I just bought some new ac delco drilled rotors to fit 80 corvette. should I turn the rotors? I was told if not it would catch air in the line. I have herd people talk about run out on new rotors. frstvette2007-11-11 18:36:14

       New rotors should not need turning....Stay away from the Chinese stuff tho....

As a rule I always tune my new rotors. The way quality of thing are going down, I would rather not chance it. They're off the car and it would only take a few mins. Better safe than sorry. IMO Good luck Jamie

Reid is right...new rotors shouldn't need to be turned...but...I have seen more than a few that were not true out of the box. Chuckin them up in the lathe, and scraping the surface never hurts. Even if they are milled from the factory, which is good, they can get damaged in shipment, and from being stacked for long periods of time.
I rarely ever install "new" rotors without at least checking them on the lathe. Besides, a light scrape will also get all of the goop off of them that is used to keep them from getting rusty in storage.

(this is what a guy sent me)     It's not the rotors that are out of spec,, it's the rotor to hub mounting faces that causes the run out when you change the rotors, not being perfectly flat. You may have the problem, you may not,, The calipers I am sending you are like new,, 10 hours like I said, you would have the same problem with a brand new set, many do, but I would like you to be up on this stuff and benifit from my year of research. it's a crap shoot and a very common problem. I would suggest having a mechanic check the rotor runout on installation to be sure it is less then 8 -1000ths of an inch < 0.008", that is why shimmimg the rotors, with shim stock between the rotors and the hub can correct the problem

Well...some of that is true, some is a bit skewed. It is true that the hub/rotor surface can cause runout problems, but so can warped rotors. That's why it would be good to have the rotors checked first, before installing them. That way, if excessive runout is encountered, you'll be fairly sure it's not the rotors.(factory specs on runout is .005", btw...less is better)
Also, the hub surface, where the rotor fits, needs to be absolutely clean, and free of any rust. The tiniest piece of dirt/rust/corrosion will cause some runout. Shimming the rotor would be the last thing to do, if runout can't be solved by any other means. Sometimes, just removing the rotor, and turning it one notch will eliminate, or make the runout less. You may even need to turn it(one notch at a time) several times to find just the right spot.

The rotor should not be turned unless a good non-directional finish is put on the rotor.  The non-directional finish helps the pads break in better and may prevent noise and provide better braking in the future.   New rotors already have this.    If they are turned, and a non directional finish is not done, use some rough (80 grit) sand paper and go over the surface.

All good advice. I always put rotors on the lathe to check them prior to installing. I want less than .005 inch run out. Like Joel, I want shimming to be the last resort. I would do thorough cleaning of the hubs with a wire wheel.  I also put a non-directional abrasion on the rotors with my small air grinder, round 2 inch pad, and light scotch brite pad. I always clean the rotors with brake cleaner to remove contaminants. Always works well.

Scott