These proving rings are made out of special steel, carefully forged to give maximum strength & machined to give the highest sensitivities commensurate with high stability and to ensure long life and accuracy. All proving rings are integral type viz. the loading (outside) bosses are forged integral with the ring body. This ensures that there is no possibility of abutment shift and consequent loss of accuracy in reading that always exists in rings with bolted abutments. The dial gauge and anvil are mounted on U-brackets clamped to the ring body by Allen set screws. The dial gauge is fitted with a special back cover and threaded bush which does not permit any shift from the original diametric alignment, coaxial with the loading boss centers, that is set at the factory before calibration. For all compression proving rings and tension compression proving rings up to and including 5 kN capacity (a little over 500 kgf) the loading bosses have female threading, 1/2" B.S.F. (16 t.p.i.) The boss faces are ground and polished and are lane parallel to within 0.02mm.total D.T.I. run-out. Caps are provided for use when loading in compression. Repeatability is as stipulated in IS : 4169-1967. The rings are supplied complete with dial gauge and our own calibration chart, packed in polished wooden boxes.