In order to preset a spring, you need to be sure that you have calculated what the set height needs to be and that there should be a guide to fit the inside of the particular spring to prevent it from being canted as it is being compressed. Unless you are properly equipped, the best way to set the spring is to put it in the gun and let it set itself to the height that is determined by the gun itself when cocked. Once assembled, cock the gun and let it set for a couple of minutes and then fire it. Once it has been cocked a couple of times it will determine its set length required perfectly.
Also when using a piece of all thread rod, you can knick the coils creating a "weak spot(s)" and possibly early spring failure.
