The front discs on my Fury are 247mm vented from a Capri. There is plenty of thickness on them, but they came with the car so they are years old. Measuring with a dial indicator, one is 0.4mm out of round and the other 0.2mm. The 0.4 one ever so slightly rubs on a new caliper, I can hear it but I can't feel it. What sort of tolerance should their be on diameter?

Regards,
Dave

I really don't think they should be rubbing the caliper!

I used a Sierra donor for my MK Indy and simply got new discs for the front. I never checked the roundness other than rotating them by hand to make sure they didn't touch anything but the new brake pads in the refurbished calipers. IIRC new discs weren't expensive and I didn't see the point in not fitting new ones.

I realise rubbing is bad I'm just trying to figure out if the disc or the caliper is out of spec.

Assuming the caliper is on sliding mounts?

Unless the slides are worn allowing the caliper to move (radially) then it has to be the discs are out of round and best replaced.

Quick scout on fleabay suggests £35-£45 per pair for Capri brake discs.

Max 0.1mm runout, 0.05mm thickness variation (as the disc is rotated, not across the disc face and 2 dti's needed to check)
I would expect juddery brakes with your given figures.

If they've been removed/refitted check the hub mounting face for cleanliness and runout, max 0.01mm there.

Dave

As said way too much run out on the disc

This isn't 'disc wobble' type run out (which I would expect to be very small), this is diameter variation.

There shouldn't be any issue with that, they're normally rough finished on the outer edge as it is not a friction face so the tolerances are not critical.
I can't think of anything apart from some escort discs are 245mm, i don't know if the caliper is slightly smaller for those slightly smaller discs, mixing the 2 might make clearances tight.

Dave

Thanks for the answers. It transpires that the holes in the mounting lugs were not accurately drilled. The lower hole on the right caliper was 1.2mm nearer the body of the caliper than the left one. They're going back.

Glad you've found an answer to this one! Certainly seems logical that the either the disc is wrong or in your case the caliper position is wrong.