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standard brake piston size?

Ian Macquarie

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Thanks Phil, that's the same info as I have (from James Whiting's site.

What I was trying to find was the data for the other calipers and pistons so that I can make an informed judgement of the likely changes in brake balance / effort required with different caliper / master cylinder combinations.


Whilst we are at it would someone like to confirm (or otherwise) that the master cylinder bores are....

Standard 5/8"

Uprated 13/16"



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The standard M14 Girling caliper is 48mm bore diameter and the Ford Sierra rear caliper is 43mm bore diameter.


If the uprated brake sizes are correct then they will give a little bit more rear axle bias to the brake system. You would then need to be careful which pads you fitted to ensure that the rear axle doesn't lock first.

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Thanks for the info on piston sizes.


I don’t follow your point about brake balance though – I had it as follows:


Caliper Dia Total piston area

Sierra rear 43 1452

M14 48 1810

Alcon front 38.1 & 34.9 2096

AP front 41.3 & 38.1 2480


I took this to give Front/rear splits of braking force (or at least force on the brake pads) to be as follows


M14 / Sierra 55% / 45%

Alcon / Sierra 59% / 41%

AP / Sierra 63% / 37%


This would mean that either of the upgrades would move the balance to more braking at the front and reduce the likelihood of locking the rears first. Is this correct or have I made some errors somewhere?




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