Is this thing bent?
 

M-739 derailleur. Sure looks bent but I see a similar, less pronounced, twist on a short cage 600 unit. I've mostly got non-indexed older derailleurs laying around so not much to compare to.


http://farm4.staticflickr.com/3734/1...94a52299_b.jpg

It look bent to me .

The cage needs to be parallel to the cogs to shift correctly. The mounting bolt also needs to be flush against the dropout. So if the cage isn't parallel to the mounting surface of the derailleur, it's bent.

I figured. I was hoping it was by design but knew it was unlikely. More for the junk pile.

Save this bent der for stealing small parts off of. Andy.

It could be camera angle. I've never seen a cage that was mis-aligned without the plates themselves being bent, which these don't appear to be.

Best bet, eyeball to see if the cage plates appear flat or not. If flat, lay on a table and press down so the inner plate is flat to the table, and look to see if the inside mounting face of the inner body seems to be parallel. If so, odds favor the RD being OK after all.

Sometimes, rarely, RDs bend within the pantograph, and it's usually pretty obvious that the upper and lower bodies are twisted with respect to each other. Otherwise, bend aluminum cages are fairly easy to straighten with some hand/eye skill and patience.

I've even saved twisted pantographs standing the upper body and mounting bolt on a hex key in a vise, and using a lever to twist to lower body back into line. Sometimes it works, sometimes the ears on either body crack, but it's worth a shot on an RD that's dead anyway -- it's especially worth a shot as a field repair in an emergency.

Everyone has their own threshold of what's rideable. I don't mind stuff that's been rescued, and especially like it when I can make zombie parts (working dead) work as good as new.

+1 this. Those 8 speed derailleur pulleys are impossible to find.