Close Clearance Dryer Vent Periscope Pipe 0"-18" - Close clearance dryer vent periscope pipes are the only way to reduce the space behind the dryer from 4 1/2" to 2 1/2". Use them to position the dryer closer to a wall. The 0"-18" periscope pipe is used when the dryer exhaust and wall exhaust are close but do not exactly line up. The excess can be easily trimmed with tin snips as shown in the picture.Note: Only the bottom part has been trimmed in the photo in an actual installation you would also trim the excess off the top.