We stock parts on models dating from the early '50's. Most part are interchangeable between various models. It depends on the age of the machine, and how much you want to do. Start by giving us the serial number, or age of your unit, and we'll explain what can be done, and the cost of doing it. About 3 degrees.
Critical angles are functions of the refractive index for a particular gem.
Main angles are choosen by convention, and are selected to give maximum brilliace. Their values are close to critical angle values, however, pavilion main angles should not be less than the critical angle of the material, or light will be refracted.