Granite 944

I just gave an extra pair of used front rotors I had, to a FLAPS, asking to have them resurfaced. I told them, yes, I know that they will most likely come out slightly under the min. spec. of .8”, but that’s ok with me, they are only going to be used temporarly, till I make other arrangements. A while after I get home, I get a phone call from the FLAPS, saying “our machine shop needs to have the hubs to go with the rotors”. Ok, I really don’t fully understand this explanation that I’m getting from them, about the rotors need to be turned as a “set” with the hubs attached to them in order to keep the runnout proper. Like I said, these are extra rotors (which, I've explained to them). I do have one decent,extra,used hub that I took to them to use for mounting the rotor on it, for the purpose of machining it. I did not have two hubs to give them, nor do I believe this, about the “set” horsepucky! I’m assuming that the rotors when mounted to the hub, are perendicular to the axle axis. I’m also assuming that if machine shop doesn’t have a regular lathe, just the drum/rotor machine, he MAY need the hub also, in order to chuck it up in his machine. I’ve heard a few times from these people of the “hard time” the machine shop is having machining my rotors one at a time on “one hub”. I’m sure if there really IS anything to this “set” horsepucky, I can just unbolt the rotor, turn it to a different position on the hub to get the least runnout possible on the rotor as verified using my dial indicator. I’m planning on installing these rotors on my “83 that I’m driving right now, and using the same hubs that are on it right now (after I install all new bearings/seals in them). I’m thinking I’m either getting a big line of BS, or, I really do need some educating on this. Do you know of, or had any experience with this type of thing? Right now, I’m thinking, I’ve got to find a decent FLAPS/machine shop, for the real important stuff.

Am I missing something here? Turning down our front rotors isn’t rocket science….is it?

Thanks!

Matt H

They must only have an on-car brake lathe.

1) They should NEVER agree to cut a rotor that will below spec even if you tell them you are going to use it as a planter. They assume ALL liabilty of doing so whether you ask them or not.

2) Yes, ideally you would cut ALL rotors on car as this gives the best possible scenario. This is probably done about 1/100th of the time because on car lathes are expensive to buy and costly to operate (i.e. one can cut a rotor MUCH faster on a brake lathe).

3) Pick up your stuff and find someplace else to do it, cutting a rotor is not rocket science.

See also #1, you run the risk of:
a) pedal pulsation once they warp (which they will esp. if they are too thin)
b) fatigue cracking/failure
c) having the piston extend too far out of the caliper housing and ruining a caliper

There are more reasons but I have to tell you this is NOT a good way to save a whopping 120 dollars. I.e. what a set of rotors will cost you.
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Granite 944

Matt, I do agree that one shouldn't run under spec. rotors, for all the reasons you've brought up here. I wasn't going to sweat it for now myself since I'm just planning on running these fresh cut rotors on the car just for this summer, just using car for street pleasure/daily driver, and I know the front pads have plenty of body left on them. (The PO had new pads put on car, but used the same rotors which are worn down pretty good, leaving deep shoulders on edges of rotors.) The right front bearing on the car is a little loose, and doesn't want to snug up well, so I just wanted to replace the bearings/seals, and thought I'd just run these rotors for now. My heartache was in the fact that the shop wants the hubs in order to turn these rotors. I thought it should be no big deal just to chuck up a rotor, dial it in on the center axis, and cut it. I can't believe their using an "on the car type machine", since this is a send out item thru a FLAPS. I thank you for your reply, I was just feeling a little pissy towards the FLAPS/machine shop for screwing around with this whole thing, so now I've got to wait till monday to get these parts. I guess in ones life, a little rain must fall? Ha Ha!

Thanks!

Matt H

Just pick them up and go to any brake repair shop and ask them to cut them. Should cost about 20 bucks.
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Chris_924s

I know for a fact Auto Zone and other FLAPS wont undercut a rotor- too much liability.

IceShark

Besides cutting the rotor below spec, I have had a brake engineer (the guys that design brakes) tell me that it is a bad idea to turn used rotors. Just use the old rotors and wear out the new pads quicker if you are trying to be cheap. I forget exactly how this went but it had something to do with the heat cycling establishing a molecular lattice in the metal that you screw up when you turn them and cut them down. Then various bad things happen. And he mentioned something about differences in hardness between the surface and deeper in the metal.

Of course this was coming from a brake engineer who wants brakes more or less perfect.

Granite 944

Yep, I just knew it.......shoulda given them to NASA, not NAPA!

Well, I'm already commited to having this shop cut them now. Late yesterday morning, the store told me the machine shop had already done the first one, but was just "complaining/uneasy" about not having another hub to mount on the other. Oh well, I told the store fella, if I thought for a second that his machine shop was going to have such a "hard time" dealing with it, I would have taken it somewhere where they know how to machine parts! Made an impression there, I'm sure! I won't be going back here again for machining (I wonder what problems they'd have doing a flywheel? ). They never had a problem with the underspec part of my request. Their closed on sundays, so monday, I'll pick them up, mic them and see how thin and well cut they are, and if not under about .750", I'll still use them at least for a little while. For $7.00 a rotor, I can't complain to much I guess. I'm just to cheap to spend the $100.00 for new rotors right now on this car. Most of these shops that turn rotors/drums do leave a very rough finish when done. If they'd just slow down the cutter feed on their machines, the final product would be much smoother. I'm sure that contributes greatly to pads wearing down quickly.