Bach 12E vs Kelly SKY-SS
The Kelly SKY-SS has a wider cup diameter.
Mouthpiece A
Bach 12E
• Trombone
Cup diameter24.50 mm / 0.9646 in
Cup depthShallow
Rim width—
Throat diameter5.85 mm / 0.2303 in
Backbore—
Finish—
Material—
Best suited for
High-volume projection — open throat for power
vs
Mouthpiece B
Kelly SKY-SS
• Trombone
LeadCup diameter24.64 mm / 0.9701 in
Cup depthShallow
Rim width—
Throat diameter—
BackboreS
Finish—
Material316 Surgical Stainless Steel
Best suited for
Professional lead trombone
Cup Diameter
The Kelly SKY-SS has a cup diameter of 24.64 mm / 0.9701 in compared to 24.50 mm / 0.9646 in on the Bach 12E — a difference of 0.14 mm / 0.0055 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a shallow cup depth, so the sound character from depth alone will be very similar.
Material
The Bach 12E is described as unspecified, and the Kelly SKY-SS as 316 Surgical Stainless Steel. Material affects mass, warmth, and how quickly the piece responds.
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.