Doing the fronts was pretty easy, but the rears are a huge PITA, so I wanted to post my own experience to help the next weekend warrior wrenching on their allroad. Although the Arnott Gen2s and Bilsteins came with illustrated pdf instructions, they are woefully inadequate. They remind me of the Bentley paper manual: e.g., it might say remove part Z, as if part Z would just fly right into your hands, when in fact it totally neglects to mention the fact that you might also have to perform X and Y to get part Z to even budge. So that is the reason for this write-up.
Disclaimer: I am NOT a mechanic, so proceed at your own risk!
ratcheting strap with metal hooks (buy a cheap one because it WILL get filthy with grease and grime)
small flat crowbar
wrenches: 10mm, 18mm (nice to have 2), 25mm (or crescent that can span 25mm)
sockets: 10mm, 13mm, 16mm (deep, or 1/2" impact), 17mm, 18mm
Put car in jack mode:
Chock wheels, use your 17mm socket to get the lug nuts off, remove wheels, place jack stand under car.
(1) Remove 10mm air fitting farthest from you.
(2) Remove 13mm bolts for upper mounting plate.
(3) Remove the two 10mm bolts for the rock shield on the lower control arm. You have to do this to access the lower shock bolt (in next step.)
(3) Remove lower shock bolt 18mm.
(4) The Arnott instructions say collapse shock by pushing down on the upper mounting plate and dislocate the top of the spring from its perch--if only it were so easy! Apparently some of you guys have done this by having someone stand on the rotors and bouncing up and down while holding the cargo rail. I wasn't so lucky, and couldn't get it to dislocate. Seeing that the shock needs to collapse several inches to dislocate, I tried something else. DON'T do this, because it doesn't work!
(5) Wedge your crow bar between the spring and the perch, and place the hooks of your ratcheting strap into the holes on the upper mount of the spring.
(6) Wrap ratcheting strap under and around the lower control arm. EDIT: I realized after writing this up that you could always try to skip ahead to step #11. I have not personally tried it, but if you have enough space to wrap the ratcheting strap around the Tee outlet of the shock, then you can skip steps 7-10! If someone tries this then let me know and I'll make the edit to the DIY.
(7) Crank the ratchet!
(8) So you think it will come out now? Nope! Still have lots more at the top of the spring that won't clear the perch. I couldn't crank the ratchet more (or at least I wasn't sure if it was strong enough to tolerate a greater load.) Even bouncing on the rotor and trying to use another strap to pull on the spring was futile. This is when the cursing started...
(9) Place a jack about under the rear differential to support the subframe. Lower the subframe by loosening the 18mm bolts (can't miss them) on either side of the upper control arm. Not sure if there is an easier way, again I am not a mechanic, but it worked. Also might want to note the position of the bolt head in relation to the large aluminum washer on the subframe bushing, so that when you torque it back on it will be close to its original position. May want to think about an alignment if you do this.
(10) With the subframe dropped about 3/4 of an inch, you'll be able to get that spring out FINALLY.
(11) Or so you think. The problem now is that the shock still wants to expand, and so to get the spring out the mounting plate still has to clear the perch. Time to wrap your trusty ratcheting strap around the shock itself, which will collapse it again to allow you to rotate it and pull it out.
(12) If you're installing new Bilsteins, skip this step. If you're going to need to keep your OEM shocks, you'll need to separate them from the spring: remove the spring cap 12mm allen, and then 16mm bolt sitting under the cap. Follow the Arnott instructions on separating.
(13) Remove 17mm nut on the Bilsteins, and place spacer on shaft.
(14) Insert Bilsteins into new air spring and torque the 17mm nut on the shaft (grip spring/shock so it doesn't spin).
(15) Replace 25mm cap and collapse the spring about 2 inches.
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(16) Thread air fitting cap onto bottom of spring. If you are holding it upside-down, it is the hole farther from you, which will be the hole closer when the spring is installed right-side up.
(17) This step is critical! If you skip this, like I did initially, the spring will not inflate and you will think Arnott sent you a defective part. You must push the bright aluminum sleeve/O-ring that is midway down the barrel of the shock up towards the air spring that you just collapsed. You will have much more leverage and space to work with if you do this while the spring is still outside the car. EDIT: make sure to lubricate the O-ring
Since I did this step after installing the spring, I had to use a screwdriver to push the sleeve up.
(18) Install new Arnott spring. To fit under the perch, I had to compress the mounting plate with the ratcheting strap around the lower control arm (de ja vu).
(19) Thread air fitting from compressor.
(20) Replace the 2 bolts on the top mounting plate of the spring.
(21) Replace the lower shock bolt. This is tricky because the shock needs to be elevated for the bolt to align with the shock bottom. Use the crow bar to lift gently -- don't want to damage the finish of the shock.
(22) Remember to re-torque the sub-frame bolts! Use a long breaker bar.
(23) Replace the rock shield.
(24) Put wheel and lug bolts back on. Remove jack stands. SLOWLY lower car (remember, the air spring has no air in it!)
(25) Start car. Kill jack mode. Let the compressor fill your new air spring!