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MG Midget and Sprite Technical - Brake calliper piston

A long time ago I converted my Mk2 Sprite from front drum brakes to discs.
Recently read a works manual and it said that the piston cutouts should be pointing downwards. My pistons don’t have cutouts! What is the purpose of the cutout, anti squeak?
Looked on the internet and some pistons have cutouts and some don’t. The ones that do have the cutouts show them on the side, pointing towards the hub, not on the bottom.
When I fit the shims I’ve always had the largest cutout on the bottom, not sure why but this would tie up with the piston cutout (if I had one!) on the bottom giving full contact between piston and shim. If the piston cutout is on the side then there is only partial contact with the shim.
As the shim large cutout is larger than the piston then does this not give same effect as a cutout in the piston?
I’ve had no issues with my brakes other than initial squeaking with Mintex 1144 pads, resolved by fitting Greenstuff. Now have Yellowstuff as recommended by EBC.

Les
Les Robinson

And another question: my brake pads overlap the OD of the disc slightly so as pads wear there is a slight rim on the pad. Is this normal?
Les
Les Robinson

Les, I ised to think the cut out was to give a slight 'angle of attack' to the pads. But what about for draining any water out of the piston?
Or better still - maybe it equalises the air pressure as the brakes get hot.





GuyW

Thought the same Guy--about giving the pad a bit of a tail end heavy approach. On some high performance multi piston calipers I've seen the leading end piston is smaller than the trailing giving a heavier tail end to the pad and the pads wear quite evenly--It might be a long term pad material wear evening out fix where if the piston didn't have a lead cutout the nose end of the pad might wear more than the tail--??
It's interesting though, MGA's have them downwards but MGB have the cutout towards the inside.

willy
William Revit

Yes, it's because of the Too-Inn that keeps the brake from biting and because of the wear. With the first disc brakes such as on the Healey 100 S, asymmetrical pads were indeed needed. They were not there at first, resulting in uneven wear.




Flip Brühl

I know that is the theory, but I am not totally convinced. Whatever angle is created by the cut out, whether a toe in or toe out, wear would pretty quickly nullify that and face of the pad will become parallel to the disk surface.

I think it works slightly differently to that. The pad doesn't have a toe in, or if it does that is quickly lost. But the cut out does mean that the pressure that the piston exerts is concentrated in the area of no cut out, creating a 'leading edge' effect, probably preventing snatching or maybe reducing brake squeal.
GuyW

My pistons do not have cutouts, looking at pictures on internet they are the same as some others for sale ie ones from MGOC. The ones from MOSS have cutout in the wrong place.
Now the shim has a large cutout which is bigger than the piston, so if placed at the bottom then this means only part of the piston area is in effect pushing on the pad. Granted the shim thickness is less than the piston cutout depth but not sure that matters.
My brakes work fine but then again have never had pistons with cutouts so can’t directly compare.
Les
Les Robinson

This thread was discussed between 09/08/2024 and 11/08/2024

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