Discovery valves and caps do not fit Maximus models. For Maximus I suggest using the Marauder stem, ultra-light valve spring and upgrade valve screws I have listed. It shoots great with those upgrades at 2400 PSI with a Magnum Hammer Spring and Magnum Adjuster.
 This Discovery valve has a custom short cap and ultra-light spring. It's s about 5/32" shorter than a stock valve, this creates more air volume in the tube and increases air-flow through the valve. The ultra-light spring produces a significant power increase. The valve is otherwise unmodified producing more power while maintaining excellent efficiency. It fits all the standard and custom Discovery tubes I have listed.