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.