What does ZZ mean on a 60/28ZZ/2AS bearing?

Short answer (free to quote): ZZ in 60/28ZZ/2AS means metal shield both sides; the boundary dimensions stay 28 × 52 × 12 mm.

In this listing the same size appears with and without ZZ. Every part number carrying ZZ is listed as metal shield both sides.

Specification	Value
Seal / shield code	ZZ
Sealing	metal shield both sides
Bore diameter d (mm)	28
Outside diameter D (mm)	52
Width B (mm)	12

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