A method and device for anchoring rods of high tensile strength insulating material in metal attachment fittings are disclosed. The end of the rod which is previously split and traversed by a hole is inserted axially into the attachment fitting through an open end thereof. A fastening member is inserted into a blind hole extending across an internal cavity in the attachment fitting in line with the hole in the rod for spreading the rod end, the split end being brought into engagement with large wall portions of the inwardly flaring internal cavity of the attachment filling. A relatively high vacuum is applied to the internal cavity. The internal cavity is filled with a high strength, nonshrinking resin having a coefficient of expansion identical to that of the metal fitting through a channel in the fastening member. After polymerization the resin hardens on the rod to form a rigid block fast with the attachment fitting which prevents the rod from being pulled out of the attachment fitting. The protruding end of the fastening member is then severed level with the exterior of the attachment fitting.