Hi, I have searched but I can not discover if the Shimano 600, ST 6400 STI, 8 Speed shifters I have can properly function another rear derailleur.

I am interested in swapping out the RD-6401 rear derailleur with a mountain bike rear derailleur to run larger cogs.

I am in San Francisco and would like to use a 8 speed 32 or 34 rear cassette.

Is this possible and what type of rear derailleur should I be looking for?

Thanks for the help!

No problem, as long as you don't use a 10s (Dynasis) MTB RD.

You can use just about any Shimano RD except the above, or a RD-7400 (8s Dura Ace). And possibly those belonging to groups using the aero STIs, which have a slightly different cable pull.

I don't know MTB stuff too well, but AFAIK XT is roughly equivalent in quality to 600/Ultegra.

Something like this in a medium cage would be pretty appropriate, I guess.

If you want to be fancy about it and your bike isn't a triple, you could probably swap the cage out for a short one off a road derailer, but you'd need to find one with the same cage mounting setup (there are 3 or 4 different styles).

Actually come to think of it, I have just such a RD here, from a still-born project:



You can have it for AU$30 + $20 shipping (Aus Post Registered Post International or Pack and Track International), which today comes in at just under US$45 total.

Missing a little paint as you can see, but it works a treat. I think Deore is equivalent to 105.

Thanks for your response. So it sounds like any 8 or 9 speed RD will work just fine. Especially the Deore, correct? Thanks for the help!

yes, any 8 or 9 (or 10, for that matter) speed derailer will work. You want "mountain bike" components because they are designed for a wider range of gears and will properly track over larger sprockets.

In the late 80's and much of the 90's, Shimano's 2nd- and 3rd-best mtb groups were Deore XT and Deore LT.
Nowadays "Deore" signifies a mid-level group, and "XT" signifies the 2nd-from-the-top group.

I have a 1994 (8-speed era) Deore XT rear derailer in my parts bin, PM me if interested.

10s MTB stuff is called Dynasis, has a different cable pull and won't work.

No, any grade of stuff will work. But since your bike uses Shimano's second-best road stuff, you probably want something fairly nice to go with that like Deore or XT. They come in long and medium cage versions, you want the medium cage unless you have three chainrings.

But you could be stingy and just slap a Tourney on for $15. I don't think there's much actual downside to a long cage except for the goofy looks, and if you buy a Tourney you're obviously not concerned about that aspect.