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MG MGA - disc pads

Have struggled to fit new pads. Even with special spreader tool can only get 3 on. On the last one the piston still protrudes about 2mm and the new pad wont push in. I dont think its seized. Is it possible to hone a couple of mm off the new pad say with an emery stone?
H L Davy

Does the last piston behave like the others, protrusion wise? If not, there is a problem here, maybe something inside the bore stopping it retracting. Did you install B pistons and remove the guide pin, if so is there a bit of the pin left?
If all pistons are retracting the same, then there must be a misalignment on the caliper s to the disc. if you got 2 pads in on one wheel, then they should fit the other, if they are all the same thickness.
Art Pearse

H L, It would be dangerous to remove any of the pad material unless you did it for the four pads. Even then it would have to be machined off to maintain even thickness.

Stay safe and investigate the cause of the problem - please! Check out Arts' advice above.
BM Gannon

You should be able to get every piston returned so that the surface is flush with the outer dust seal. I don't know what your special spreader tool is, but I usually use a C-clamp. The piston should return without much effort. I am still using unmodified MGA pistons. If you have one piston that is acting irregularly, I would start thinking about a caliper rebuild, not sanding down a brake pad. If more than one piston will not return all the way, it could be a plugged vent hole in the reservoir filler cap or a deteriorating rubber brake hose.

Art, Not to hijack the thread, but what's involved in removing the guide pin if I ever wanted to switch to MGA pistons? It sounds like a lot of people are doing this.
Mark J Michalak

insert a couple of the old pads in the caliper housing (to restrict excessive travel of the piston) and press your brake pedal to move the piston outward.
Attempt to retract it completely. As stated by Art and Barry, there is a problem if the piston does not completely retract. Insure that your clamping device is centered on the piston, so as not to cock it, which would cause this problem. Check your seal(and also under the seal)to see whether there is seal damage and whether there is corrosion under the seal on the piston. If so, you have to rebuild or replace the caliper.
As Barry stated, stay safe. Even MGs are no fun to drive if the brakes don't work.
Mike Parker

Mark,I did mine a few years ago. I had bought a set of oem pistons when I first restored my car at great expense. After they got pitted,I was not get them again. I taped up the bore to protect it and used a dremel with a cut off disc. I have heard that the pin could be pulled out also. If I remember right I think I got a set of stainless MGB pistons.
gary starr

I first tried with a C clamp, but I could not force the pistons back enough except on one. Nor was it wide enough to cover right across the piston. I bought a caliper piston spreader which has two flat ears which push outwards flat across the pistons and can be used with a 1/2 in drive. With this I got two more on but even with those pistons right back I had to tap the new pads in. The last one I just cannot shove right back. I smeared red rubber grease on the pistons while protruding and hope that maybe it will ease in a couple of days. I am very interested in the references to MGB pistons. I don't know if I have them. Are they a straight swap? I dont understand the references to guide pins. How could I tell if I have a bit left in without dismantling the calipers? Do yopu have to change the calipers and pistons together to MGB? One thing I am sure about. The pistons are not stainless. One other thing that puzzles me. Why is there a cut out on the lower half of the pistons? Surely this prevents the whole circle of the piston pressing on the back of the pads? What is its purpose? regards to all.
H L Davy

Mark, the centre guide pins are press fit. Take a hammer and drift and carefully bend it sideways back and forth to loosen it in the hole, then you can pull it out. The stainless MGB pistons have no centre guide but otherwise dimensionally the same. The MGA pistons used the guide pins (plus some other fittings ?) to retract after braking, but with experience it was found that pistons retract enough with the seals anyway so the centre pin arrangement is not required. To get at the pins you need to split the calipers and remove the pistons.
I too am curious about the cut out on the piston face.
Art Pearse

If the piston does not move back you can be sure it doesn't move forward either! In other words your brakes are not fully effective. If you still have everything on the car, follow Mikes advice and press the brake with all pads fitted except the stuck piston. That should get it to move. If you can move it out, (freeing it off) you should be able to push it back in.

However, this could be a good time to remove and refurbish the calipers. (you have no real choice if the piston will not move). Caliper seals are often overlooked when the rest of the brake system is refurbished, but they are not so difficult and the myth about splitting them is just that!

You will find all the information you need on Barney's site including a link to lower priced MGA pistons. MGB pistons are cheaper still and interchangeable, with the removal of the guide pins.
Neil McGurk

I assume that the cut out section on the piston face, which appears on many different types of disc brakes, is for turning the piston when it is sticking.
When fitting new pads on a modern Volkswagen car the retracting tool is threaded to turn the piston as it retracts and so the piston has to have that cut out section.

M F Anderson

Although it is not suitable for an MGA I have attached an image of a piston retracting tool for a Volkswagen. It shows how it turns the piston while it pushes it back. If you can turn the MGA piston it may be easier to push it back.


M F Anderson

Mick, right from Lockheed's instructions for replacing the seals- "These "cutaways" portions are there to assist in the prevention of brake squeal,therefore it is important to ensure that when refitting they are positioned correctly within the caliper bore to obtain maximum benefit"
gary starr

The VW rear pistons HAVE TO rotate as they move, or they do not budge. Its in the design. Found that out after lots of cursing and swearing before consulting the internet!
Art Pearse

Position the notch at the bottom, as per instructions in the Workshop Manual. See section MM page MM.5 here:

The notch is to let water out and air in for drying after road splash. Dirt and/or rust in this joint promotes vibration of the pads (brake squeal).

It is best to clean the mating metal surfaces well (sanding if necessary), and apply a dab of high temperature RTV brake anti-squeal adhesive, available at any local auto parts store. It often comes packaged with new brake pads in small flat tubes, or is available in larger squeeze bottles. A few ounces may last a lifetime.
Barney Gaylord

This thread was discussed between 22/04/2009 and 24/04/2009

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