Hella 355120811 Two-Piece Vented Brake Rotor, 340mm 6x120

The Hella 355120811 is a two-piece vented brake rotor, and it's the kind of part you fit once and forget about. The friction ring is gray cast iron with a zinc coating, so the areas the pads don't sweep stay clean instead of blooming orange after the first wet week. The hat, the center section that bolts to the hub, is aluminum. That combination is what OE engineers reach for when they want good heat capacity in the ring without carrying dead weight up front where it hurts steering feel and ride quality.

Here's what the two-piece layout actually buys you. The cast iron ring does the hard work, soaking up heat under braking and shedding it through the internal vanes. The aluminum hat handles the mounting and shaves rotating and unsprung weight off the corner. Less mass spinning at the wheel means the suspension works a little easier over bumps and the brakes have less inertia to fight. It's a design you usually see on heavier, faster cars where the front brakes take a beating, which is exactly where this 340mm rotor lives.

The dimensions are straightforward. You're looking at a 340mm outside diameter, a 30mm nominal thickness, and a 73.20mm overall height from the mounting face to the outer edge of the ring. The center bore, or hub size, is 79mm and the bolt pattern is 6 x 120mm. Those numbers matter. A rotor that's a hair off on hub bore won't seat true on the hub, and the wrong bolt pattern simply won't mount. Cross-check the outside diameter and thickness against the rotor coming off the car, or against your OE part number, before you order. Hella cataloged this one under a BMW application, but the fit is defined by those measurements, so verify them.

The ring is internally vented. Two friction faces with vanes cast between them pull air through the rotor as it spins, which pulls heat out far faster than a solid disc. If you tow, run mountain grades, or just drive a heavy vehicle hard into stoplights, vented rotors are what keep the pads from cooking and the pedal from going soft. This isn't a budget solid blank. It's a proper performance-oriented casting.

A few honest notes on install. This listing is for a single rotor, so if you're doing the axle you'll want two. And even if only one side looks worn, replacing rotors as an axle pair is the right call. It keeps braking balanced side to side and stops the car from pulling under hard stops. Bed the new rotor in with fresh pads following a normal bed-in cycle, a series of moderate stops from speed to lay down an even transfer layer. Don't sit on the pedal at a light while everything's still glowing hot, or you'll print pad material and set up a pulsation later.

Two-piece rotors like this generally aren't meant to be resurfaced. The friction ring is thinner than a heavy one-piece casting and the assembly is built to a tighter tolerance, so when the ring reaches its wear limit you replace the rotor rather than turn it on a lathe. Check the minimum thickness marking before reusing anything that's already seen miles. When you fit fresh, run new pads too, and give the hub face a quick wire-brush so the rotor sits flat with no rust flakes trapped behind it. That one step prevents most of the runout complaints people chase later.

Bottom line: if you need a direct-fit front rotor with real heat capacity and OE-style construction, the 355120811 covers it. Zinc-coated iron ring, aluminum hat, vented core, sold as a single so you order exactly what the job needs.