Steel Doors

The 3/8" stubby will work if you install it in a 3/4" "donut". The "donut" is a 3/8" to 3/4" adapter. You don't want the switch or magnet to sit directly next to steel. The "donut" allows the magnetic field to spread out as it was designed to do. If you intall a 3/8" directly into steel, the magnetic field will be very weak and the switch will be unreliable. They also make 3/4" switches with the "donut" collar built in.