I have had one of mine apart. If memory serves me correct, I believe the front is glass. The black trim ring around the front of the gauge that holds the lense on is either brass or copper thats been painted black. All you have to do is carefully pry the trim ring away from the gauge to get the face off. If you do it carefully enough it can be done without any noticable damage. Then when you reinstall the gauge, the marks from it being separated are placed against the opening of the gauge cup or pod.