Bicycle Mechanics - Swapping Rear Derailleur Cages?

I'm looking to swap the SGS (long) cage on my Shimano XT M771 rear derailleur with a GS (medium) cage off of a Shimano XT M771. The derailleurs are pretty much the same minus the cage length, so will this be doable? If so, how would I go about getting the parts for the changeover?

I'm certain it's doable. You can prolly swap the cage from a whole bunch of Shimano derailleurs.

Whether you can get the cage as a spare is another thing... NFI. But it doesn't sound too urgent...

I recently replaced an Ultegra short cage with a medium cage from a 105 that had a broken knuckle. It was pretty straightforward, and the resulting derailleur looks and works as it should.

Awesome, I'm wondering where I'd be able to find a replacement cage. Do you think Shimano sells the inner plates?

that's great for sure.

I'm looking to swap the SGS (long) cage on my Shimano XT M771 rear derailleur with a GS (medium) cage off of a Shimano XT M771.

After you do that, what do you think you will have accomplished?

I doubt that you would be able to find the parts as replacement/spare parts. most component compaanies don't work that way. your best bet is to find a broken donor derailer like Chris did.

Retro Grouch's question is a good one. After all of that work, what will you have accomplished or improved?

As to replacement parts, they generally aren't available and your only source would be to cannibalize another derailleur.

When I read the question I assumed the OP had the units in his possession and one or both where broken.

The cage takes up the slack in the chain on a chainring. The long cage is to take the slack in the chain on a triple between the small ring and the large chain ring; on a double, a short cage is used; compacts can use any combination though I find medium tend to work the best.

If so, how would I go about getting the parts for the changeover?There lies the issue.. I got an Olympus Campag short cage MTB from the mid 80's, in the late 80's ..
and not long after, early in the 90's, got a long cage plate pair..
.. fitted it on my Touring bike, Triple crank..
26t [50t outer 24t granny] wrap needed..

Campag, though, is (was) better about small parts than Shimano..

I'd expect just getting the RD you need as a whole the path of least resistance..

Campag, though, is (was) better about small parts than Shimano....
Yeah, that reputation lingers despite it not having been correct (except for brifter repairs) for years.

How many years? I got some derailleur bits... 10 years ago

Shimano does not offer a road derailleur with a medium cage, 105 derailleurs are either short or long. There are some Shimano medium cage mountain type derailleurs however.

That is incorrect. Here's a quote direct from Shimano's own website (http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/105_5700.html) about their 105 (road) rear derailleurs "The new derailleur is available in both short and medium cage lengths."

Shimano calls the road RDs short and medium cages, and the MTB versions are medium or long cages. I believe the road medium cage and MTB medium cage are about the same length, which means that this naming system is useful because it is consistent, but I've never done a direct comparison between the MTB medium and road medium cages, so I can't verify this.

There's no correlation as far as cage "lengths" between road and mtn derailers. Each size is specific to the application, each type is different.
I.e., between XT and Ultegra there have always been 4 different cages (at least since 1996, and I'm not counting the Shadow models).

They can be swapped about, I've done it, but can anyone confirm that the upper and lower (A and B) pivot springs are the same stiffness for all the different cages?
If not, one would want to swap both springs to go with the particular cage being put into service.

That is incorrect. Here's a quote direct from Shimano's own website (http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/105_5700.html) about their 105 (road) rear derailleurs "The new derailleur is available in both short and medium cage lengths."

Shimano calls the road RDs short and medium cages, and the MTB versions are medium or long cages. I believe the road medium cage and MTB medium cage are about the same length, which means that this naming system is useful because it is consistent, but I've never done a direct comparison between the MTB medium and road medium cages, so I can't verify this.

Ok, the 105 medium cage is a new design. Before Shimano road rear derailleurs have always been short and long, not medium. Campagnolo has had a medium for several years.

To answer everyone's questions I don't actually have another derailleur to work with. I did manage to order up the inner and outer plates from Shimano through my LBS. Once they get in I'll have the shop swap the plates and do some other work. As for the reasoning, there are a few reasons for doing it. Weight is a silly reason, but it would weigh a tiny bit less. I am removing the granny ring off of my triple front and don't really need such a long cage for my 12-26 cassette. Also, it would certainly look a lot nicer on there. Realistically these aren't huge reasons and there is no major benefit, but it does give me something to do. :)

As for the reasoning, there are a few reasons for doing it. Weight is a silly reason, but it would weigh a tiny bit less. I am removing the granny ring off of my triple front and don't really need such a long cage for my 12-26 cassette. Also, it would certainly look a lot nicer on there. Realistically these aren't huge reasons and there is no major benefit, but it does give me something to do. :)

Oh oh! Those are CBTD (Compulsive Bicycle Tinkerers Disorder) symptoms. There's no known cure so you might as well just go along and enjoy it. Don't ask me how I know.

(FWIW, once I got the idea into my head I'm sure I would have done the same thing.)

Having a shorter cage is beneficial when using a compact crank as tension on the middle to small ring shift will be better when bouncing around on a trail lessening a chance of dropping the chain. That's my opinion anyways. Same reason applies for preventing chain slap on downhills while coasting.

To answer everyone's questions I don't actually have another derailleur to work with. I did manage to order up the inner and outer plates from Shimano through my LBS. Once they get in I'll have the shop swap the plates and do some other work. As for the reasoning, there are a few reasons for doing it. Weight is a silly reason, but it would weigh a tiny bit less. I am removing the granny ring off of my triple front and don't really need such a long cage for my 12-26 cassette. Also, it would certainly look a lot nicer on there. Realistically these aren't huge reasons and there is no major benefit, but it does give me something to do. :)

Well, to really look better a new derailleur would have done the job, and you could have done the work yourself.

As to the weight, you are really getting obsessive. You might as well carry a tablespoon less water in your bottle as shave grams off with smaller side plates. Your speed would have increased immensely more if you had spent the time used on this forum, plus the time sourcing the parts, and the time you will spend traveling to and from the LBS with your bike in actually riding instead!

Ok, the 105 medium cage is a new design. Before Shimano road rear derailleurs have always been short and long, not medium. Campagnolo has had a medium for several years.

Not so "new" actually. My quote was about the 5700 series, but they used the same nomenclature for the 5600 series also (i.e., short and medium, not long). It does appear that this wasn't the case for the 5500 series, but that was discontinued 5 or 6 years ago.

Not so "new" actually. My quote was about the 5700 series, but they used the same nomenclature for the 5600 series also (i.e., short and medium, not long). It does appear that this wasn't the case for the 5500 series, but that was discontinued 5 or 6 years ago.

This is interesting but also frustrating. The current Shimano "tech docs" show that the 5700 and 6700 RD's are either short or long, not medium: http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/RD/EV-RD-5700-3042_v1_m56577569830702474.pdf
What we need to know is what the real differences are in cage lengths and wrap capacity specs. I'm going to keep looking for this info.
Thanks for pointing out that I may not be correct. I like to post accurate information but sometimes the manufacturers don't make that easy.
The new 105 spec sheet shows the 5700 GS-S to have a capacity of 39 teeth, typical for a long cage RD and same as the Ultegra 6700. The 105 and Ultegra short cage RD's have capacities of 33 teeth typical for short cage RD's: http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/road/105_5700/product.-code-RD-5700-GS-S.-type-rd_road.html
So what is the capacity of the medium cage? And where do we find the specs?