Glad to hear that you are happy with the Aerotronics folks. I am not installing an autopilot right now, but may in the future. I just did an approach to minimums in a twin (I was right seat) while bringing my aunt home and it was done by the autopilot! I was sold.
Finally home from my trip to help with my aunt. I can say nothing but the best things about Aerotronics. They were great to deal with and Jason Smith, who handles most of the experimental stuff was a real help in planning and design. He gave me lots of ideas to make the panel cheaper, but even better. Now remember, "cheaper" is a relative term. My cost was @ $18,000, but when I compared what I was getting with just buying the equipment and trying to do it all myself, I thought it was very fair. I only have about 9 wires to connect and some antennas to install and the AHARS to mount and I can run the system. Dynon makes upgrades easy, too.
In answer to your guestions. Yes the stiffener is hollow, the drill bit just cleans out a clean hole so the rod will slip in easily. I used steel rod on the rear because that what was laying around the shop, I think I used about 8 oz of epoxy, Then I decided to do the front and bought some aluminum rod, this time I used some very good construction adhesive, found at Lowes, its cheaper and more of it. You can drill some 1/16 holes down the stiffener, to see epoxy oozing out and let you know you have it down there. taper the end of the rod,so it wont push all the epoxy out in front of the rod as you push it in. I think one more important thing is to change the location of the hole in the bottom latch for the rear latch pin to give it more travel. This will insure the rear of the door will stay latched.I also added a bracket under the gas strut attachment point to spread the load on the door frame. Too date very happy with the mods, the door is much more ridged, I do not think it will fail again.
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