Feeling Mullen Gag bit

The correct size of the bit

You can measure the size of the bit by determining the width of the mouthpiece. You measure the mouthpiece between the bit rings. Figure A shows by the arrow what exactly you need to measure.

How do you measure the thickness of the mouthpiece?

You measure the thickness of the mouthpiece a little bit next to the bit rings. Picture B shows the arrow where you can measure the thickness of the mouthpiece.