The 6221, 6223 and most other 6220 series electric strike models are designated for use only for 1 ¾" thick double doors.
There are three exceptions;
6224 and 6226 Closed Back models can be used on 1 ¾ or 2 ¼" Hollow Metal Doors or Aluminum doors
6224AL Closed Back model can be used on 1 ¾ or 2 ¼" thick Aluminum Doors.
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