Mountain Biking - Rear derailleur question

I have a Giant MTB that I use for a daily 30 mile round trip commute. It doesn't like to down shift so I replaced the chain, replaced the shifter cable, and adjusted the derailleur. The Derore rear derailleur seems to have play in the area where it bolts to the dropout (hanger). It feels like it is loose but it isn't. I thought it may have a worn bushing. I have a used Ultegra 6600 rear derailleur and was wondering if it will work on a 7 speed MTB. I don't see why it wouldn't work but I thought I would ask. Thanks.

Rear derailleurs do wear out at the pivot bolt- they can develop enough play to be hesitant to shift, especially shifting to a larger cog.

Any Shimano rear derailleur would only work if it has a long enough cage to wrap all that chain from the wide range of gearing usually found on MTB's. You are unlikely to find a long cage (SGS) on a Ultegra RD.

I would suggest a new Deore rear derailleur. Here's one for for $19, a bargain!

http://www.jensonusa.com/store/product/RD507A00-Shimano%20Deore%20M510%20Rear%20Derailleur%20Oe.aspx?SSAID=176195

Thanks for the reply. I run a 11-19, 7 speed cassette so I don't have the wide range of gearing. That is a good price on the Deore. Is it possible to rebuild the worn derailleur?

Yes but not worth it; jockey pulleys, a spring.... an old DuraAce for old time's sake but... you actually really USE your bike, utility, not personal appeal is important to you. It's a Deore so naahhh

So since I have the 11-19 cassette I don't actually need a long cage, right? Would the Ultegra shift better than the Deore?

So since I have the 11-19 cassette I don't actually need a long cage, right? Would the Ultegra shift better than the Deore?

Do you have a triple up front? If so, what chainrings? You might still need a long cage for the capacity.

I would guess that the Ultegra would shift better, but that is just a guess based on my experience with 6700 on my road bike.

I do have a triple in the front not sure about the gearing. Regular MTB gearing I assume. I put the Ultegra derailleur on today and it shifted fine on the stand. I will try it out on the commute in the morning. I may need to take a link or two out of the chain.