3.1.1 The base must be built atop a stable structure, designed to meet the function, geometry and tolerances outlined in Section 3.5. 3.1.2 The material for cast aluminum bases shall be SAE 315 or equivalent, with a BHN of 74. The average yield strength shall be 180 MPa and a tensile strength of 240 MPa. 3.1.3 All steel bases shall be stress relieved and rust proofed 3.1.4 All aluminum bases shall be normalized, and protected against surface corrosion.