TromboneChat

Doug Elliott wrote: Loosening stuck mouthpiece parts:
 
You need a rawhide or wood mallet and a 9/16" box-end wrench for large shanks, or 1/2” for small shanks.  14mm and 13mm box-end wrenches will also work.
 
For the rim, one of two methods may work:
Tap around the top surface of the rim - sometimes that's all that's needed to loosen it.  If that doesn't work, hold the mouthpiece in your left hand with the rim facing away from you, and with the mallet hit the right edge of the rim downward with a glancing blow.  That puts force in the unscrewing direction.  Now turn the mouthpiece slightly and hit it again. Keep doing that until it loosens.  You might have to hit it pretty hard.  The wood or rawhide has enough friction to do the job but it won't hurt the plating.  Rubber mallets just bounce, plastic and anything harder can dent it.  Only use wood or rawhide.
 
For the shank, put the box end wrench on the hex.  Hit the opposite end of the wrench with the mallet, in the correct unscrewing direction. (Normal threads, turn left to loosen)
 
IMPACT, not brute force, is what works.

A little bit of Teflon tape on the threads will keep it from sticking.  Oil or grease doesn’t really help.

I've had so many questions about this that I decided to post instructions. I attached a PDF with these instruction plus pictures.

Loosening stuck mouthpiece parts:

You need a rawhide or wood mallet and a 9/16" box-end wrench for large shanks, or 1/2" for small shanks. 14mm and 13mm box-end wrenches will also work.

For the rim, one of two methods may work:
Tap around the top surface of the rim - sometimes that's all that's needed to loosen it. If that doesn't work, hold the mouthpiece in your left hand with the rim facing away from you, and with the mallet hit the right edge of the rim downward with a glancing blow. That puts force in the unscrewing direction. Now turn the mouthpiece slightly and hit it again. Keep doing that until it loosens. You might have to hit it pretty hard. The wood or rawhide has enough friction to do the job but it won't hurt the plating. Rubber mallets just bounce, plastic and anything harder can dent it. Only use wood or rawhide.

For the shank, put the box end wrench on the hex. Hit the opposite end of the wrench with the mallet, in the correct unscrewing direction. (Normal threads, turn left to loosen)

IMPACT, not brute force, is what works.

A little bit of Teflon tape on the threads will keep it from sticking. Oil or grease doesn't really help.