Brass extruder gear is the standard stock driver gear in metal or plastic extruder that comes with stock Ender 3.
Brass is the stock one where it will wear out faster, but it grip the filament just right and won’t over grip it. May need to replace after long usage.
For Stainless Steel, it’s much suitable for abrasive filament or even standard filament, can increase the life of the drive gear, but don’t over tight it. It may cause the material to flow inconsistent and the print will be under or over extrude at certain level.
Material: Stainless Steel / Brass
Inner Diameter: 5mm
Outer Diameter: 11mm
Stainless Steel / Brass Drive Gear x1