True! I need a micrometer as Kenny s suggested. (but so expensive for my purpose.) I tried measuring with a ruler and then convert to millimeters but not positive i got it right. The new carburetor I purchased is 3/4 of an inch wide O.D. That came to 20mm from an online calculator. However, another website said that every centimeter on the ruler =10mm. That would make it 30mm. But that seemed really wrong . Confusing.