Figure 4-199.  Inspection hole. 1/4" minimum 1" maximum 7 " Skin Reinforcement (doubler) plate Access hole cover—thickness of skin Reinforcement material—Alclad$^{TM}$ 2024-T3  same gauge or one gauge heavier Rivets—material thickness of 0.040"  or less, use $^{1}$/8" rivets Rivets—material thickness greater than  0.040" use $^{5}$/32" rivets Access hole—clean, smooth, and round; length is minimum of 7" to match the reinforcement doubler that is being installed. Plate nut 4-115