Aluminum tubing is not measured like steel tubing. Two inches ID or OD is EXACTLY two inches. They WILL NOT slide into each other. You will need to mill 1/32" off each face of the male tube to make it fit. Even then, if the receivers are not perfectly parallel, the cross member will still not go in very easily. I solved that problem by over sizing the four bolt holes for the receivers and used nylock nuts left very slightly loose. There's just enough play to make them go in, a little spray lub won't hurt.