I left some new rotors outside for a few weeks before beginning to install them today. I thought they were sealed in plastic, but turns out they were just wrapped and some rain water got in and sat. I know rust isn’t usually a big deal, but these are pretty bad. Should I toss em?
Also, no idea why one is dark. Any ideas?
I’d clean them off with brake cleaner then sand off the rust. See how they look then. I doubt they’re beyond help.
I would try scrubbing them down with some fine emery cloth rather than scrapping them. As for the difference in color, that could be due to the rotors being from different production runs or manufacturers and possibly the darker one having a rust preventative treatment on it.
Both rotors should be cleaned thoroughly with carb or brake cleaner even in cases when they’re used right out of the box.
My gut feeling is the rotors will be fine with some cleaning and a few stops.
If that happened for just a week or two I’d just clean them with a wire brush and do the install. The rotors, when they are on the car, can get splashed every time you go over a puddle, so a little water won’t hurt them enough to put them out of spec in most cases. If you are uncertain, no harm done to take them to a local shop and ask their opinion. They could better judge the surface roughness and measure thickness and run-out.
If needed a good machine shop could take a minimal amount off.
They don’t need turning, this is very minimal rust. Clean them with brake cleaner to get the storage grease off then sand them lightly and you are good to go.
I would just clean them with brake cleaner and do a light sanding with a coarse ScotchBrite pad on a palm sander if you have one.
They will polish up fine after a dozen good stops.
Yosemite
Can anyone explain to me why rotors are not made of stainless steel? For a conservative driver in high salt areas, I think most of the rotor wear is due to rust. I once had a pair with rust holes completely through the rotor.
Cost and effectiveness, I bet the current ones work better. When cost is no object they go to carbon/ceramic, not stainless.
I didn’t know about ceramic rotors. But the cost difference between SS and steel is low, unless there is some other factor. Certainly I’d pay several dollars additional for SS if it were available.
SS is about $0.16 per pound as opposed to $0.03. If the rotors weigh 10 pounds each, that is $1.30 added cost.
SS is somewhat inferior to cast iron in a couple important areas. SS has lower thermal conductivity and a lower friction coefficient. So it doesn’t make as good a base material for rotors. You commonly find SS parts in the motorcycle world because they are more visible and so the tradeoff is worth it. You can buy coated rotors that have the friction surface machined if rust is a big concern for your area/usage profile…for me, they wear out before they rust away so having them be SS wouldn’t really be a good tradeoff.
I have a suspicion that rust plays an important part in the wear even in heavy usage cases. The rotor can develop a thin, not very visible, coating of rust over night, then the first use in the morning would wear that off quickly. The result (I hypothesize) is faster wear then you would get with a stainless rotor.
But this is a suspicion only. I’m not a metallurgist.
I remember as a very young boy running a magnet through the shoulder material by the road. The captured metal filings were always heaviest near the mailbox…
Flash rust scrub loss may be hard to quantify 
By nature, SS will wear slower than cast iron due to its lower friction coefficient. They will last longer just because of that. But at the cost of less stopping power when its apples to apples comparison otherwise. To compensate for less stopping power, then they spec out a more aggressive friction base and you’re back at square one…no free lunch for you!
If SS rotors were a good, economic idea, you’d see them in the market place. The aftermarket would be full of them.
good point.
but how many decades did it take before rustproof mufflers became standard? I seem to remember in the 90s that mufflers lasted 20k miles or so. Now they last well over 100k.
Here’s what Braketech has to say, regarding motorcycle rotors:
"Over the years, cast iron has proven itself to be an excellent rotor material providing superior “feel” and producing a higher coefficient of friction at the friction couple. If you’re looking for genuinely improved performance in terms of both braking power and modulation, then this is for you. If rain and or high humidity is the rule and rust potential a concern, you may wish to consider the alternative in our new AXIS range which is now offered in premium 400 series Stainless Steel as well. "
I would take them back and get a replacement, no sense worrying about pitting and get a proper part, as that one is not up to specs. It is garbage, and you should not have to feel responsible for receiving less than acceptable.
SS is a harder material to manufacture, cut, and machine. So there’d have to be a pretty good reason to use it for a simple function like brake rotors. The part where the pads meet the rotor gets wiped clean multiple times every day the car is driven, so rust isn’t a problem in that area usually. And the part where the pad doesn’t meet up with the rotor, yes that rusts, but it is just along for the ride. It performs no function. @BillRussell I’m surprised you had your rotors rust through, I presume you mean in the area where the pad hits the rotor, right? Seems like that wouldn’t happen unless the car wasn’t driven on a regular basis.
" It is garbage, and you should not have to feel responsible for receiving less than acceptable. "
?? No one should be surprised that rotors rust when left out in the rain. Nothing wrong with them.
George, the car frequently sat for a few-4 days. That gives the rust to start. If it gets deep enough in a spot (perhaps due to some slight metallurgical defect) before the next use, it may continue to stay ahead of wear until it rusts all the way through.
“The part where the pads meet the rotor gets wiped clean multiple times every day the car is driven” but that accelerates wear, as rust has very little strength and will wear off much quicker than the original steel would have. In other words, a mil of steel changed to rust will wear off much quicker than a mil of steel unrusted.