Bach 6½AL (small shank) vs Schilke 51D
The Schilke 51D has a wider cup diameter, the Bach 6½AL (small shank) features a deeper cup, the Schilke 51D has a larger throat opening.
Cup Diameter
The Schilke 51D has a cup diameter of 25.55 mm / 1.0059 in compared to 25.40 mm / 1.0000 in on the Bach 6½AL (small shank) — a difference of 0.15 mm / 0.0059 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 6½AL (small shank) has a deep cup while the Schilke 51D has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Schilke 51D has a wider throat (7.04 mm / 0.2772 in vs 6.63 mm / 0.2610 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.