Don't use rivets, even though that is what Volvo used. The mud flaps tie in to the plastic on the bumpers and other parts of the car, and if you work on the car you're always having to take them off. Go down to a hardware like Lowe's or Home Depot store and buy the correct diameter and length stainless steel bolts. Don't use regular steel, even coated, as the salt on the road will corrode them and make them difficult to remove. Clearance on an 850 is always an issue, and if you have to replace the mud flaps the stainless bolts make the job all that much easier. The rivets were a cost-cutting measure by Volvo, which should have known better and used bolts like they did on all their earlier models.