I boughtt a Shimano SM-MA-R160 Disc Brake Mount Adaptor for my bike.
It came with these black plastic things in the lower right corner of the photo below. I have a bunch and don't know what are they used for:
The stop rings go over the head of the bolt. The grooves on the ring line up with grooves on the bolt head and encourage it to keep from rattling loose. They are simply snapped or pushed over the head of the bolt once it is tightened in place.
I suspect that in some country, they may be a requirement for manufacturers. However, I haven't ever actually used them on any of my rigs. Some loctite and regular maintenance are what I tend to rely on. But, if you have had a bolt rattle out on you before, perhaps they could make you sleep easier at night.
Be sure to attach the caps when attaching the brake caliper fixing bolts.
They're called stop rings, and are intended to stop caliper bolts from loosening off due to vibration. I've never actually tried to use one - I just apply a medium strength (blue) threadlocker to the caliper bolts instead.
Stop ring. Should have been in the instructions.
Hopefully not very important if you have a lot.
Search on part # Y-8C5 11000
