The Pro Jump F40 Bouncing Stilt Spring features the following specifications... Suitable For - The Junior Pro-Jump F-Series Black Spring is designed to fit the F-Series Pro-Jump Stilts as a replacement or upgrade part Compatible With - This stilt spring MAY also be compatible with other brands of Powerbocks (however we can not guarantee this ) including PoweriZer, PowerStrider, Fly Jumper, Jolly Jumper, Air Trekker, Air Runner, Speed Jumper, PowerStrider, 7 League Boots, PowerSkip and PoweriSer. If you are unsure after checking the photos and description, the Pro-Jump Forum (search Google) may be able to advise if you post photos of your Jumping Stilts. Hi-Tech Construction - Specially produced for Pro-Jump, the F40 black springs are manufactured using a carbon and glass fibre mix for the best balance of strength and flexibility Weight - The springs weigh in at approximately 360g each Quantity Supplied - One spring is supplied What You Need To Fit - 4 bolts (not supplied) are needed to fit the spring to the frame Warranty - The Children's F-Series Powerbock Spring comes with a full 1 month warranty for piece of mind Please Note: If you are buying a single spring to replace only one of the springs on your stilts, please check the number on the bottom end of the springs. This number ('30' upto '70') is the spring strength. To ensure your stilt springs are balanced choose the matching strength number from the size selection above ONLY if the spring has not had much usage. However, if the the springs have had a lot of usage it becomes difficult to correctly match the springs and you may want to replace the broken spring with a spring that is a few kg's lower than your original. In most cases we recommend if your springs have had a lot of usage you purchase a pair of springs rather than just one. Please click the Size Guide tab above for further details.