I have an MGB GT 1980 with a Mazda MX5 engine and gearbox. I am using the standard MG clutch master cylinder with the mx5 slave.

The pedal pressure is quite hard and I am wondering if changing the master cylinder which has a 3/4 bore for a girling with a 5/8 bore would help.

Do you know if the girling has the same bolt configuration as the mg. Do you have any other bright ideas please? Thanks Colin

Switching from a 3/4" to a 5/8" bore will reduce pedal effort by increasing hydraulic leverage, although it will also increase pedal travel slightly. This should help soften the pedal feel when using the MX5 slave cylinder with your MG's clutch system.

As for compatibility:
The Girling master cylinder (GMC-10-I) has a 2.25” (57.15mm) bolt spacing, centre-to-centre, in a vertical (north-south) configuration.

The original MGB master cylinder typically uses a horizontal (east-west) bolt pattern, so the Girling may not be a direct bolt-on. You may need to fabricate or purchase an adapter plate or modify the mounting.

A couple of “bright ideas” to consider:
Check for binding: Make sure the clutch fork and slave cylinder travel aren't restricted or misaligned, which can increase pedal effort.

Pedal ratio: The mechanical advantage of the pedal itself matters—double-check that you're getting optimal leverage from your clutch pedal.

Alternative master cylinders: There may be other 5/8" or 0.7" bore options more compatible with the MGB mounting pattern if you want a more direct swap.

Let me know if you'd like help exploring compatible master cylinders, or if you're open to slight modification for a better pedal feel.

22nd April 2025