The milling makes the hole exactly the same as all new X-valves and original classic valves. These valves all accept the classic, ULT and retro on-off assemblies. The ULT just isn't designed to work in the classic valve. When using the on-off assemblies in these valves, you need two orings above the on-off top except for the ULT which has the outer oring in a groove around the top of the assembly.
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