Just before a very long ride, I installed ZTechnik Z6150 Accessory Mount and the Suburban Machinery BB3 Bar Backs on my 2007 BMW K1200GT. I use the mount for a Garmin Zumo 660 GPS.
Suburban Machinery makes a different bar back (BB4) for the K1300GT. There are other brands of handlebar extenders as well (Helibar and Verholen). It is unclear whether my results would apply to them.
I had problems with the original installation, just before the long ride. It was obvious that I needed longer screws to fit my Suburban Machinery bar backs. Inside the ZTechnik package, there was a note, saying that users with bar backs should call a toll free number. It was the holiday weekend, and there wasn't time to call and get replacement screws, so I purchased screws from a local supplier (Hillco Fasteners, http://www.hillcofasteners.com)
. They had stainless steel M8 x 45 mm screws, and non-stainless M8 x 50mm screws. They were out of the M8 x 50 mm stainless screws that day.
The aluminum spacers (sleeves / wide washers) would not fit over the 45 mm stainless screws from Hillco, because the "neck" of the screw was too wide. I couldn't test them. The aluminum spacers were a very tight fit over the non-stainless 50mm screws, and I used them for the long ride. When I returned, the screws had already started to rust, and it was challenging to remove the spacers without marring them.
After the ride, I called ZTechnik, and they sent me pairs of M8x50 and M8x55 stainless screws. I told them that I would test them and report back.
The M8 x 55 (55 mm) screws are too long. They bottom-out before the mount is tight. The M8 x 50 (50 mm) screws make a tight fit on my K1200GT. The mount and handlebars feel secure. I worry that they may be close to bottoming out, but they were good for me. In my opinion, M8 x 45 mm screws should also work. I have not tested them.
* M8x50 mm stainless screws worked well with my setup.
* M8x55 mm stainless screws are definitely too long.
* M8x45 mm stainless screws may be better, but were untested.
* Some brands of M8 screws (stainless and and non-stainless) have a "neck" that is too wide for the aluminum spacers (sleeves) that come with the ZTechnik mount. The screws supplied by ZTechnik did not have that problem.
* There are two other brands of handlebar extenders for the K1200GT (Helibar and Verholen). They were untested.
* The Suburban Machinery BB4 bar back for the K1300GT was also untested.