I bought this 4" to 2 1/2" adapter to be able to attach a shop vac to a new band saw that has a 4" outlet.  This adapter is exactly the same size as the outlet, so it won't go inside or outside of the outlet.  I will probably keep it and duct tape it in place as it I can't seem to find one any different.  I know they exist because I got one with the chip collector on my DeWalt planer.