I have bought a secondhand Trex 450 pro in pieces along with a V2 and lots of spare parts.
I have assembled the heli and tried a test hover but the nose kept turning to the left.
It seems that with the tail bellcrank at 90 degrees to the boom the tail blades are pitched the wrong way!
Does the bellcrank look right to you in the pictures?
Thanks for any help

Put the servo rod at 90 degrees

Put the slider in the center of its throw.

See what it looks like then.

It looks like you need to adjust your mechanical trim. Loosen the servo mount and slide it until the tail rotor is angled about 5 degrees, blowing air to the right side of the heli. Try to hover with HH off. Adjust your trim till it holds relatively steady at chest height. Then land and move the servo again until the blades are in that same position with the trim at zero.
If that doesn't fix it your gyro may be reversed for your application.

Joaquin

Thanks guys, It's a 450 pro so there is no way to slide the tail servo anywhere.
My argument is that when the tail blades are set to blow some air to the right side of the heli I.e. when in the neutral position in a normal hover, the bellcrank should be at 90 degrees to the boom so that there is equal throw left and right for the tailblades.

Finless Bob identifies the problem here http://www.helifreak.com/showthread.php?t=132747