This stainless steel spring fits the valve, stem and check perfectly for 13XX and related models. The second photo shows it with the US-made valve stem and check, the third photo shows it with the Chinese-made stem and check. It's just stiff enough to allow the valve to seal consistently on the first pump for consistent power at any number of pumps, too light of a spring and you start getting inconsistent FPS from one shot to the next. If you have a hammer spring longer than 1.6" or are using a power adjuster or spring spacer to preload the hammer spring, you may need to cock the gun to pull the hammer away from the valve stem before pumping or the valve won't seal or the FPS may be inconsistent.

 Many have asked if this is the same spring we have listed as a light trigger spring for 13XX and 22XX, it's the same spring except the ends have been ground flat to optimize valve performance.