For the Relativistic Heavy Ion Collider (RHIC) Spin Project, super-conducting helical dipole magnets are being constructed. These magnets will be used in `Siberian Snakes' and `Spin Rotators', which manipulate spin direction of proton beams in RHIC. The dipole field in these magnets rotates 360° and is required to reach a magnetic field strength of more than 4.0 T. The bore radius of the coils and the magnetic length of the magnets are 50 and 2400 mm, respectively. To ascertain the performance of these magnets, which are built using a new "coil in a slot" technique, a half-length model has been fabricated and tested. The quench performance, field uniformity and rotation angle have been investigated. The measured values in the model magnet agreed well with field calculations. These results demonstrate the adequacy of the fabrication method adopted in the model magnet.