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MG MGF Technical - Fitting your own front brake discs...

Many positive comments have been made following a simple write up I did re fitting front brake pads. Recently the issue of front brake discs has arisen and the expensive quotes for changing them by some dealers and other outlets. I know that most of you are quite capable to doing a disc and pad change so to add to the previous pad change notes which are somewhere in the archive, (someone remind me of the thread) I have decided that a few notes on a disc change would help many.Fitting discs is very simple as long as you have a few good tools and patience.

The first part is the same as for a pad change including retractin the caliper piston fully. Now instead of just removing the lower caliper to carrier bolt (that enables the caliper to be swung up to remove the pads, you remove the top one also) The caliper is now able to be lifted away and have some wire of strong string in order that you can tie it back and let the wire support the weight of the caliper so that the flexible hydraulic hose is neither under tension or twisted.

You then need a 19mm socket, possibly with a reasonably long handle, to undo and remove the two bolts that hold the caliper carrier to the hub. Note that there is a small amount of thread lock on the threads of these which makes the bolt removal stiffer. Once both bolts are removed the carrfier will drop away. Take this opportunity to wire brush away brake dust.

The disc can now be removed but first you have to remove the two cross headed screws that are countersunk into the disc face alongside the wheel studs. Do not try and skimp here and have the right cross headed bit that fits snugly into the screw head. Often an impact driver is needed to crack the corrosion seal, but once loosened they usually unscrew easily.

With the grub screws removed the dis is held on only by an interference fit against the hub drive flange. Use a soft headed (copper) mallet to gently tap the outer edge of the dis, turn the disc 180 degrees and hit the other side. repeat this until the disc has been dislodged. It is quite possible that some more energy than a simple tap will be required, but don't use a steel hammer against the disc for risk of shattering and splinters. If you only have a steel hammer of any weight, use a piece of wood as a buffer.

Once removed you will see that the edge of the drive flange is a neat fit into the inside bell of the disc. Wire brush the drive flange clean and smear a thin layer of Coppaslip or similar anti sieze compound on the edge and face of the drive flange.

take the new disc and use a brake cleaning spray or carb cleaner to clean the protective grease from the disc friction faces. fit the disc to the hub and refiit the screws. Here also a dab of coppaslip on the threads helps long term. The screws do not have to be mega tight, just nipped up as all they do is locate the disc whilst the wheel is removed.

Refit the caliper carrier with a dab of thread lock for each bolt. Torque for these is 85Nm. Fit the new pads, again I find a dab of Coppaslip on the contact edges and one behind each contact point for the caliper piston and the two outer caliper feet works well. Refit the caliper, making sure that you don't twist the hose, and tighten the bolts to 45Nm. A spot of thread lock on each bolt thread replaces that which was on originally.

Finally pump up the brakes to take up the slack and bring the pads into contact with the new disc, before moving to the other side. This helps stop the potentially paint damaging loss of fluid when both front caliper pistons are fully retracted and the displaced fluid overflows the master cylinder.

When the other side is done test the operating of the brakes before a test drive. The bear in mind that if you have previously done a pad change and found the initial brake response to be cr-p, then with a disc change expect worse. For this reason you need to have a few slow and steady brake applications. i.e. drive in second gear at about 20mph and for several hundred yards do left foot braking that is enough to bring the car speed down to 10pmh without altering the throttle position. Then venture on the open road and treat the brakes gently. This means no 70mph to 0 stops for a couple of hundred miles, unless circumstances dictate. Otherwise warped discs can and do occur and your in a worse state than before the change.

I hope this assists some in attempting the disc change as well as the pad change. I hate to see such over inflated prices for jobs that for once the manufacturers have made a doddle.


Roger Parker

This thread was discussed on 01/07/2000

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