be seen later the difference in diameter between id and eod is a factor which controls the degree of misalignment that the expansion unit can accommodate the inner ring 28a 28a 39 is a stiffening ring made of metal and has a inner diameter which is greater than the outer diameter pd of pipes 2 3 but which is less than the outer diameter of inner ring 27a 27a 39 outer ring 28c 28c 39 has the same width as outer ring 26c 26c 39 of guide ring 26 26 39 and inner ring 27a 27a 39 of interior sealing ring 27 27 39 inner ring 28a 28a 39 has the same width as inner ring 26a 26a 39 of guide ring 26 26 39 and outer ring 27c 27c 39 of interior sealing ring 27 27 39 so that the side surfaces 28f 28f 39 of outer ring 28c 28c 39 protrude said distance l beyond the side surfaces 28d 28d 39 of inner ring 28a 28a 39 in the embodiment shown in fig2 a further interior sealing ring 27 27 39 is positioned next to each of the exterior sealing rings 28 28 39 an annular cover plate 29 is mounted on each end of housing 21 in any suitable manner for example by bolts 30 threaded into holes 31 in housing 21 the inner diameter cpd of cover plate 29 is greater than pipe outer diameter pd.