Carb size question: diameter measured at the top or bottom?

[JP911]

When measuring a carb for size (like 40 vs. 46) do you measure the diameter at the top of the carb where it meets the velocity stack or at the bottom where it mates to the manifold?

[Henry Schmidt]

40 vs 46 mm refers to the diameter at the butterfly.
Because the butterfly is not actually round (more oval) you must measure the bore not the butterfly.
Measuring at the bottom would be close enough.

[JP911]

Thanks Henry!