It is intended to be clamped and bolted to the firewall and a reinforcing plate on the cabin side through a 5/8″ hole. Use the steel AN924-6 bulkhead nut included along with a #8 screw and AN363 all metal stop nut to prevent flange rotation. These items are sold separately. Construct the 4″ X 4″ (or larger) reinforcing plate from 2024-T3 .065 aluminum sheet and rivet it along its edges with MS20470AD-4 rivets to the cabin side of the firewall to give the typically thin firewall material more stiffness. Do not omit this reinforcement plate. Otherwise normal vibration may loosen the fitting mounting. Aluminum plate and rivets are not included with mounting flange.