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MGB and GTV8 Front brake upgrade
Geoff King in Kincardineshire has sent in a useful upgrade to the front brakes on his V8 Roadster conversion. (May 05)

Powerful four piston calipers with vented discs can easily be retrofitted to the MGB or GTV8 using components from specialists such as Wilwood, however, the following notes refer to a cheaper alternative using readily available standard parts.

Four piston calipers with vented discs. (Photos: Geoff King)

Four piston Rover SD1 calipers (see above) or the calipers from an Austin Princess bolt directly on to the MGB or MGBGTV8 stub axle; both calipers have the same distance between mounting holes but thin wall sleeves or stepped bolts are required for the metric holes. Special hoses are also required with an imperial fitting on one end and metric at the other to connect the Metric caliper to the existing Imperial MGB hydraulic system. Sleeves and hoses are available from Dave Vale at V8 Conversions. The dust shield can be left off, to increase cooling, or it can be cut slightly to clear the larger caliper. The calipers bolt up with no other modification and it looks as if that was how MG built it originally.

The standard MGB and GTV8 discs are 10 ¾ inch diameter (273 mm), Peugeot 505 Turbo vented discs are also 10 ¾ inch diameter (273 mm) with the correct offset to centre in the Rover SD1 or Princess calipers and simply require re-drilling for the MGB bolt PCD. The disc spigot bore is very slightly larger than the MGB hub and ideally it should be sleeved to minimise the risk of radial runout of the disc.

Note: The RV8 vented disc is smaller at 10 5/8 inch (270mm) diameter and the Rover SD1 vented or solid discs are considerably smaller at 10 1/8 (258 mm); fortunately neither will fit the MGB hub as the offset is incompatible.


The area of the four Princess caliper pistons is the same as the two MGB pistons, therefore, pedal travel is unaffected. The SD1 has slightly larger pistons which need more fluid to move the same distance so there will be an increased pedal travel if used with the original master cylinder, although, in practice this additional pedal travel would probably go unnoticed. The late rubber bumper MGB master cylinder with the integral servo has a larger bore and is a better match for SD1 calipers but to fit this to an earlier car it is necessary to change the whole pedal box assembly.

Four piston calipers are bigger and heavier than the MGB ones so unsprung weight is increased, however, the difference is not huge and for road use can be ignored - and anyone concerned about unsprung weight should try weighing a rear axle assembly, but that is another story.

Although the calipers and discs fit on the stub axles very easily care must be taken when fitting the wheels. SD1 calipers won't fit behind Rostyle wheels without using ¼ inch spacers, so trial fit and check for clearance if using wire wheels, aftermarket alloys or original GTV8 wheels.

With any front brake upgrade the front/rear brake balance should be considered and the use of the smaller roadster rear brake cylinders in a GT backplate would reduce premature rear lock-up.

Finally, no matter how good the brakes are they will only be as good as the tyres at stopping the car. To avoid compromising safety renew old tyres, even if they have considerably more than the legal tread depth. The tyre might look brand new, but rubber degrades over time irrespective of whether it is being used or not, the tyre will lose flexibility and handling and grip will be reduced.