Support BladeForums! Paid memberships don't see ads! The Umnumzaan is my favorite out of large & small 21’s and small Inkosi (I don’t have a large Inkosi...yet). The orings are 1/8 id x 1/4 od, a size commonly available at hardware stores. The material from CRK is polyurethane, I believe, whereas hardware stores usually stock Buna-N or occasionally Viton. I’m not sure how much that really matters, as I’ve never gotten around to getting replacements from CRK. The HW store ones are cheep and seem to work well enough. I replace them every now and again, maybe once or twice a year during a rebuild. I love the aesthetics of the knife, especially the blade grinds. I also love how sturdy it feels in hand. Most of all I love the unique feel & motion of deployment (although like many, my thumb did rebel during break in). With the pivot adjusted just right, it thumb-flips open with a secure but simultaneously soft and muted “thug.” Just awesome. JJ, when you finally pick up your first Umnumzaan, stick with it until both the knife and your thumb have had ample chance to adjust to one another, if needed. Totally worth it.