I'm using three of these RAM U-bolt mounts on a Trek DS 8.5 hybrid bicycle. Although the 31.8mm diameter handlebars have a tapered diamater rather than a fixed diameter, I was able to tighten this mount down securely with no problem by using the smaller of the two U-bolts. The larger 1.5" U-bolt also fit nicely over the fattest 31.8mm section of the handlebars. The V notch on the bottom of the mount combined with the U-bolt allows easy tightening against a round surface with increasing diamater. I use it to hold a Garmin Oregon 450, a Samsung Galaxy Player WiFi 5.0, and a flashlight/headlight. I appreciate that the U-bolts I received in this kit are painted black: The black hardware blend better with my black handlebars.
One of my favorite features of RAM products in the ball-and-socket joint. Once you tighten it down, it stays put. However, there's still a little bit of flex in the rubber balls so that whole setup absorbs some shock. I appreciate that on my mountain bike.
The kit includes (see additional uploaded picture):
The aluminum mount with 1" rubber ball
2 black U-bolts--a smaller 1" width one and a larger 1.5" width
2 nuts with nylon friction inserts
2 rubber nubs to cover the U-bolt ends
1 plastic adapter insert to narrow the U-bolt down for use on handlebars as small as 1/2" in diameter
Be careful with part numbers. RAM-B-231Z items seem to have multiple configurations on RAM's website. Some hardware is zinc coated, some is stainless steel, and some is black powder coated. I ordered RAM-B-231ZU from Amazon retailer GPSCheap and received the parts as indicated above. I ordered another RAM-B-231ZU from GPSCity and received the zinc coated 1" U-bolt only.
The rubber nubs were too loose to use in my application, partly because their inner diameter is too large and partly because I didn't have enough of the U-bolts exposed for them to be very useful or secure. Also, the kit does not come with washers (-1 star) and you'll likely scratch the mount with the nuts as you tighten if you don't add your own washers. I added washers in order to protect the mount. Be cautious when installing: If you're metal against metal, you may cause some scratching with the U-bolt as you position it.