I have the Pro Design filter system on my Raptor with the foam filter, not being a fan of gauze type filters in the AZ dust. I assume you find the K&N a tad large for the airbox as was my foam filter. It is necessary to trim off the fins in the bottom of the airbox enough for the filter to fit easily. The fins were designed to support the mickey mouse stock filter & serve no purpose with the Pro Design system so you may trim them off as much as you wish.
Be sure to have the O-ring fully seated within the billet adaptor before mounting to the air box. I used some Vaseline to hold it in the groove.
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Thanks for the info hot_shoe_cv. I called Pro Design this morning and found out that there should have been some instruction with my kit that I did not get. I understand your concern about the dessert dust and the K&N filter, but with an outerwear on the filter you should be alright...