Solid PTFE, with metal band and choker spring. Stem diameter: 3/8" (0,375"), valve guide diameter: 0,500"; over all length of seal: 0,507"; outside diameter of seal: 0,600". Kibblewhite Precision Machining custom valve stem seals are a quality product and suitable for any and all custom application. Note: Below mentioned valve stem diameters are for reference only. - 36-84 B.T. = 3/8" (.3750") - 57-83 XL intake = 0.3090" - 57-83 XL exhaust = 0.3384" - 84-04 B.T.; 86-03 XL = 5/16" (.311") - 05-16 TCA/B; 04-22 XL; 08-12 XR = 7mm (.2760") - 02-17 V-Rod = 6mm (0.2345") - 17-24 M8 = 6mm (0.2345") Installation note: Valve guides made of PTFE require another fitment technique than 'regular' or non-PTFE valve guides, this is due to the PTFE material. They are typically 0.020” (0.50mm) smaller than the valve stem and guide top they’re designed to fit. - The guide top should have a decent chamfer - The valve stem should have a decent chamfer, but a mylar seal installation protector / installation capsule (501823 green 5/16" / 0.3125", 501824 Yellow 11/32" / 0.34375", 501825 3/8" / 0.375") is advised for easier installation over the valve stem - You have to kind of roll the seal around the valve tip while exerting downward pressure to get it started - Don't push directly on the very top of the seal to get it on to the guide, you’ll collapse the sealing area and tear the seal - A 12 point 12mm socket catches the shoulder of the guide pretty well and can be used to push the seal on to the guide until it just bottoms out. The choker wire around sealing area and the band around the area that fits the guide, combined with the hardness of the material itself makes the seal a tight fit and little harder to install than regular Viton seals. PTFE material does not have 'memory' and therefore is subject to creep. This creep allows the seal to conform to mating surfaces. Because of the ‘creep’ characteristic, the seal relaxes a bit after installation, and doesn’t drag on the valve stem as hard as it does during installation. The twin-spring design provides continuous tension on the valve guide and the valve stem of the valve during it's lifetime.