Cross-drilled rotors can't be turned????

I'm getting down to thin pads, and had the tech check them out. They are still good for several track miles, but getting close to replacement. He also stated that there is quite a glaze on the rotors, and that he "strongly" recommends replacing the rotors as well. Says cross-drilled rotors can not be turned. True???? If so, we are talking buckoo bucks here. Probably eat up my GT3 seat or PSE budget allotment, maybe both.

True.

true dat. hehe. Just use a mic on them I would be surprised if they are that bad. I thought the same and did it and they were fine

Thanks for the replies. Whats a "mic"?

Well, according to the Porsche Tech Manual... Yes, they can be machined. Here are the specs: Brake disc thickness, new -- front 28 mm, rear 24 mm Minimum brake disc thickness after machining* -- front 26.6 mm, rear 22.6 mm (* The brake disc must only be reworked symmetrically, i.e. from both sides evenly.) Brake disc wear limit -- front 26.0 mm, rear 22.0 mm Peak-to-valley surface roughness of the brake disc after machining, max. -- 0.006 mm Thickness tolerance of the brake discs, max. -- 0.02 mm Lateral runout of the brake disc, max. -- 0.03 mm Lateral runout of the wheel hub, max. -- 0.03 mm Lateral runout of the brake disc when installed, max. -- 0.06 mm

mic = micrometer I read a post on team.net's vintage racer forum that asked this question. According to those who answered, it is no problem to turn a cross-drilled (or Porsche's 'cross-cast') rotor on a standard auto machine shop brake rotor lathe. One writer got this info from "Scott Bethke at Brake Materials".

I replaced the four rotors on my 996 after they got cooked at a DE event. They should not use up your seat or exhaust budget... only half of it. <img border="0" title="" alt="[Wink]" src="wink.gif" />

Can't turn perforated rotors??? Whew - who dreamed that one up? Take a close look @ your rotors. Notice that the "outside" edge of each perforation is nicely chamfered? That's there for two reasons: First, to reduce the chance of a pad edge catching, and Second, to allow for machining! If you dig around, you can find some perforated rotors without this bevelling. Those rotors are extremely difficult to turn. J

Glazed rotors are generally a result of overheating. There are two things you can do to prevent this in the future. One get a front brake cooling kit. The best is the ducts that are used on the Cup and RS cars. Parts are available from PMNA but some cutting of the fender liners is required. The other option is an aftermarket cooling kit from CarGraphic, FVD, or one of the other tuners. Second, keep the holes clean. Periodically use a punch or screwdriver and clean the brake dust out of the holes. This will allow the rotors to run cooler. Most dealers don't turn rotors but you may find a brake specialty shop that has the right equipment and the skill to do this job. New front rotors should be about $200 per. Karl

Sounds like a good DIY project ... lots of posts on changing the pads and the rotors are easy to do while you're in there!

I vaguely recall a recommendation that Porsche rotors not be turned but be machined on a Blanchard Grinder. That ring a bell with anyone else?

Thanks for the replies, all. It seems my particular Porsche serviceman either does not do machining, or does not want my specific rotors machined for one reason or another. Karl S, thanks for the heads up on how to prevent this in the future. Will definitely look into cooling kits. Hope your GT3 comes out to you soon!

I have heard mixed things about this basically amounting to the fact that if turned and not chamfered with the holes a bit after or before???, then they are open to cracking moreso.

Palting -- When I was looking for a car, i spoke with a number of Porsche Service people about the brakes. Positions ranged from, we change the pads and rotors at the same time (no exceptions) to we change the rotors when they need to be changed. I dont know for a fact, but I would guess Service Departments make a decision one way or another but that they don't have the machining capabilities.

Talked to the service GM. Thought I'd give an update to thank those that responded. The rotors are quite worn. Not down to minimum, but close. There is no chamfering at the holes anymore, and there is a definite high ridge where the pads do not contact and are not worn. Whether I want to or not, it seems they DO need to be changed. Another thing I picked up by talking to other car nuts. Pads only change is acceptable, but regarded as a cheapskate thing to do. I guess brakes is the last thing we all want to be a cheapskete on. Thanks for the replies

I don't think buying new rotors with every pad change is necessary but you should turn the rotors. Like Loren posted, you have about 2mm of rotor thickness to work with, and if machining drops them below that tolerence, then replace them. Frugal is not cheapskate, but safety should never be compromised.

Yup, That's what the service GM is saying. My rotors are worn. New pads will wear the rotors down beyond minimum before the new pads need replacing. He said they can be turned, but he wouldn't recommend it for my rotors. Guess I've been doing too much threshold braking.

I've worked on cars for many years, you definitely do not need new rotors with every pad change/ brake job. As long as they are in spec it's ok to use the original rotors. It's not a question of someone being cheap or being a sucker, it's very straight forward. If they are, or soon will be, out of spec then replace the rotors and if not then go ahead with a simple pad change.

During DE seasons I go through 3 sets of front pads before the rotors need to be replaced, which is about twice a season for me (fresh at the start and replaced about half way through). Maybe with normal street use, pads and rotors would need to be replaced at the same time but this is not the case with mostly track use. Karl

What do they mean when they say rotors are glazed? I know it has to do with overheating, but what else? Does the pad material get melted into the rotor? Does the rotor surface get so hot that it gets polished so smooth that brakes don't work well?

Yes, the rotors overheat and the pad material bonds to the rotor, causing it to get a blue tint and lose a significant amount of friction. Glazed rotors have to be surfaced or replaced. Karl

The blanchard griding does ring a bell. It was discussed by a few engineers in the 928 areas. Remember that the nice round curve from the surface of the rotor to the hole will be "edged" just slightly if it is "ground" or "turned". I supposed from many readings of many performance books that an edge is a stress riser, "problem" waiting to happen.

That's a great point. Any perforated rotor must have the edges of the holes beveled. This reduces rotor stresses/cracking, and also can keep the pad material from "catching" and tearing free of the pad surface. This was an issue with some GM disc brakes in the late 80's/to mid 90's. J