Home > GR Mouthpieces School > Gap / Blow Resistance
The Gap is a very critical area of the match between player, mouthpiece and instrument.
Variation in this system due to the taper in the receiver, age of the instrument, size of mouthpiece shank, and poor tolerances by manufactures can be an area for trouble.

GR Technologies has defined the Gap components and their working relationship
These components are the Mouthpiece Shank, Receiver, and Leadpipe.

1. Distance from the end of the receiver to the leadpipe venturi.
2. Engagement of the mouthpiece shank into the leadpipe.
3. Exit diameter of the mouthpiece shank.
4. Leadpipe venturi.
5. Exit wall thickness or the mouthpiece shank.
6. Internal diameter of the receiver.
We use a formula to set gap. A change of .001" on the diameter of the shank will cause a difference of about .020" in the gap distance.
You may need to take your horn and have a professional measure it. If you do it at home make sure you do it several times and your numbers repeat.
If these numbers are set exact to this formula you will have the optimum or Zero Gap.
If the gap is greater it is a positive (+) gap measurement and the result will be a bit more resistance, although, the horn will slot and speak well. If the gap is less it is a minus (-) gap measurement and the result will cause less resistance, as well as, difficulty slotting. Some players like this condition and prefer the gap to the minus side.
If your gap is measured correctly and you find it within
.030" of the Zero positions you should be fine.
Formula: Exit wall of mouthpiece (#5) multiplied times 5.
Example, exit wall of mouthpiece = .025" X 5=. 125" .
Next take Venturi (#4) and subtract it from the Receiver ID (#6). You will need to divide this number by “2” to find the” effective wall thickness” of the leadpipe, and finally multiply the” effective wall thickness” by 1.5.
Example, Venturi is .345" and Receiver ID is .385. Subtract .345 from .385 and divide it by 2, you get an effective leadpipe wall of .020, multiply it by 1.5 =.030" . Now add the .125 and .030 and you get a Zero Gap of .155.
We find this formula to work very well. If you are getting good results there is no need for change. If you make a change measure the gap and calculate the change. It is not expensive to have the Gap adjusted. We recommend you set your gap and leave it alone. Constant change in this area will cause your playing to be insecure.

There are companies out there who manufacture "sleeves" which fit on to an altered mouthpiece shank. These sleeves are supposed to offer the player options in gap adjustment. Our finding is that the sleeves on the market today do not meet their advertised spec, plus they have a major design flaw. These sleeves are secured to the mouthpiece shank using a rubber O ring. This O ring acts as an insulator which deadens accoustical feedback to the player, inhibiting response and overall performance. This is why sleeves are seldom used by working professionals. Your best option is a custom mouthpiece with the shank designed specifically for your horn, or have the horn adjusted to gap properly with your preferred mouthpiece manufacturer.

Blow Resistance

Blow resistance is the acoustical resistance created by the mouthpiece when playing the instrument.
  • Such resistance depends on the specific design of the cylindrical bore and the backbore.
  • The blow resistance number that we use is a ratio, which is a multiple of the trumpet bore velocity head.
  • The mouthpiece blow resistance affects the acoustical impedance of the resonating air column.
  • We use this when analyzing mouthpieces and matching the player to the mouthpiece.
  • The bore size alone doesn't provide enough information to accurately make a match of the mouthpiece, horn, and player.
  • Blow resistance is a function of diameter and length.
  • We also calculate the volume of each mouthpiece.
  • This includes the cup, bore and backbore.