While it's possible in some cases, it's not going to be cheaper than buying a whole new chainring in both time and money. Buy a new one, sell the old one

Due to severe chondromalacia knee pain I want to try 165mm crank lengths on my road bike. I currently have Ultegra 6700 crankset and bottom bracket. I wanted to just change the crank arms not the whole crankset, but I cannot find new shimano crank arms only for sale. Is it not possible to change just the arms? You have to buy a whole new crankset with new chain rings?

Daniel

 

Yes it is possible to just change the crank arms with out buying new chain rings and BB. You can use 105, Ultegra or Dura Ace. Just make sure you have the correct bolt circle diameter for your rings as there are three different types of cranksets on the market (doubles, triples and compacts).

 

Except for some minor wear-and-tear parts, Shimano doesn't permit internet parts sales. But every part on the linked exploded view that has a Shimano Code Number can be ordered through a local bike shop. Check prices before ordering though—as said, it may be cheaper to just buy a new crank.
http://techdocs.shimano.com/media/t...cs/content/cycle/EV/bikecomponents/FC/EV-FC-6700-2913_v1_m56577569830670817.pdf