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MG MGB Technical - What does the cutaway in the calliper piston do?

I am replacing the seals and pistons in the front brakes (2 are very rusty, so I can't re-use them)

I have a set of replacement pistons, that have no cutaway- they are completely flat on the pad side.
The ones that came off have a notch cut out of the pad side, that faces the hub according to the manual.

Will it make any difference if I use the 4 without notches as a set?

Why is there a notch anyway, and why does it face the hub?
Martin Layton

Answering my own question.

They are for anti-squeal. Modern cars use a tin shim between the pad and piston that doesn't cover all the piston.

Luckily, the new ones are only slightly magnetic, so they are stainless, I've milled a 10 thou' step across them.

Martin Layton

Martin. How did you mill the .010" step in them? Simply mount the piston in a milling machine vise and use a standard end mill to make the cut? Carbide or HSS end mill?

It would be worth having in the archives in case any one should need to do that job again. Besides, I should be interested in learning how to do the job in case I should ever need to do it myself.

As to the pistons. Were they a shiny, silvery color (raw steel) or had they been blackened? The last ones I saw, of the latter type, had a somewhat mottled finish as if the blackening bath were not designed for that grade of steel, a situation more common when blackening stainless steels than when blackening carbon steels.

Any information would be appreciated. Thanks.

Les
Les Bengtson

The reliefs are to alter the distribution of pressure across the pad, keeping them from going tapered. It seems to be not clear whether or how well it works, as there are differing applications re orientation; possibly to do with arcane matters of bracket flex, friction coefficients and the exact materials used. There may or may not be an effect on squeal - I can remember when disc brakes squealed at random and everything was tried with varying success. The depth of cutaway may also be a factor - .002-5 might act as a damper for vibration, .010 would just change the pressure map.
Les, it's a simple milling operation, and the usual rules apply. Very sharp positive rake tools on SS or you get work hardening which makes a bad cut, or no cut. Cobalt HS cutters best for light cuts, and take it all off at once, as a couple thou at a time will work harden it to death. Do not feed too slowly in an attempt to get a good finish - you won't, and it will give the same work hardening. A fly cutter with a very sharp HS bit at very high speed frequently works well and is cheap. Might be easier to do as a grinding op, with silicon carbide or diamond wheels and coolant.
FRM
FR Millmore

I just clamped a small 4-jaw chuck in the milling vice and held the piston by the lowest edge wrapped in emery cloth to avoid marking it.
Then I ran across it with a 5/8 HSS cutter in one pass, undid two jaws, and replaced with the next piston. By always undoing the same two jaws, the pistons have to be in the same place for cutting. (and I end up with a matching set)
Then the fray needs cleaning off with the dremmel.

The Indian stainless steel is free cutting, don't know what grade it is but the magnet will just about stick to it. It was plain grey, with polished outer surface, no finish.

I don't know if I've done the right thing, but I'm happier having pistons that look the same as the ones that came out.
I wouldn't have machined plated steel, as it would have made them go rusty from the cut edge.

Why I thought that they were anti squeal was my internet research led me to a document on brake problems and solutions, and the only problem that misalignment of the cut-out causes is squeal, also there are no anti-squeal shims fitted.


Now I need more brake fluid as I knocked the bottle over whilst I had the lid off to get fresh fluid to lubricate the seals. (I poured some fluid in the cap, and carefully placed the open bottle where there was no way that I could knock it over, then knocked it over!)

I will finish it off tomorrow!


Martin Layton

Martin and Fletcher. Many thanks.

Martin. I used a silicone grease to lubricate the seals. The pistons slid in quite easily. I had six NOS chrome plated pistons and used four of them on this project. Hence, my interest in how well the currently available ones are. I need to build up another caliper set, but wanted to get this car on the road and see how well everything worked. This is the first time I have rebuilt calipers and split them. They seem to be working very well now. But, I want to test them our for about six months before coming to any firm conclusions.

Les
Les Bengtson

Les,

I'm in the middle of rebuilding my calipers right now. I got the Lockheed/Girling & Delphi (one of each) full caliper rebuild kits. Both kits came with all of the seals (except for the seal between the halves) and pistons. The kits appear identical with the exception of the box they came in. Will need to double check, but pretty sure the pistons are chromed steel as were the originals.

The Girling kit was bought about 3 years ago and the Delphi was acquired early last year - both through Moss. If memory serves, the price was comparable to getting the parts individually.

They should be done this weekend. Will let you know what I find.

Tom
Tom Sotomayor

This might be a grandmothers and eggs scenario, but you don't ever need to split the 2 halves, unless you are fitting one of those spacers to accommodate a wider disc.

To remove pistons.

oil the pistons with brake fluid, and get one fully in, with levers and a g clamp, then clamp it in with an engineers clamp, or wire.

Place in a tub, cover with rag, and connect an air line to the fluid inlet, 120psi or so. DON'T try to see what is happening with the air line connected! When it goes "pop" turn off the air. Remove the seals and replace the piston. clamp it in and repeat to pop the other. The piston without seals will fit well enough to pop the other, and then shake out.

Clean up the grooves. Fit both fluid seals, lubricate with clean fluid, or red rubber grease.
Fit the dust seals to the rings, and lubricate. fit them over the hollow end of the piston, about 1/4" down. Lubricate the piston, and press in by hand. Now use a flat piece of steel and a G clamp to press the piston fully in, and press the metal ring in place. It helps to hold the caliper in the vice at this point, but not if you knock the brake fluid over!

Then fit to the car, bleed everything, and find that you have no front brakes! (See my other thread)
Martin Layton

Martin,
I like the 'grandmother and eggs' comment!
Been doing this for a little while (25+ years) - my caliper halves ALWAYS get split. It's far easier (for me anyway) to get the bores rust free when there is clear access to them.

It's curious to me though that so many are reticent to split the halves. Rebuilders do this all the time. How do I know? A friend bought some rebuilt calipers where the outside piston on one would not extend. The problem? It was the outside half for the opposite side and had no drilling for the fluid passage - it was on the other flange.

This set is getting cleaned up using electrolytic rust removal and it's working very well. They are coming out nice and clean inside and out. A lick of paint and they look like brand new.

My preference is to always use the red rubber grease. Parts can be rebuilt and set aside without the worry of residual brake fluid collecting moisture from the air and causing them to rust before they ever get used.

Besides being messy, brake fluid is a poison absorbed through cuts in the skin. I always have cuts on my hands. Gloves don't help much since they easily get holes or get torn.

But this comes down to 'horses for courses'. Everyone deals with this the way that is best for them.

Tom
Tom Sotomayor

Tom. My rebuild kits are Centrix brand, made in Oz. They contain the square O ring for replacement when the halves are split.

I decided to split the calipers for two reasons:

First, to see what was involved.

Second, because John Twist had written that it was impossible to fully clean them without splitting them.

After having done so, I am a believer and will continue to do so on my rebuilds. I do agree about the use of grease on reassembly rather than brake fluid.

Since we are speaking of some machining processes, I used the lathe to make up two round pieces, about .375" thick, which fit into the holes for the pistons and seals. A minor diameter of the piston's diameter and a major diameter of the seal's diameter. Two pieces, one with a piece of 3/8" rod attached and the other with a hole that will slip over the rod.

As the calipers are being put together for painting, but before any seals are installed, each caliper half has a spare seal installed in the top portion (inside facing) and the tool, using a spring to keep both halves tight in the bores, are held in place while the two halves are bolted together. When done, we have an assembled caliper with a plug into each piston opening, allowing the assembly to be painted easily, but making sure none of the paint gets into the inside passages of the caliper.

I used Gun Kote to paint the calipers--baked on it is solvent resistant and not affected by brake fluid. As a coating, it forms a physical barrier between the part and atmosphere. I am finding increasing uses for it in automobiles--any part that can be baked in my old shop oven can be made much more rust resistant.

Les
Les Bengtson

Les-
What's Gun Kote?
FRM
FR Millmore

Fletcher. It is a product that was developed to coat the main gun of battle tanks, providing a barrier between the metal and the atmosphere similar to what plating does.

The part to be coated is cleaned, abrasive blasted, cleaned again, then sprayed with Gun Kote. I use a small artist's air brush. The part is allowed to dry for a hour. It is then baked at 300 deg F for an hour. I use an old toaster oven for small parts and an old kitchen oven for larger parts. Need good ventilation as the fumes are not good for you. When coated and baked, the surface is sealed and has some self lubricating properties. The coating is resistant to abrasion (more than black oxide, less than plating) and cannot be damaged by common solvents. Holds up well over a long period of time. Excellent for use on brake back plates, drums, calipers, etc.

It is made by K-G Industries and Cal Gard, both of which have websites and offer other forms of specialist coatings.

Les
Les Bengtson

Les,
Both calipers got rebuilt today. The pistons supplied in the Lockheed/Girling & Delphi kits are all chrome plated mild steel. There was no visual difference between the components, so I'll assume that no process change had been made.

I'd like to get into a more durable coating system for my parts, but lack of space keeps me from getting an oven to bake parts. So, for the last several years I've used Rustoleum Professional brand paint in spray cans. After 'curing' for several days it is reasonably resistant to most chemicals. Unfortunately with the temps back in the 20's this weekend the paint won't have a chance to really cure before they get hit with incidental brake fluid drips.
BTW - This is the same paint that's on my winter wheels. So far they've held up well for the last two winters.

I haven't made any permanent paint masks like you did. My solution has been masking tape and a cardboard circle to cover the piston bore. The halves get painted seperately, each half gets rebuilt, then they are joined using a new seal between the halves.

Tom
Tom Sotomayor

This thread was discussed between 04/04/2007 and 06/04/2007

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