GT Front Disc Rotors

For some time now, I have been investigating replacement rotors for my GT which were getting down to the minimum limit, for the second or third time. The plan was to be able to use the existing GT hubs and callipers to avoid using parts from other cars and the associated engineering hassles. This has been done by others in the past.

Finding suitable rotors with the slight conical 'hat' to clear calliper bolt heads, similar bore diameter, similar total height and 4 bolt holes has taken a lot of time searching through various catalogues, not only Australian ones. Eventually some clues came during some correspondence with Dick K. in Netherlands. They have used rotors from a Mazda 323 (or possibly a 626) - older cars, not later models. They even have these 323 rotors on a UK race Bellett (Mark). However, the relevant rotors for the 323 were no longer available in Australia from Protex or AAD.

Then there was a lightbulb moment, or more likely somebody reminded me - in the 1970's and 1980's, Mazda had an agreement with Ford to market the Mazda 323 as a Ford Laser and the Mazda 626 as a Ford Telstar, although with a few modifications. There was also the Ford Escort and Capri. Maybe these Fords had a better chance of having rotors that may be suitable and still available here. Four rotors looked like they would be suitable - Protex DR113, SDR118, SDR119 and SDR932. Eventually settled on RDA118 (different manufacturer?) as it was readily available. It is listed for Ford Escort Mk2 76-81. Currently (11/12/23) there are about 30 of these available in the Keysborough warehouse.

As this RDA118 rotor has different dimensions to the original Bellett one, some lathe work was necessary to fit this rotor to the hub and to manufacture a spacer ring. This spacer fits between the mounting face of the hub and the new rotor to enable the disc surface to fit in the centre of the calliper. The total height then matches the original rotor at 37mm.

Firstly, the slightly tapered rough casting surface on the hub was skimmed (<1mm) close to the original rotor mounting face to provide a smooth surface of 71.0mm to hold the new rotor. This did not reduce the structural integrity of the hub as it is approximately 12mm thick at this point. Clean off any rust around the area of the original rotor mounting without altering the diameter which is 74mm.

The centre hole diameter of the new rotor was increased to 71.0mm to match the machined surface of the hub. The outside diameter was reduced from 247mm to 237mm to match the original. The thickness of the RDA118 was 12.5mm so this had to be reduced to fit the calliper gap of 14mm and allow some clearance. It could be reduced to 12.0mm but the decision was made to go down to 11.0mm. This was done on the hat side only so as not to affect the groove near the hat, hence the structural integrity of the rotor was not reduced. The mounting holes are spaced at 92mm across the diagonal which does not match the holes on the hub at 94mm. They are also not threaded like the Bellett rotors. Therefore, new holes were drilled and tapped for 10mm x 1.25mm pitch bolts at 94mm across diagonal.

Finally a spacer was made to enable the disc to fit in the centre of the calliper. Dimensions -
OD : 115mm
ID : 74mm (to fit hub original rotor position)
Width : 10mm (this would need to be adjusted if other disc thickness was used as mentioned above)
Holes : 10.1mm at 94mm diagonal spacing to match hub and rotor

As I don't have a lathe nor the experience to do this sort of work, it was done by someone in Ringwood who manufactures and modifies cars for racing - he does know what he's doing and has some interesting stuff there.

Before assembling everything, the callipers were rejuvenated with new pistons and seals. They were obtained through Yahoo Japan auctions via https://buyee.jp and are still available. Pistons do not have the hole for the pin so this was removed from the bottom of the calliper cylinder. I have just found that the piston rubber boots are also available as a calliper overhaul kit from the same web site. The hubs also received new bearings and inner seal - no problems getting these parts locally. New brake hoses were fitted all around, made to utilise the existing metal line connections.

The new rotors have been on the car for a while now, bedded in and working well. As we all know, some pedal pressure is still needed to stop in a hurry without any power boost.

If anybody wants more info about this, please contact me.

Cheers, Barrie.
 

Great work, and there is enough stock/supply of these discs worldwide.
Gr Dick