[MattSAU2XR8]

I finally ended up getting myself one of these gauges and its been very informative. Executive summary is:
- New PBR slotted 322 fronts had 0.08 and 0.04 mm runout straight out of the box, on clean hubs, and not altered by fitting the disc in a different position.
- RDA 322 mm slotteds that have done 5000 km had 0.05 and 0.12 mm of runout.
- It seems possible to reduce a small amount of runout by sanding the back of the disc where it mates with the hub, i.e. from 0.08 to 0.04 mm, which may help reduce disc thickness variation down the track, and this would only require sanding the surface back by about 0.015 mm which is fairly easy given iron cuts easily.

Long version below.

I've been having some ongoing brake brake shudder after fitting RDA slotted front discs and Ferodo pads and bedding them in. I think now that the RDAs did have some runout when new (spinning the wheel after bolting it back up made a tch, tch, tch sound as though the pads were touching intermittently, although the old rears additionally warped during the bedding in process - The Ferodo front pads require higher pedal pressure which had been working the rears harder. So initially I planned to just replace the front again, but now it looks like I need rears as well.

To use the gauge for front discs you need to put some spacers under the wheel nuts to allow them to hold the disc against the hub, I used large sockets and pinched them up very gently with a socket bar. Ideally the sockets will be of larger diameter than the countersinks in the disc holes. Also a spare disc sitting on a towel or rag makes a good surface to mount the (magnetic) gauge on. For rears you'd additionally need some good wheel chocks since the handbrake would need to be off for the disc to rotate.

I'd initially figured it was the front brakes and tested the discs (5000 km old) and found that one had 0.04 mm of runout and the other 0.12 (which is still incidentally within RDA's fairly generous tolerance, DBA recommend no more than 0.04 mm). I took multiple readings at each point and repeatability was within 0.01 mm.

Next up I commenced fitting a new set of PBR slotted discs (made here in Aust) and found that one of them had 0.09 mm of runout which I could only reduce to 0.07 mm by refitting it in a different position. I'd already cleaned the hub (which has only done 11,000 km) and so next step was to gently sand back the inside of the disc where it fits against the hub, about 1/3 of the circumference related to the side of the disc that is 'sticking out' too far. Only had to do the outer part of the discs mounting surface since the hub has an inner and outer ridge that bear on the disc, not a flat plane all the way across. And because the iron has a rough surface its easy to cut it back a bit. I had been hoping for 0.02 mm runout but in the end I only managed to get both discs within 0.04 mm of true, at least as far as the outer face is concerned. Hopefully this will be good enough.

So I guess the moral of the story is that different companies specify different tolerances (eg. 0.04 for DBA and 0.15 for RDA) and not all new discs are true, i.e. 0.08 mm runout on brand new PBR discs, although whether this small amount would be noticeable when driving remains to be seen. I quite like the sanding idea because having bought new discs and fitted them up to the car, its too much of a headache to put the old ones back on in order to return the new ones and exchange them for more discs out of the same factory which may still have runout.

The other interesting thing I noticed is that I still have the vibration immediately after fitting the new front discs and some new rear Ferodo pads. In fact as soon as I got the brakes warmed up it was worse than before. So I think the shudder I was getting was in fact the rear brakes all along. So now I'm tossing up between DBA T3 rears or just upgrading to territory rears which are a heap thicker. Leaning towards the T3s since the car stops OK, shudder notwithstanding, and I'm trying to stop myself from doing excessive mods :-) I still have the RDA fronts I removed and may re-use them with a bit of sanding in the future, assuming changing out the rears solves my problem for now.

Life would be much simpler if Ford had fitted BA GT fronts and BF V8 rears in teh first place. At time of build this couldn't possibly cost more than an extra $200 and would have been a good selling point over Holdens :-( And then there wouldn't need to be brake threads on Ford Forums.