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MG MGA - Caliper piston cut-away orientation

During the front hub replacement job, I noticed that the cut-away portions of the pistons in the brake caliper are oriented such that the inner piston has the cut-away portion facing forward while the outer piston has it facing toward the top. The shop manual cautions that they should both be facing downwards. Should I be concerned about this? Anyone know the theory behind why the manual directs their placement in such a manner?
Rick deOlazarra


It's certainly an odd one and opinion seems to vary throughout the forums - all cars, not just MGs. Suggestions about the cutaway position vary from providing even pressure; providing a drain for moisture and debris; and eliminating brake squeal. My MGB calipers have the cut-out facing towards the caliper mounting holes. i.e. inwards towards the axle.


Steve Gyles

I think the cut-out in the piston is there to bias the braking force towards the trailing edge of the brake pad which would have the effect of reducing any pad chatter/judder or squeal if you were going fast enough.

I likened it in some way to when sharpening a chisel on a revolving grinding wheel; if you place the chisel on the leading edge of the wheel it gives much more vibration than when placed on the wheels trailing edge.

But I may be wrong!!.................Mike
m.j. moore


That's also one of the theories. My list was not complete!

I just found the following comment which is interesting: "....the reason given to me by a Mintex brake engineer a few years back was they are there to minimise the area for heat transfer from the pad
back into the fluid (at the expense of overheating the pads)."

My interpretation of that is that the cut-away will be closest to the brake bleed nipple/drilled fluid pathways. That would be sort of logical for the MGB caliper and also would put maximum pressure towards the outside of the disc, which would also seems sensible for maximum braking efficiency.

Of course, all these orientation diagrams for our cars were done in the days of asbestos pads. Are they still relevant (or even more so) for the later more efficient pads?


Steve Gyles

Sounds like it could be anyone's guess as to what benefits compelled the designers to orient the cut-away this way or that & to instruct those who replace the pistons to follow suit.
Thankfully, it's vague enough so that I can convince myself that it's no big deal to just leave them as is; the brakes have been working just fine. If I replace the seals and/or pistons, I'll orient them accordingly. Appreciate the comments; thanks.
Rick deOlazarra

This thread was discussed between 05/02/2016 and 07/02/2016

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