Just ordered one from mcmaster. The 35mm one is an astonishingly perfect size for my Roco w/ Progressive Ti spring. 35mm ID, 52mm OD. The preload collar has a 34.47mm lip to seat the spring, which has a 51.56mm OD. I may need to make a small delrin piece to extend the lip on the preload collar just a tiny bit, but that's easy.