The rockers are surface hardened. They wear through first on a line across the face. This line eventually grows deeper and is like a bump in the road for the valve following the cam. The adjustment is still achievable, but the the bump causes the thing to sound as though the gapwere larger. Or something like that.
The rockers are easily removed with a special spring depressor. The valve is depressed and the lifter just lifts off (once the secondary spring is poped off.
Bosch Master, ASE Master, L1
33 years MB technician