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MG MGB Technical - 4-pot brake used austin princess/SD1

I would like to know where to find used austin princess/sd1 4-pot brake calipers and vented discs. If anyone can help I would be very greatful. Thanks

christos nanouris

I have also been researching SD1 brakes. I know the twin pots are a straight swap with sleaved bolts. but what I cant ubderstand is why no one seems to use the 4 pots which have the same mounts. is it because of fluid displacemnt problems from the master cyl. i hope this adds to your discussion.
Great site

Hi, this spring I bought used Princess brake calipers for my 1966 B. The only source is now days scrap yard but if you are not from UK as I am not, then you can look at e-bay (UK one). I bought calipers for 50 GBP plus p&p but I spend also 60 for total rebuild, and you will have to make new hoses (very cheap if you decide to do yourself). Anyway if you do not want to make your hand dirty, costs for Princess upgrade will be very close to Willwood upgrade.
rgd toni


Princess 4 pots have appox same piston area as B set up, whearas the SD1 4 pots appox 15% (2 pots 11%) increase, which changes brake balance and has potential to increase stopping distances depending upon other changes to the brake system.



I got my set from a colletor of Pricesses, yes that exists. They were sandblasted and eight new pistons were turned from stainless steel. A replacement set of rubber o-rings inserted and the whole setup was painted. I went for the solid V8 discs and I discarded the option of using vented discs since I donīt race my V8 roadster. The braking power is a slight improvement over the standard B-disc. My largest gains sofar, is a delayed brake fade compared with the originals. If you choose the vented disc from a Peugeot 604? with the same disc diameter ie 273mm, you will further delay brake fade but gain nothing in braking power. Brake pad size and disc diameter is the formula to look for. I am very happy with my 273m solid MGBGTV8 discs, which gives me plenty of front braking power, so great that my rear wheels lock up. Eventually I need downgrade my rear setup and thus regaining front/aft brake balance.


Paul is right about the slightly bigger area of the 4-pot SD1 brakes, but they still seem to work OK. Geoff King has them on his V8 using the later type of master cylinder and the pedal travel is not excessive.

Mike Howlett

I use 4-pot non-vented SD1 calipers with solid V8 discs; Mike Barnfather has vented using Peugeot 505 turbo discs.

Paul, I assume you mean with the larger piston area the fronts can lock up earlier increasing the stopping distance? On mine the rear Sierra discs lock up just before the fronts if severely tested but in normal use the balance feels fine. I donít have a long pedal despite the larger pistons but I did pay particular attention to minimise disc runout so the pads donít get knocked back into the calipers.

As a simple and cheap upgrade using readily available standard parts (new from Rimmer Bros) I can recommend 4-pot SD1 calipers. They bolt straight on with thin wall sleeves on the bolts and metric to imperial hoses from Dave Vale at V8 Conversions.

Geoff King

You have all been most helpful and I now have a good idea of what direction to go.
It also seems as if the only changes really nessasry on the rear is to down size the calipers to Roadster spec?


Depends on tyre grip, pads compounds, piston spec and disc size as well as many other factors that change weight transfer under braking. However the main objective of brake system is to dissipate heat, hence the vented option if pad fade is an issue.


No point having great big calipers unless you've got great big tyres, and something heavy to keep them stuck to the road. You'll just lock up the wheels.
The MG calipers and are right for the size of the car. The big problem is that as these solid disks heat up you get brake fade. You need to cool those disks.
Solutions- Easiest (cheapest) first.

Remove the dust sheilds/splash guards at the back and duct cooling air toward them.

Drill lots of holes in disks (much as motor bikes have) This increases the surface area availiable for cooling. A little less area for gripping though.

Use GTV8 disks, which are thicker and can soak up more heat. Need to use the inside half of triumph 2000 or 2500 calipers and the outer MG half (to get a wider gap).

SD1 calipers will grip more evenly, being 4 pot) but are designed for smaller disks (14mm smaller in diameter). I assume that the master in SD1's is of a narrower bore and the pedal of a longer travel, in order to grip harder, to make up for 14mm less leverage. They also must fade more quickly than MG brakes, less surface to radiate.

Princess Calipers will go over the thicker GTV8 disks. 4 pot, more even grip, larger by about 12mm in surface, better grip. Straight bolt on operation.

Unlike the MG (& triumph) calipers the princess calipers split evenly down the middle. This means you can add 10 mm spacers, allowing a much thicker ventilated disk. These are what became RV8 calipers.
It is common to use peugeot 504 GTi, 505 Turbo or 604 ventilated disks. You need to redrill the bolt holes, to match MGB, no drama. The ventilated disks radiate heat very efficiently which is why most modern cars have them.

I am using Volvo calipers from a DL I believe. Not to be confused with earlier solid disk calipers, these are designed for ventilated disks. Large surface area 4 pot with the same pedal travel as MGB. There is a draw back, the mounting holes need to be redrilled to accomodate the wider MG bolt pattern. Internally they are crossed drilled so that they have a single outlet. Other wise you need use a T piece. I am using nissan ventilated disks with a spacer bolted to them. This was bought as a kitfrom MG workshops here in Melbourne. The Kit has a long history and easy approval from authorities. ADR reg' numbers on stainless adaptor hoses etc. They work very well indeed with no brake fade I can detect.
More on this in the archives.
Willwood calipers seem to becomeing more competitive (pricewise) and now have rubber seals for road use.
Heard a rumor the Ford falcon calipers are suitiable.

Instead of replacing the rear cylinders, you can install a brake proportioning valve in the fluid line running to the rear brakes. Most of the ones I have seen are fully adjustable so that you can achieve just the right balance for your driving situation. One of the suppliers is summit racing. Ray

This thread was discussed between 17/09/2005 and 20/09/2005

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