The enclosure cap fits into the top of the upper vertical enclosure pole connecting this with the circular G3 rod that holds the top of the enclosure netting in shape This cap is designed to fit a 25mm diameter upright pole and connect it with an 8mm G3 rod.Genuine JumpKing OEM spare parts. Select the right size for your trampoline for an easy fit. Extend the life of your trampoline. Size 8mm x 25mm