A true mandrel bender, as used by exhaust system manufacturers (not muffler shops) consists of the following: An inner radius die, that is almost a perfect "size on size" with the O.D. of the pipe being bent---the die is round, and has "half" of the pipe shape scalloped out of the outside diameter. The outside diameter of the "die" determines what the inner radius of the bend in the pipe will be. Two hydraulic "grippers" which close on the pipe on either side of the area which will be bent, and a mechanism which holds one gripper stationary, and swings the other "gripper" through an arc which will hold the pipe in contact with the "radius die".---And most important of all, a series of "slugs" which have an outside diameter slightly less than the I.D. of the pipe being formed. These "slugs" are not very long---less than an inch, and are all hooked together with a flexible spring which keeps them together. Before the bend is started, these slugs are inserted into the pipe, and positioned exactly where the bend will occur. As the pipe is bent through whatever angle is programmed into the machine mechanism, the slugs keep the inner wall of the pipe from collapsing and deforming. A flexible "pusher rod" is then inserted into one end of the pipe, and the chain of "slugs" is pushed out the other end. When I first seen one operating, I thought the slugs would all be jammed tight inside the bent pipe, but they weren't--they slid out very easily. Thes mandrel bends are beautifull, with no deformation whatever.