I had a roof mounted bike rack system on my 6, but I thought it took away from the looks of the car. I decided to figure out a way to take advantage of the stock tow hook mounts in the rear. They are 28" apart.. very close to the 27" spacing of the roof mounted bar positions. There was absolutely no support online trying to figure out the bolt and thread spacing of the tow hooks.
I can definitively tell that they are 20mm bolts with 2.5mm thread pitch spacing.
After a ton of searching, I found a class 12.9 bolt with these specs in a 400mm length. About 16". With a clean 17mm Allen key head and the perfect amount of thread for this application. They were not cheap! About $74 for the two bolts shipped to my house, but worth every penny. I got them from a site called McMaster Carr. They have a great app for shopping and they bolts came in 1 day!
They fit perfectly inside 3/4" black polyethylene sprinkler pipe. Very strong, but flexible pipe. Feels very much like the plastic used in Yakima crossbars. I gave the bolts a couple coats of gloss black rustoleum to slow down the rust from the black oxide treated bolts. Hit the ends with a little anti seize so they would go in and out smoothly.
Once I figured that out it was simple. The bars will work with pretty much any roof mount bike rack. I opted for the Thule Big Mouth. I added some 3M reflective tape for increased visibility.
And before you ask, "why didn't you just get a hitch?" ... I did want to put a hitch on it. Don't like the looks. My setup can be removed in just few minutes, but is theft deterrent from the locked Thule rack. You can't take the Thule rack off when it is locked and you can't take out the bolts when the Thule rack is clamped down.
Now for a few pictures...
I added some 3M reflective tape in the last picture
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