If you're looking to tweak your power, these steel power up rings are a nice and easy method to increase your power. They are designed for 13mm springs and 9mm guides. They come as a 3 piece set with 2/4/6mm sizes. They work on most 9mm guides and with most springs. How many you can use will depend on the spring (due to different compression lengths). These are made of steel and black oxide coated, ensuring longevity and reduced friction when used with a suitable lubricant, due to the porous nature and better retention of lubricants.