Grey (unfinished) calipers are the stock part used around the world. However, in the US, it could also mean it came equipped with one of the M Sport Brakes options (see below). Following the logic in the BMW parts program, there are multiple M Sport brakes for US cars. It's not obvious which brakes BMW installed on your car. The option code remained the same (S2NHA) but there are two different sets of part numbers related to that code. You definitely want to be clear on which brakes you have because rotors of different sizes are not interchangeable. One dead giveaway is from the rear caliper - a single piston caliper is for the stock 335i brake sizes; a 2-piston Brembo caliper is for the larger brakes.
US M Sport trim package (P337A): The M Sport is one of the trim packages offered when the car is ordered (base, Luxury, Modern, Sport, or M Sport). It was around $3,000 and got you 18" wheels, M steering wheel, M Sport aerodynamic kit, Sport instrument cluster, and glossy Shadowline black exterior trim. The brakes are mostly the same as the other trim lines except for the front pads. The front pads were actually the same as used on the M Performance accessory kit. Many drivers complain that they lack feel and initial bite. Luckily all of the F30 335 calipers share the same pad shape so pads can be interchanged. The rotors and rear brakes are all the same as a standard 335i/435i.
US M Sport Brake Upgrade 1 (S2NHA) (pads only): In the BMW parts database are pads listed as "M Sport S2NHA" that fit the stock 4 piston front calipers and the
single-piston rear calipers. These also happen to be the stock base pads used in Europe. This was a $350-400 option on 2012-2013 cars and could be added to ANY trim package (even a base model). The Euro pads have much more bite and dust a lot more than the stock pads. The bite makes the brakes feel stronger but the dust can be excessive for some people. The rotors are blank - no drilling, dimpling, or slotting.
Specs:
- Front: Brembo 4-piston calipers, grey/unfinished; 340mm blank brake rotors
- Rear: OEM (Bosch, Ate, etc) 1-piston calipers, grey/unfinished; 330mm blank brake rotors
M Sport Brake Upgrade 2 (S2NH/S2NHA) (pads, calipers, rotors): Sometime during 2012-2013 production BMW changed the M Sport Brake option to include larger front and rear rotors, a different front caliper (still 4-piston Brembo), and 2-piston Brembo rear calipers. It was a $650 option but still called "M Sport S2NHA" and could be added to ANY trim package (even a base model). This kit was the same as the M Sport offered in Canada/Europe. The calipers should be blue with an ///M logo but there's a slim chance they could be grey/unfinished with no logo. The front pads were a similar aggressive Euro-spec pad as above. The rotors are blank but the dimpled/slotted rotors from the M Performance brake kits are a direct fit if you want to change their appearance.
Specs:
- Front: Brembo 4-piston calipers, grey or blue; 370mm blank brake rotors
- Rear: Brembo 2-piston calipers, grey or blue; 345mm blank brake rotors