The idea here is to make lubricating the linear guide bearings easier. These go on pretty tight initially but quickly break in. I am in the process of testing them out long-term. I start with just one side of the bearing. Let those caps break in. Then put on the other side.
NOTE: This is an untested, experimental modification to the MGN12 linear guide bearing. Your mileage may vary. You will need to change from M2x10 to M2x12 screws. When lubricating, be sure the shuttle isn't directly over a hole in the rail. Your lube will just go down the hole then. Position the shuttle entirely over a contiguous flat section.