Using the Shimano Ulterga 6800 RD mid-cage for parts, will i be able to use the cage to bolt up to the Dura Ace 9000 RD to turn it into a mid-cage RD to use with Ulterga new 11-32 cassette?

True mechanics have not necessarily tried or spec'd every combo of parts mashups (often it's the garage mechanics who have the time and inclination to experiment) but the geometry/dimensions that allow a derailleur to handle a larger cog are not primarily in the cage, whose function is to take up chain slack.

what cny bikeman said is true
lbs techs seldom have time for experiments of idle curiosity
there is more money to be made by selling a whole derailleur
than by taking two hours to rebuild an old one

anyways
i seriously doubt this is possible
the main problem is that shimano derailleurs
are not designed to be servicable
beyond replacing jockey wheels and maybe bolts

so even if the derailleur parts mate together properly
which is a big if
it will be tough to get the thing reassembled and functioning

however
if you have the two derailleurs
you can go ahead and try
and report back here

also
what i want to know is
the reason you cant just use the ultegra derailleur

edit
also like cny bikeman said
a long cage will not allow larger cogs
for that you need a long body
like a mtb deralleur

All true.

However, people have reported that they have successfully used both a mid cage and long cage Ultegra RD with an 11/32 cassette. Mid cage may require installation of a longer B screw (or removing it and reinstalling from the hanger side to get more length), long cage seems to handle it just fine.

Hope they are correct, as I'm planning to use a 6600 long cage RD with an 11/32 combo on my century build.

Or the OP could simply buy a RD designed to handle an 11-32 cassette. In any case, disassembling an ultegra RD to move the arms over to a dura ace body doesn't seem to make much sense. If the ultegra is doing its job, why waste your time on this project? If it's not, it's hard to believe this will make an improvement.

That is not something "true LBS bike mechanics" would deal with. More like folks who live bikes and spend their boss' time on the interweb thinking up such nonsense. If you don't believe me go in on the next really nice Saturday (I'm thinking 10 or 11 AM) and ask, see what he says...

The capability for using a 32t cog with any Ultegra derailleur has more to do with the length of the derailleur hanger than the length of the cage.

The cage length makes no difference to the maximum sprocket size. I would hazard a guess that the cages are not interchangeable anyway.

- Joel

^^ Agreed, as well as whether it's a horizontal or vertical dropout- horizontal typically gives you more room to play with.

Read an interesting post recently talking about body (as opposed to cage) geometry and how much it contributes to proper shifting on larger cassette cogs. Pretty cool analysis.

I OTOH, bought a Campag Euclid/Olympus RD from the era when Campag was trying to compete
in the MTB sector. then dropped the idea.. late 80's they were moving into close outs ..

Those were able to replace the pulley cage, and so found another one Medium cage ,

bought the long cage pieces from the shop's vendors .. Its on my touring bike, still ..
have the medium one to sell.. [inner and outer plate]

NB Shimano really makes just putting a whole new RD on, the More efficient use of the shop time ,
as their parts supply chain, through wholesale distributors, is whole boxed RD's .

I have done this operation a few times, because I am too cheap to buy new derailleurs. I have merged a good cage from one Shimano derailleur to the scavenged body from another.

Shimano derailleur parts are surprisingly interchangable, even between the mountain (not the latest 10-speed stuff) and their road lines. I have created long-cage road derailleurs by merging mountain cages to road bodies.

However, when it comes to the derailleur parts, XTR and Dura-Ace units are quite different. The cage attachment is incompatible with Ultegra/XT and the lower lines. I suggest you use the Ultegra as-is.

Bottom line? Buy the right derailleur for the right application.
You don't want that DR to blow up 40k from home on a rainy Sunday afternoon...

Lots of truth here. As someone who works in a small shop that generally goes way beyond what most will take apart, it is very tough to justify the time it takes to do this on the clock. Personal projects, sure we have all tried weird mash ups but there are some days that the sales floor is so overwhelmed that I rarely get to touch a wrench and end up going in after hours to get us out of the weeds with scheduled repairs, today is a great example. I went in at 6:30am to get some repairs done before we opened because on Fridays we are lucky to turn many wrenches, we still have so many repairs that I am about to head back into the shop here in 30 mins or so to simply try to keep us caught up. Obviously part of the problem is we are to quick to say we will get it done by the weekend but quick turn around and willingness to try and fix things before replacing is why other shops send people to us for tough service issues.

With that being said, there is more to how big of a cog a derailleur can handle than simply cage length. Try it if you are bored but I personally think it will be a waste of time and not really be any improvement.

Do you have a link to that post?
Thanks.