Trevor has a good point. If the thing will only be shown with the O-ring in place maybe you don't need the groove. But, if you did, you could do as Kris indicated. Sometimes the most straightforward way to do it is to draw the negative space, intersect it with the board. then you simply remove what you don't need. That's how I did it here. [image: 8583246533_103b3fbb9f_z.jpg]