A top-seated mouthpiece is illustrated below:

A better-seating replacement may be made by increasing the diameter of the end of the shank/stem. However, this requires one to know what the appropriate diameter is and it’s a bit of a challenge to ascertain this. Measuring the inside diameter of the receiver at the appropriate point is exceptionally difficult, even when you have the special equipment needed.

One way is to build up the end diameter of the stem/shank with tape until it is a snug fit in the receiver. The diagram below shows the idea when working with a receiver with a stepped ending:

To fo this, you may use the tape from your kitchen drawer and progressively build up the end of the shank/stem diameter as shown in the illustration below:

In this illustration, insulating tape was used so that it showed up well against the mouthpiece body but finer tape will give more precision. Also, the shorter the width of the tape the better as it is the diameter at the very end of the shank/stem which you need to know.

Before you put such a taped-up mouthpiece into the receiver of an instrument, you should make sure that the tape is sticking tightly to the mouthpiece so as not to leave it behind when you remove the mouthpiece from the receiver. If a small section of tape like that shown above, is insufficient to provide a snug fit and the mouthpiece still wobbles, you’ll need to need to put a longer piece of tape around the mouthpiece exit diameter and one which passed over halfway round the diameter. You may build the tape up slowly until the mouthpiece fits snugly into the receiver and you can feel it seating tightly.

Once this is achieved, you will need to measure the diameter of the mouthpiece end, over the tape. To do this, you may use a digital vernier caliper. For details of such devices, please see the entry under Digital Vernier Calipers

The anvil or measuring surfaces of digital calipers have two different sections, one being wedge-shaped and one flat. You should use the flat portion of the caliper to measure the diameter as the tape is soft and the wedge-shaped portion will dig into the tape. That said, the flat section will also tend to dig in if you apply too much pressure on the tape so the process will call for a certain amount of delicate manipulation of the calipers.

If the receiver you’re dealing with has a smooth entry into the instrument body, then you’re finished at this stage and may go onto the section below: Adjusting the Shank/Stem Taper.

Checking the Mouthpiece Depth Seating

Once the mouthpiece taper has been dealt with, the next task is to check how deeply the shank/stem seats inside the receiver. The diagram below shows two conditions: one where the mouthpiece seating is acceptable and one where the gap between the mouthpiece’s lower edge and the receiver lip is somewhat excessive.

Adjusting the Shank/Stem Taper