I don't think "nobles" refers to quality; I think it refers to importance.

This expression seems to be primarily North African; at least the examples I find seem to come from there. This is true of the following very useful document on industrial Pièce de Rechange (PDR). It classifies parts in three groups, of ascending order of importance, judging by the graphic on p. 8, "Catégorisation de la PdR", which has an arrow with three buttons on it of increasing size, labelled "Pièces Standards", "Composants & Accessoires" and "Pièces Nobles". The last of these also has the label "Sûreté & Fiabilité Complexité Engin.". So Pièces Nobles seem to be the most complex and important.
Examples are given on the next three pages:
Catégorie 1 : Pièces Standards
Joints, roulements, filtres, reniflards, boulons, vis [nuts and bolts, you might say]
Catégorie 2 : Composants & Accessoires
butées, filtres à air turbines, boulonnerie spécifique, paliers, accouplements, pompes de lubrification
Catégorie 3 : Pièces nobles
Arbres, chemises, roues, diffuseurs"
http://www.sonatrach.com/PDF/besoins.pdf

Well, I think the last of these corresponds to what are known as critical parts or spares.

"While no common, industry-wide accepted definition of “Critical Spares” exists at this time, these items generally have the following characteristics:
— They are critical to the ongoing operation of the asset they support
— They are generally equipment components or systems rather than individual parts
— They typically have long lead times
— They are generally high-dollar items"
http://www.scottmadden.com/?a=strm&aid=449

"Genox and our established network of partners/distributors worldwide hold large quantities of spare parts including blades, screens, belts, electrical parts and other critical spares."
http://www.genoxtech.com/Services.aspx

"Bin stock items are materials that have little individual value with high volume usage. The examples might be small bolts, nuts, washers, cotter pins, etc. [...]
Critical or insurance spares are those items that may not have much usage, but due to order, manufacture, and delivery times must be kept in stock in case they are needed. [...]"
http://reliabilityweb.com/index.php/articles/maintenance_inv...