Reinstalling the rear under tray / shield on your DB9 is easy. I am assuming you’ve already removed it for some other service task (like changing your rear differential fluid), and you can check out my previous post and video on how to remove the rear under tray.
While I had the tray off my car, I took the time to clean it up so it looked good as new. This was simply using my pressure washer to clean it off (you could use a hose and a scrubby brush equally well), and then after it was dry I treated it to an application of Meguiar’s Ultimate Black Plastic Restorer to make it look as good as new. Check out the results!
The only tools you’ll need are the same as taking it off the car, a T30 Torx socket.
Installation is simply holding the tray up in place (it’s light so this isn’t very hard) and reinstalling the nine (9) T30 Torx screws (Aston Martin P/N 6G33-100134-AB available for about $0.35 each online if you’ve lost one). I put one in the center of the back row first loosely, then one in the middle of the front row to hold it up while I crawl around under it doing the rest. Don’t tighten any of them until you have all the screws started (you may need to wiggle it around a bit to line up the holes).
I couldn’t find a specifically listed torque spec in the Official Aston Martin Workshop Manual (there is no section on removing the under trays surprisingly since its referenced at least 20+ times), but in the back they listed the generic torque for an M8 screw (which these are) as 10 Nm (7 ft-lbs). [If someone knows better – please comment so I can update this] I used my judgement and went with firmly snug since the tray is just HDPE plastic.
Here is a short video of the process I followed: