I purchased these to splice two 4" fittings together.  The outside edge is just a bit less than 4".  Perfect I thought.  But when I tried to connect the pieces together they only slid in about 1/2"  You see they are tapered and get bigger the further away from the end you go.  This made my connections very unstable.  I am not happy with this product but was finally able to use it after spending hours sanding the pieces so they would actually mate with my other fittings.