I bought a used Specialized Allez A1 at a swap for 250. Bike rides nice, looks nice, but it had Tiagra rear de. Also the deraillure hanger and maybe deraillure were bent or misaligned. I managed to get the alignment pretty close and then installed a slightly used 105 I got on ebay. I didn't even break the chain...just took of the sprocket on the long arm, put in the chain and reassembled. Worked like a charm! A little tweaking of the limit screws and barrel nut and I was off and running. I'll leave the fine tuning to an expert...but I got it pretty close allready. My main problem now is the front derallure which tends to rub a little when the rear is in low gear.