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MG Midget and Sprite Technical - Cooper S Wheel Cylinders

I run my midget with 3/4" bore Cooper S rear wheel cylinders.
Anyone ever tried 5/8", I don't run with a servo and GMC151 master cylinder.

Peter King

I used the smaller S cylinders (15mm?) on the racer in order to stop the rear brakes from frying.

Worked well.
Dave O'Neill 2

Thanks Dave.
Do you bolt them in to save movement?
Remove roll pin and tap?
Peter King

Yes, M5 button head socket screw.
Dave O'Neill 2


I know this is not quite the question you asked, but you can get 4 different wheel cylinders for Minis:

11/16" (0.68) - Unipart no GWC 1129;
5/8" (0.7), GWC1101;
3/4" (0.75), GWC1102;
15/16" (0.9375), GWC101.

Note 2 different sizes for Cooper S (which itself came in 3 engine sizes). Car info in:

M Wood

And then there's GWC1103 (13/16") and GWC1122 (7/8") of the MGB, also identical on the otuside just different in the bore. So 6 different sizes to chose from.
Important point is that all these have the roll pin in a different place to warn you that if it doesn't match then there's someing amiss. Though Darwin types ignore such warning and achive massive imbalance left to right ...

(At the risk of sounding picky, 5/8 is 0.625", but if you are anything like me the 0.7 is a bit of temporary brain fade)
Paul Walbran


Good info - even more to choose from! Thanks for correcting my fractions - brain fade (hopefully the only fade on this thread).

Also if playing with rear wheel cylinder sizes, you may want to fit a fixed or adjustable valve to limit fluid movement to rear brakes under hard braking:

This raises what braking characteristics you want, what do MG Midget Challenge racing cars run in the less modified classes, as well as sprint and hillclimb cars, if the 3/4" cylinder is fine, and scope to alter things with rear and front friction material? (Plus tyre types, sizes and pressures and wheel sizes, suspension settings..)

M Wood

Search 'Brake pressure valves'
For 3 versions plus union

M Wood

C & C parts :

This is the site where I buy brake parts. They manufacture them and much better than original. I guess that they deliver to MxSS Mxnni spares etc

Flip Brühl

This thread was discussed between 28/09/2014 and 05/10/2014

MG Midget and Sprite Technical index

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