Its the springs that limit lift, the retainers will clear .600 but springs will bind at .575 and honestly, when you get past .550" you need stiffer springs and different lifters.
Its the retainers and locks that are made for 8mm or 3/8 stems, not springs. If you tried to use 3/8 retainers and locks on 8mm valve the valve would pull out of the retainer and lock when you took spring compressor off. Its 5/16's thats .0025" smaller than 8mm, 3/8's is .0625 larger.
