I'm in need of some technical assiatance. Here is the issue: I am
building up a new bike and cannot get the bike to shift properly down
into its smallest cog(12t).

Components:
All NEW Dura Ace 10 speed Rear Der, chain, shifters, cables, 12-25
cassette(10s)
Shimano compact crankset.

Mavic Ksyrium SL wheels
Pinarello Paris Carbon frame

I have backed off the limit adjustment screw completely on the der. to
allow it to travel as far as possible outwards. Cable tension is not
limiting the movement of the der outwards.

To me it appears that there is a spacing issue. Either the cassette is
spaced too far out on the hub body or possibly the Der. hanger is too
thin..not allowing the der to pull the chain far enough to drop it
onto the 12T cog.

I minimized the spacer by 0.5mm behind the cassette on the hub body
and even added a 0.5mm washer on the Der mounting bolt to space out the
der on the tab. This gave the der some more throw to pull the chain
down. The shifting is improved and successful into the 12 in the stand
but still stalls under pressure when test riding and won't drop into
the 12.

Suggestions??

any help is greatly appreciated