Bellmouth-Style Stacks Improvements in flow, torque and Spun from 1/8" -3mm- thick 6061 Aluminum Available with 3", 4" or 5" inlet I.D. Sold as each 3" I.D. Bellmouth OD: 6.000" 4" Bellmouth OD: 7.000" 5" I.D. Bellmouth OD: 8.750"