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MG Midget and Sprite Technical - Brake caliper pistons. Are these ok?


I bought some new pistons for my brake calipers but they're different to the old ones and I'd like to know why and if they're ok.

The original had a level surface where they contact the brake pad, like all pistons, but the new ones have part of it machined of. See picture.

This seems odd because the piston will not apply an even pressure on the brake pad. I checked the part number in the Moss catalogue and they look correct 17H7960.

Could someone please tell me what's going on?

Thanks,Greg

Greg D

Greg, The stepped ones are the correct version. It reduces brake pad squeal and snatch.
Guy Oneandahalf Sprites

Greg
As Guy says these are the correct ones, but IIRC they need fitting in a correct orientation, I just can't remember off the top of my head. Guessing I would think the cutout needs to be towards the front of the car.... but I'm sure someone will be along soon to tell us.
Ian
Ian Webb '73 GAN5

Hi, the notch faces down (facing towards the rotation of the rotor, and since our calipers go rearward of the wheel centerline, that's down)

The piston should be rotated so that the notch edges are at roughly 3 o'clock and 8 o'clock.

By the way, if you also have pad shims, you'll see they've got a large, oddly shaped hole that mimics this and you can use that as a guide (the shims go with the large hole down, small hole up).


Norm
Norm Kerr

Thanks guys, very helpful. I don't have pad shims but I understand what your saying Norm about the rotation of them.

I'm surprised the uneven surface on the piston doesn't cock the piston making it jam or cause uneven pad wear

I never had a problem with the brakes snatching but I assume if they have changed the design it must have been a problem.

Greg
Greg D

Greg as I understand it the "cocking up" action sets a certain amount of slightly angled pressure due to the cutaway which enables the pads to grip better under rotation

And of course I won't be surprised or upset to find I am way off the mark, if it can be better explained.

Looking at the calipers on my Ford Monjo it seems as if a similar action is imparted by the inner and outer sections of it having the ability to float a little as pressure is applied

By the way the calipers pistons I took out of my "unmessed with 1966's" brakes back in '89 had the notches too
Bill 1

Greg as I understand it the "cocking up" action sets a certain amount of slightly angled pressure due to the cutaway which enables the pads to grip better under rotation

And of course I won't be surprised or upset to find I am way off the mark, if it can be better explained.

Looking at the calipers on my Ford Monjo it seems as if a similar action is imparted by the inner and outer sections of it having the ability to float a little as pressure is applied
Bill 1

I doubt whether anyone really knows.

BMC had the same type of piston for both the B and the midget, they differ in size only by 1/8" or so. For the Midget as above the manual says fit the cut-away facing down - but for the B the manual says fit the cut-away facing inwards.

Figure that!!
Paul Walbran

As aul says these cut outs are extremely confusing.

In the Factory manual the description simply states note the position of the cutout. Looking at the drawing it is possible to see that the cutout face the top ie towards the bleed nipple. This means that it is away from the direction of travel of the disk

When I did my MGA I actually fitted MGB caliper pistons into the MGA calipers. In the MGB manual the piston cutout as stated by Paul faces the axle (centre) whilst the MGA workshop manual suggests that the cut away should face downwards or in other words facing the direction of disk rotation (opposite to the Midget)

With all this in mind I think it is fair to say it is a total mystery!
Robert (Bob) Midget Turbo

On bicycles one sets the brake blocks at a very slight angle in relation to the contact surface (rim) such that the trailing edge contacts first. This prevents them from squealing. Maybe the step is to produce the same effect.

OTOH, as brake pads have a pretty rigid backing, maybe the step is there to maintain a clear slot and equalise pressure inside the cup of the piston. With temperatures produced there must be some strange pressure changes.

Or maybe it is something to do with control of expansion stresses in the rim of the piston, again with changing temperatures.
Guy Oneandahalf Sprites

IIRC this is related to the reason that calipers with more than one piston per side often have pistons that differ in size, more obvious on the likes of 6 piston racing calipers with 3 pistons per side where they reduce in diameter in the direction of rotation IIRC. I asked the question many years ago at a racing company and it was mentioned that the reason was to do with the heat build-up from the leading edge of the pad to trailing edge and the differing pressure applied to the pad compensated.

Regarding the factory piston illustration, IIRC it was mentioned here that one such illustration was incorrect and showed the piston the wrong way round.
David Billington

What has been said about the cut out at the leading edge (bottom) makes sense as the most force and initial contact will be at the trailing edge and so less likely to grab and should release better.

I'm aware that there is varying piston sizes on some 4 and 6 spot calipers for teh same reason.

My orignial ones ('69 GAN4) were flat like all the twin opposed calipers I've seen before and they worked just fine. To make a change like this is just a recipe for problems. With the original design no problem, but now the're rotation specific. If it's done correctly perhaps there may be a small benefit but if not and the gap was in the wrong place at the trailing edge then when the brake was applied the inital force is at the leading edge and the brakes are more likley to grab and not realise as well.

I wonder how often they have gone in the correct way. I'll just put them in with the gap on the leading edge and hope it's right.

The KISS principal has certainly not been applied here.
Greg D

"The KISS principal has certainly not been applied here":

the issue is the elimination of asbestos from brake pads, which has lead to a whole lot of brake issues, like squealing.

brake squealing was not as much an issue when these cars were built, but since then, using the latest production car countermeasures for things like this just makes sense, although, arguably, it is not as simple as it once was, I'll grant you that!



Norm
Norm Kerr

This thread was discussed between 02/12/2010 and 03/12/2010

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