Heavy-duty aluminum I-Beam frame with large, protected top read window and shock absorbing end caps. Accurate 360 degree vials set directly into frame, easily read from all angles, and never need adjustment. Large screw-on vial covers with protective lenses allow maximum visibility. Machined working surfaces. Pipe groove for use on rounded surfaces. Replaceable vials and covers. 2-1/2" x 1" frame.