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Although Mazda calls it a pulley boss, it differs from the typical boss, which is a raised surface. In this case it is a block of material functioning as a flange and shim, or separator.

After the introduction of the big nose crankshaft, the timing belt cog has only one job, to rotate the timing belt. Mazda introduced an additional part they call a "pulley boss" (Figure 5). The pulley boss is a sturdy cast hub that forms a mounting flange on the front of the crankshaft. It is bolted to the crankshaft. The timing belt cog is bolted to the back of the boss and the accessory pulleys are bolted to the front of the boss
http://www.miata.net/garage/crankshaft.html

This sounds like it is a custom part, but this is my guess. I found some translations where shaft boss is given as "cubo del eje."

http://automotivebee.com/glossary/b.htm
boss: An extension or strengthened section that holds the end of a pin or shaft. For example, the holes in the piston through which the piston pin is placed would easily break the thin walls of the piston when under pressure. The area around the hole (on the inner side of the piston) are strengthened to prevent breakage. This area is the piston boss. Also see gudgeon pin boss, horn boss, and piston pin boss.

ESPAÑOL - [ Translate this page ]**cubo del eje. shaft boss.** cubrechumacera. chafing batten. cubrejunta interior de los ángulos. bossom pieces. cuerda. chord. cuerpo de proa. forebody ...
www.uniovi.es/marina/utils/diccionario/c_ESPANOL.htm


LA REVOLUCIONARIA POLEA CAM y 1/2 - [ Translate this page ]
... Cuando haya quitado el tetón del cable de control, gire el módulo unos 180 grados
hasta que su parte abierta pueda ser desencajada del **cubo de la polea** base. ...
www.cerraarchery.com/Tecnica_ archivos/Cam%20y%20media.htm

OK