I've just done that same mod! The spring I used was out of a retractable ball pen, you know, push the button and the tip emerges type pen! And I must say it is spot on!! Just be a little careful on the adjustment of the screw that is in front of the trigger. Too far in and the gun won't cock. Give the bolt a few back and forths while screwing it out until it catches, then screw it out another half turn to a turn to be on the safe side! You may want to cock it without a pellet and then give the gun a few taps around the stock and trigger area to make sure the gun won't accidentally fire! Good luck on the mod, it's easy to do and changes the dynamics of the disco dramatically! :)