An air cycle machine includes a bearing housing with a shaft bore. A rotor shaft is arranged in the shaft bore and includes knife edges extending from the rotor shaft. A seal engages the shaft bore and includes an outer diameter and an inner diameter providing a seal land axially aligned with the knife edges. A radial clearance between the seal land and the knife edges is nominally 0.006 inch. The inner diameter is arranged between lateral sides of the seal that define a width. In one example, a tapered surface extending at an angle relative to the outer diameter from the lateral side to the seal land, the tapered surface facing the large end. The outer diameter to the inner diameter provides a first ratio of 1.25-1.28, and the inner diameter to the width provides a second ratio of 4.33-5.58.