These days there are basically three types of ways to fit a crystal. Each will require its own specialized techniques to properly install. (Removing can usually be accomplished by pressing with the thumbs on the inside…)Due to the lack of documentation it may be an appropriate time to consider the establishment a thread of threads for crystal fitting. Thoughts, watchmakers?
1) Friction fitting
This is the method almost all quality sapphire and mineral glasses are installed. The gasket (11) is normally a hard nylon or similar material, and can be either "L" shaped as shown or "I" shaped. The case has a slight undercut or angle cut in the vertical wall so the gasket will flow into it and lock itself in place. Simple friction between the glass and the gasket holds the glass in place. The size of the gasket is critical in the fitting of this type, it has to be the right thickness and the right diameter. To install you need a press that pushes on the outer edge of the glass or uniformly over the entire surface, and the press face and the case need to be parallel, otherwise the glass will become cocked.
2) Tension ring fitting (r - external, l - internal)
These two are the more common type of crystal fittings for the average diver's, or highly water tight watch, with a plastic crystal. Although, both use a "tension ring" (shown in pink) the installation methods are quite different.
The external tension ring (most commonly seen on Rolexes with plastic crystals) is easily installed by placing the skirt of the crystal on the case so it goes around the raised wall and the tension ring is pressed on. The tension ring compresses the plastic into a good seal, and friction holds the entire assembly together. The press should only contact the metal ring.
The internal tension ring (most common on the rest of the world's plastic crystalled diver's and diver style watches, as well as anything with over 3 ATM water resistance), works by having the plastic skirt squeezed between the case wall and the internal ring (under compression, so 'tension ring' is a misnomer). To install these you should use a press with cone shaped dies. The die wall should come in contact with the crystal at an angle so the force applied by the press both pushes the crystal down as well as tries to reduce the diameter, or roll the skirt under. This helps get the crystal skirt started in the opening in the case.
3) Mechanical locking (r - clamped, l - wedge)
This method is interesting as this method has the highest pressure resistance and the lowest.
The clamped method is the most complicated method to manufacture as the clamp is a separate part and there has to be threads, or a bayonet fitting in the case. But it is the easiest to install, drop in the gasket, the crystal and clamp them together. This method along with the "L" gasket friction fitting offer the best water resistance.
The wedge is the weakest, as far as water resistance goes, and is the only one that uses the "claw"-type crystal tool. To install the skirt must be mechanically compressed to reduce the diameter by approximately .20 to .30 mm then inserted. When the mechanical compression is relaxed the skirt's upper coned shape will force the bottom down into contact with the top surface of the case creating a seal. This is also the most commonly messed up crystal installation. Most people pick the wrong size crystal (one or two sizes too small) and the crystal does not wedge downward with much, if any force. If the crystal can be spun in the case, the crystal is too small. (Some glass in pocket watches are installed in a similar manner, although the ring is usually warmed instead of trying to compress the glass.)
Both of these methods were/are popular with front loaders.
4) Other less common methods (these days at any rate)
Glue. Older square or non-round crystals are usually glued in place.
Seiko. In keeping with doing things different, many older Seikos use mineral glass, a soft gasket and a external, snap on clamp to secure the crystal. The external clamp presses down on the glass pushing it into a soft "L" shaped gasket crerating the seal. The clamp snaps on to the case.