If you want, you can get an AMPS mount with a 1" ball that is a direct replacement for the back of the Garmin unit - pull out the 4 screws and the ball joint comes off. I've also seen a 1"ball to 17mm ball that 'might' work (RAM part). I did the AMPS plate on my Garmin.
I recently picked up the Garmin RV890 and my unit did not come with a 1 inch ball mount. It is also NOT 17mm which I ordered only to learn that it is much too small. I'm guessing it's a 20mm ball which seems to be an uncommon size. Installation instructions for the BuiltRite bracket are spot on. I opted for the metal brace for the back side of the dash mount that BuiltRite offers which will reduce the likelihood of damage to the dash plastic and allow me to load more weight without worry.
The 20mm adapter I ordered arrived today and it is also too small to mount to the RV890. I've ordered a 1 inch double ball adapter as recommended by sprayhawk808.
