18N Specs: ID of 18mm / .709 inches OD 26mm / 1.024 inches Rim Width 4mm / .158 inches ID at rim/ cup joint 17mm / .669 inches Details about Wedge Horn Screw Rims: Wedge French horn mouthpiece rims are available with 750-36 threads (compatible with Moosewood, Giardinelli, Osmun, and Houser mouthpieces), and with _ÇÎÎÇÎ¢ÎÎÎ£Î_ÇÎ±metric_ÇÎÎÇÎ¢ÎÎÎ£ÎÎÇÎ 800-36 threads (compatible with PHC, Paxman, and similar European mouthpieces). Rims are available in four inner diameters (ID), ranging for 17mm (.669 inches) to 18mm (.709 inches). The ID of Wedge mouthpieces is measured at a depth of .04 inches into the cup, measured from the crest of the rim. Rim widths range from 3.75 mm (XN) to 5 mm (W). 750-36 Rims The diameter where the rim joins the cup is 16.5mm (.650 inches) for the 17mm to 17.75mm rims and 17mm (.669 inches) for the 18mm rims. The 16.5mm joint diameter work best with most 750-36 threaded cups from Moosewood and other manufacturers, although they can also be used with larger cups, producing a slight overhang. 18 ID mm rims have a 17mm joint diameter. They work best with Osmun wide cups and Moosewood BR and BW rims. The rim height above the joint is .11 inches or 2.79mm on the 750-36 rims.