Abstract: An insert (50) for a holder (22) in a post-treatment unit (10) for exhaust gases of a combustion engine, has a socket (52) for a treatment component (70) intended to have flowing through it the exhaust gases which are to be treated, a connector (54) for holding the insert (50) in position in the holder (22), and a seal (82) for gastight closure of the socket (52) relative to the holder (22). The seal includes a sealing ring (82) on the socket (52) for cooperation in self-centering seat engagement with an opposite sealing ring (84) in the holder.