The most precise RGS instruments ever made. Tolerances are less than 20 microns made of the finest quality steel with lightweight color-coded handles. The companion to the RGS 1-2, the “RGS Three Four” preparation calibration instrument completes your calibration set. Dr. Stevenson’s prep evaluation device will easily show you the ideal and maximum allowable occlusal clearance of All-Ceramic, FGC, PFM and Onlay preparations, and the ideal shoulder dimension on Ceramic restorations of all types. The width of the instrument accurately measures class II amalgam isthmus width, and the length assesses adequate axial wall height. The small end of the instrument measures 1.0 mm in diameter and 3.0 mm in length, and the larger end of the instrument measures 1.5 mm in diameter and 4.0 mm in length. Used by thousands of dentists, students and several dental schools, this guide serves to train your eye to within 0.1 mm of ideal in your quest towards preparation mastery.