Will this rear mech run a 27t cog with a 34t front ring?

[EPOJoe]

It's a Dura-Ace ten speed (not sure what series) and I've already tried it with a 28t- won't work without contacting the upper pulley no matter what I do with the stop screw. So, before I buy a 27t cassette, should it work with a 27t when in the 34t front ring of a compact crank? I'm running it on a 25t cassette now with no issues (d'uh) but Mt. Umunhum convinced me I need a few more teeth. :)

[smead]

That's a short cage 7800 RD, and it should work with a 27T; however, it is not the stop screw you need to adjust, it is the "b" screw. I had to turn mine a few times to get the upper pulley off the 27T cog when moving from a 12-25 to 12-27. If you do that, I would expect it might work with a 28T as well.

With the 34 on front, however, you may not have enough capacity ...

[dem]

Assuming that's an RD-7800 Short Cage, it's borderline.. might be a bit floppy in small-small.

Max cog was 27T, capacity was 29T

27-11 = 16T
50-34 = 16T

So 16+16 = 32T capacity required

I'd try it.

[weisan]

.

[EPOJoe]

Thanks guys. Yeah, it was the "b" screw I was working with (brain fart). I'll give it a shot and see what happens.

[Kontact]

A big factor is the length of the derailleur hanger in some working and some not.

[bjf]

Same derailleur, same issue. I took the B screw to a hardware store and got one that's longer (same size and thread, obviously). That did the trick. 28-34 no problem.

[kramnnim]

You could also put the B screw in backwards

[Ralph]

You could take a couple links out of the chain.....that might pull the pulleys away from the cog.....just make sure you don't shift big to big and rip the RD off. I would only do this for an occasional use of a big cassette that is outside of RD specs.

[oldpotatoe]

You could also put the B screw in backwards

Yup, and make sure you have just enough chain length for big-big but not one link more..It should work.