a smooth change.

I've found it works very well and shifts are seamless 99% of the time and clunk for a tooth or two going into the big ring that 1% of the time. So infrequent as to be barely noticeable and at it's worst, it is still better than my old Ultegra on my winter bike.

I'm sure that, if I can do this, you'll be able to make the gap smaller no problems. When I did it, it took ten minutes for the change if that. All of the new DA9000 rings have the same bolt circle diameter so are interchangeable.

Bigger inner ring shouldn't cause issues. 42T inner used to be a common climbing ring! For flatter races like Paris Roubaix many riders go with 46 or 48T inners, from what I gather.

You should just be able to buy a new inner ring and switch it over