Disclosed is a mechanical clamp useful for providing access to the interior of an enclosure such as a pressure vessel. The mechanical clamp is an apparatus having a cap having a convex side and a concave side wherein the concave side is adapted for positioning adjacent the wall of the enclosure. The apparatus further includes at least one rod attached securely in the cap for passing through at least one hole in the wall of the enclosure wherein each rod has a first end attached securely in the cap and a second end and each rod is threaded along a portion of the rod including the second end. The apparatus further includes a nut for attaching to the second end of each rod for securing the cap in place adjacent to the wall of the enclosure. An opening is provided in the cap so that a probe can be passed there through. Methods for applying the mechanical clamp are also disclosed.