Big Horn 11448 4-Inch by 2-1/4-Inch Tapered Adapter. The small end of 11448 has an inside diameter of 2-1/4 inch which duplicates the 2-1/4-inch opening on your Shop-Vac. Insert the Shop-Vac brand hose to the vacuum and the other end to the small end diameter of this fitting to power the dust collection system. These fittings provide a step down from a larger main line to smaller branch lines. This step-down increases air velocity near the machine where it’s needed most. This reducer connects to a standard 4 inch dust collection hose and should be secured with a hose clamp.