Each CQ comes with 4 short screws and 4 longer screws. The longer screws are used for the 1 1/2 inch vents, the short screws are used for the 2 and 3 inch vents. However, its fully acceptable to put all in for a more secure mount if desired.
Also only 3 screws are normally used. This is because the pitch of the roof makes putting the screw in the back problematic. If it can be reached, go ahead and install. But if its not, use two on the side and one on the front. If totally tightened into the vent it will be centered.
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