With the long cage rear mech could you run an 11-40 or 11-42 rear cassette with the 48/31 chain set? Can get my head around it!!

Not officially.

I’m running 50/34 with a 11-40T cassette and a standard Ultegra long rear mech. Outwith Shimanos documented limits but it works fine.

48/31 is a 17T difference so might still work (50/34 is only 16T).

I doubt it will work with 11-42T cassette though.

I wondered if it would be too far a stretch at 42. But it should be ok with a 40 then. 🤔

Might. Only think I had to do with mine was that I need 118 link chain but that’s more down to chainstay length.

The other thing that might help you would be a Roadlink.

Yeah expected the longer chain, didn’t think of road link. Cool cheers!

Mine doesn’t seem overly stressed with 40 running 48/32. Might work. I say might.