I have just put a new Ultegra 12-30 10-speed cassette on as a replacement for the old Tiagra 11-28. The bike has a full Dura-Ace 7800 groupset.

Before the change, the chain was skipping on the second largest cog, and sometimes skipping it altogether.

Now, everything seems to be working fine in the largest 4 cogs, but then the chain skips one or two and won’t go into the smallest AT ALL.

I have played with the cable tension, and the limiting screw, but even if I manually push the derailleur into the smallest cog, pedalling forward shifts it back up again.

What am I doing wrong? Is there a compatibility issue with the cassette?

Could be an adjustment issue, but have you fitted a new chain too? If not, how many miles on the old one?

Yes, the chain is new as well.

The 7800 mech technicaly only worked with a <27t bottom gear so it might just be that you’re not going to get away with it even with the B-screw wound right in but then it’ll be too far away at the small end (maybe).

If the chains new as well then the mech hanger or drop-out is bent which causes the indexing to be out at one end of the cassette or the other, so if you tweek the cable tension it should work in the high gears but not the low ones.

Okay, I have done what I think is everything I can, but now the chain is not staying in the second- and third largest cogs, and it still won’t shift into the smallest.

It’ll be LBS time if I don’t get this sorted by sundown…

Fit new cables (inner and outer)
Ensure mech hanger is straight (with the correct tool)

Failing to do either of these things could have you faffing around aimlessly for ages.

Didnt you fall off the bike earlier?? Bent hanger?

Didnt you fall off the bike earlier?? Bent hanger?

Different bike. When I fell then, it was only my elbow that got damaged.

Have you got a directional chain on the wrong way round ?