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MG MGB Technical - Brake Master Cylinder Bore Diameter

What is the diameter of the bore of the brake master cylinder on an MGB?
This is the non-servo, dual hydraulic line type of 1968-1974.

Mick Anderson

Dear Mick,

I am quite sure that it is 3/4". The size is cast into the cylinder on the side.

Shareef Hassan

A bore size of 3/4" presents a problem.
Several people are considering using MGB dual line brake master cylinders on MGA's.
The MGA, both pushrod and Twin Cam, use brake master cylinders with a 7/8" bore.
Can someone check a non-servo, dual line, brake master cylinder for markings of the bore size?
These should be on the MGB 1968-1974.

Mick Anderson


On my MGB (1970) I have a non-servo brake MC, it is a dual hydraulic line type. The size casted on the side is 3/4".

Due to a leakage I had to dismantle the MC.
The primary piston moves in a 1" bore and the secondary piston in a 3/4" bore.
I use the MGB workshop manual PN AKD3259 14th edition describing this MC.

Remi B

Wilwood has a 7/8" dual cylinder unit, you might want to consider.
Bob Muenchausen


In United brake cat. for those years a tandem cylinder is 3/4" #1035. For later applications it is 13/16".

Jean Guy Catford

This thread was discussed between 09/08/2006 and 11/08/2006

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