Test heads consist of upper and lower cylindrical segments having inside radius of curvature of 3" for 6" sample 2" and for 4" sample. Lower segment is mounted on a base two perpendicular guide rods extend vertically from the base. One guide rod is larger than the other, with a correspondingly larger guide sleeve in the upper segment to assure correct assembly. Guide sleeves in the upper segment bring the two sections together without appreciable binding or loose motion on the guide rods