Can 1/4 inch plexiglass be used to insulate the spark ring ( brake drum) on a coil tester?
Yes. Be sure the hardware is well clear of the frame however. If the bolts holding the sparkring are seperated from the frame by more than the gap between your pointer and ring you are in good shape. If that distance is less than the value between the pointer and the ring that is where all the sparks will occur.
The original piece used on an Allen tester using a brake drum was a piece that looked like 1/4" tempered Masonite and that's what I used in my HCCT pros that used a brake drum. Bob
Thats' "HCCT reproduced copies"