Machining brake disc diameter

[Mark Crowe]

I have some 993 front disc's ( part no; 993 351 045 10) on a a 930 turbo that i am trying to locate under 16 inch Fuchs. It originally had ATE calipers and skinny disc's)

The diameter is 322 mm, and I was thinking of machining down the diameter on the edge to reduce the diameter, and allow the caliper ( Big Red 993 Turbo) closer in to wards the hub, thus giving more clearance between the rim and caliper.

Any thoughts on this? Dangers of shattering disc whilst machining? Cracks later on?

I understand that the thicker disc allows better heat dissipation, I am using this car for moderate road use, not racing, and I think compared to what the car started with, the reduction of small amount of material will not pose a significant change in feel/ stopping in this relatively light application.

I was thinking of taking 100 mm off the diameter, not sure if there is any limit to how close to the center of the hub the caliper can go. The idea is to minimise the spacing of the rim out from the hub to allow the rim to clear the caliper with the 16 inch rims

[Bill Verburg]

If you have 2 piece 322x32mm rotors they are from a 993tt.
Look at this pic of the 322mm rotor and a big red caliper


Notice how close the caliper already is to the hub?

Next issue, 930s w/ 16" wheels used 304x32mm rotors to get the 322 down to 304 you need to reduce the diameter by 18mm, ie 9mm must be machined off the outer edge.

Now, measure 9mm and note the proximity to the holes? You do not want the holes that close to an edge. I know that people do this but it is not good practice.

last, even if you used the correct 304x32mm rotors(by the way n/a 993 or '78-80 930 are correct)the width of the Brembo calipers(928 S4/993 n/a or 964 3.6t all work w/ the 304s) is such that you even w/ 944 Fuchs you usually need a wheel spacer.

[Mark Crowe]

Thanks Bill,

The way we had the caliper lying on the disc led us to believe there was some "
fat" we could take off, but looking again and at your photos, it is not the case.

As we cannot get 944 Fuchs, any recommendations on a spacer width to allow the standard 16 x 7 Fuchs to be used? We worked out a total spacer thickness of approx 52 mm from the hub, basically double the standard spacer thickness- , so standard spacer + additional one, not sure if this is too much?
As we would need to change studs to longer ones , might it not be better to go adapter + spacer?

[Bill Verburg]

I dislike the use of spacers, especially so on the front. You are changing the scrub radius a lot w/ very detrimental effects on steering.

When you change the suspension geometry you are potentally opening a can of worms.

Besides that using a 1" spacer will require 930 fenders.

My advice, use 993 calipers and 304mm rotors or straight 930 stuff

[Quicksilver]

As a side note to Bill's comments...
If you do machine the outside diameter of a rotor you will need to get it rebalanced because you have machined of the asymmetrical machining where it was balanced at the factory.

Either way Bill's suggestions are way better.

[lucittm]

So if I understand this post...
One-piece 965-351-043-00 = Two-piece 993-351-045-10.

The rotor is the same dimensionally, it is lighter and it costs less?

Seems like a no brainer... I'll take two.

One question, do all of these rotors develop the ridge at the outer edge like mine do after some wear? Or is that caused by the wrong type/size of pad.

OK, another question, is there a consensus regarding the front pads for these 911 turbo brakes (spirited street use only)? There seem to be an unlimited variety of colors, prices, compounds, etc. available.

Thanks,
Mark

[Bill Verburg]

Originally Posted by lucittm

So if I understand this post...
One-piece 965-351-043-00 = Two-piece 993-351-045-10.

The rotor is the same dimensionally, it is lighter and it costs less?

Seems like a no brainer... I'll take two.

One question, do all of these rotors develop the ridge at the outer edge like mine do after some wear? Or is that caused by the wrong type/size of pad.

OK, another question, is there a consensus regarding the front pads for these 911 turbo brakes (spirited street use only)? There seem to be an unlimited variety of colors, prices, compounds, etc. available.

Thanks,
Mark

993 and 964 rotors are not interchangeable because of the o/s. You can use the 2 piece 993tt rotor on a 964 if you change the o/s of the caliper adaptor or if you use 993 struts

they all develop the ridge, that's just normal rotor wear, the pad does not go to the edge

you won't find a consensus because different people value different qualities in a pad, for strictly street use I use Textar, but Jurid and Mintex are good too, they are all quiet, some dust less than others