The Mini Cooper R53 has a three groove valve keeper/lock design. They "lock" into 3 grooves in the valve stem at the top and hold the retainers in place. From the factory, there tolerance is so loose, that while it does hold the valve, it does so in a very "sloppy" manner. The valve moves around so much, that it slowly removes metal, and wears both the valve and keeper, again, so much, that I have seen them all but completely worn way.
What we do is hand lapp the contact sides of each the 32 keeper halves, and remove about 0.25 of a millimeter in order to retain the valves tighter, and hold them with a more properly placed contact area.
The result of the lapping will be to raise the keeper slightly up and increase the contact area on both the valve stem, and inside the retainer itself. This will make for a stronger valvetrain, with less vibration, and oscillation of the valve at high Rim's.
Contact us regarding modifications to your Mini Cooper valve keepers, and rocker arms.