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You can not imagine how much work is involved to adapt the 635 airdam to an E9. To start with the 6er airdam is mounted to the bumper, a very different bumper from the E9, and its wider and the wrong shape for an E9. The one I sell cost me a significant amount of money to adapt to the E9 and it is specific to the E9, will not fit a 635. Paul Cain I believe did the original adaptation of one to an E9, but his mounted to his car;s bumper. Mine attaches to the front valence and can thus be used with or without the front bumper. If you try adapting one of these 6er airdams to your E9, all I can say is good luck. Lotta work to save about $200.
As for fitting the bumper over an airdam, there are other ways to do it, the front bumper is in three pieces and once off you can loosen the joints between the middle and ends and you will be surprised how wide they can be made within the original parts tolerances. Just grinding a small amount from the underside of the ends will make it clear as well and not be visible. Or notch the airdam itseld, they will all work. IMO, looks a lot better without a bumper. Anyone who purchases my fiberglass parts are welcome to contact me for help and advice, glad to help.