DOS

In my drawer of busted parts, I have an Ultegra small cage (double) derailleur that went through my wheel when derailleur hanger snapped. The body is toast (the opening for the mounting bolt is oval rather than round), but the cage, oddly, is not bent. I also have a 105 long cage (triple) derailleur the cage for which is bent and jockey wheels shot. Is there any reason I couldn't put short cage from Ultegra derailleur on the body of the 105 triple derailleur to make one functioning short cage derailleur?

time bandit

Should work fine.

fietsbob

105 RD are not all the same, for decades.. perhaps if they are from the same year,
and other than the cage length . identical .

Retro Grouch 

Since you can't use either derailleur the way it is now, what do you have to lose by trying?

Mondo734

I think this thread should be not about you asking if it would work but rather if it works once you try it! Like Retro Grouch said since you cant you either one now as is you have nothing to loose by trying it.

DOS

I'll give it a try; although not sure when I'd get around to installing on an actual bike. I was just sort of curious whether I could cobble one functional component over two nonfunctional ones I have in a drawer.

But a related question, also mostly curiosity. The Ultegra mounting/pivot bolt is same size and dimensions as the 105 so two bolts are certainly interchangeable. However, the Ultegra bolt has rubber o-ring in the middle of the bolt (centered between bolt head and threads), while the 105 bolt has the machined groove for an o-ring but no o-ring. What purpose do you reckon this o-ring serves? Clearly, the derailleurs pivot correctly with or without the o-ring; although it does seem to make the Ultegra bolt fit more snugly in the derailleur body.

Edited to add: Here is a picture of Ultegra bolt (this is for 6700, I am dealing with older 6500 but bolt is visually identical). https://www.bikepedia.com/pa/Item.aspx?itemid=431148.

The bolt from 105 derailleur is same except for o-ring in middle.