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MG MGA - Disc brake piston cup in wrong

I had the front calipers in my 1600 rebuilt over the winter. I just picked them up and discovered that they put the piston in with the cutaway pointed in toward the center of the brake disk rather than straight down in the direction of travel.

I know the cutaway is there to reduce noise. Will this orientation cause a problem?

Is there any way to twist the piston while its in place to orient the cutaway correctly?

As always, thanks for the help.

Tysen
Tysen

Hi Tysen. Yes, a cutaway that is in the wrong position will likely contribute to brake pad squeal, and the caliper piston should be properly repositioned. You could try this: You can purchase expandable rubber plugs used for plumbing purposes at a hardware or plumbing store. Pick an expandable rubber plug that will just fit inside the caliper piston. Then tighten the rubber plug so it firmly grips the inside of the piston. In the process, bolt on a thin flat piece of steel bar to the rubber plug. This steel bar can then be used as a handle to twist the piston in it's bore. I have never used this method myself, but hear that it works. Cheers! GLenn
Glenn

Hi Tysen, The above note was written assuming that you installed MGB cup type caliper pistons, and not the "proper" stock MGA caliper pistons. This trick would not work with original MGA caliper pistons, since they are a different design than the MGB cup type pistons. If you have original MGA caliper pistons, you will likely need to use compressed air or hydraulic pressure to push the pistons out of their bores. Once the pistons have been removed, they can easily be repositioned. You will only be able to remove one piston at a time if the caliper halves remains bolted together. Cheers, GLenn
Glenn

Glen,

Fortunately it is the MGB cup.

I can evision the concept that you suggest, but have never seen the rubber plug you are referring to....does it have name or alternative use I can refer to?

I wonder how tight it is in the rubber seal and if twisting will somehow damage the seals.

Thanks
Tysen

You could also try an expandable rubber freeze plug if you con find one the right size (might be cheeper.) You should do no daage to the seals by simply turning the piston in its bore.
J.H. Hall

Hi Tysen. I only know that the expandable rubber plug is used for plumbing work, presumably to plug an open pipe. I dont know the official name of this rubber plug, but it should be easy to find in any hardware or plumbing shop. Basically, it looks like a short rubber cylinder, with a threaded stud sticking out of one end. Tightening the nut on the stud compresses the cylinder, forcing it to expand. Hope this helps. Glenn
Glenn

The same contraption that Glenn describes is used also as a bolt-in freeze (core) plug for engine blocks. Available in several sizes at larger auto supply stores.

Cheers - - Alec

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Alec Darnall

Well done gentlemen, well done.

The freeze plug or expansion plug in 1 7/8 worked like a charm.

Installing and bleeding this weekend.

With any luck she'll be road ready on Sunday.....now all I need is some weather.

Thanks All!
Tysen

Tysen

I fitted some exchange reconditioned calipers on my 1600 and they squealed really badly. Eventually I discovered all the pistons had been fitted at exactly 180 degrees to where they should have been.

Managed to rotate all pistons to correct position with brake flexes still connected. Now they are as quiet as a lamb.

See the drawing in the workshop manual, it shows the pistons position very clearly.

Pete
Pete Tipping

Hi, Pete - there was a big controversy a few weeks ago on the MGB forum about the correct orientation of the cups. The MGB technical manual specified that they should have the cutaway notch in the pistons pointed inward, toward the hub.

The midget manual was very specific that the cutaway should face toward the oncoming rotor surface, and this seems to be supported by most other sources on the internet. Since the calipers on the Midget are behind the axle, this meant that the cutaway should face toward the road surface beneath the car.

Now we seem to have a third source. Could you please elaborate on what the MGA manual recommends, and how you have your's oriented now, as it seems to work for you?

Thanks in advance - - Alec
Alec Darnall

Alec you are quite correct the MGB manual suggests different positions for the cut away section to that of the MGA manual. The MGA "workshop" manual says the cut away section should be at the bottom and therefore opposite the bleed nipple. Thus when in use the caliper being at the rear of the wheel the cutawy would be towards the disc rotation. The MGB on the other hand is shown as the cutawy being towards the inside of the disc and thus vertical. I used MGB pistons in my calipers and set them as per the MGA manual. I do not have any squealling problems, would I have had, had I installed the pistons as per MGB?
Bob (robert) I am coming out!! yes I once owned an MGB!!

Hi, Bob - that was the controversy. All that I was able to find on the web specified that the cutout for several different cars must face toward the oncoming rotor surface. In fact, the Porsche site specified a 20 degree cant to the alignment, and even offered a template you could use, apparently because the outer edge of the rotor would be coming at the piston at a slightly higher velocity (linear not rotational).

The deduced reason is so that the leading edge of the brake pad has less pressure on it, and that causes the pad to wear more evenly, and to not "dig in" to the rotor surface due to the inevitable "camming" action of the pad itself. That high speed "chattering" is what we hear as brake squeal.

But the MGB crowd was almost universally sold on the "toward the hub" method that the "official" manual recommended. I really can't blame them, but - since that thread was about brake squealing - well, I just don't know.

Pete's post was the first one I saw that seemed to cover the "before and after" effects of cup rotation, and that is the reason I was curious for some more information. Looks like you're another one that can speak from experience.

Now, if you would just rotate them the wrong way to check ...... <grin> - - Thanks for the post - - Alec
Alec Darnall

Alec

Get knotted!
Go screw those bits of "Midge" back together...

Bob

Exactly right - a drawing - Fig MM.4 is shown in the manual in Section MM.5, that's how I discovered it.

To rotate the pistons, with the calipers removed, pistons right back and dangling on the fex, I could just get a pair of mole grips to grab the central pillar in the middle of the piston and gradually I worked them round. It is a right pain even with new recon calipers (Alec).

Pete
Pete Tipping

Thanks, Pete, for the clarification. You're right, of course, and I need to get off my fat ar*e and get my baby back together. This living vicariously through the trials and tribulations of others is fun, but it doesn't get me back in the driver's seat.

So, my conclusion is that the MGB manual is just wrong, and that someone took too long a lunch hour that day in the technical writing division.

Cheers, and thanks again - - Alec
Alec Darnall

This thread was discussed between 19/03/2006 and 25/03/2006

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