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MG MGB GT V8 Factory Originals Technical - PRINCESS CALLIPERS

I am thinking about using 4 pot callipers. Any advice on how to convert from two feed lines to one (t piece in the brake line to both inlets or a pipe from one of the bleed nipples to other inlets) and what disc"s (solid or grooved) and pads (are the Greenstuff as good as they are made out to be ?) should i use ? Is it worth the effort as the book How To Convert Your B To V8 Power say"s that the pad sizes are about the same, so is there any difference in braking after converting ?
Many thanks Neal

I dont believe you will get many answers from the "experts" they have answered this question before and don't like having to repeat themselves you will find a lot of good advice in the archives on princess calipers. But as a novice I believe that Princess caliper do have a slightly larger pads the reason most people fit them is to use the thicker v8 brake disk. As for are they better for braking I will tell you when I have fitted mine in about 3 weeks. Any other questions I am quite happy to chat afraid I am not an expert though only just learning

Mick Black
Michael Black

I think that you have the latest edition of that book which may have a picture of my Princess caliper with vented discs shown.

Firstly the result is far better than a comparison pof pad size, which incidentally the Princess has around 10% approx greater surface area. Secondly the provision of 4 pistons spreads the clamping laod over a wider area thus providing a better clamp onto the disc. The result is a more powerful brake when simply replacing the V8 caliper.

To fit to a standard MGB needs a coouple of things doing. Firstly as you say the Princess caliper is a twin feed to two separate circuits each feeding a pair of the pistons either side of the disc. An ideal situation would be to use each as designed and create a true split braking system so you always retain good front braking if a circuit fails. This is a more complicated system to pipe up and needs a tandem master cylinder.

The common route to convert to a single feed is to bridge between the bleed nipple of one circuit and feed that into the second feed apperture. My system uses formed metal pipes and standard ends (professionally done by Hi Spec Motorsport from Dartford, Kent) This allows the external pipes to be routes neatly and close to the caliper body. I have recently seen a specialist high performance race car with the smae principle using braded and flexible hoses - I didn't like that at all. Externally routed metal pipe is the norm for many AP Racing and other big brake companies calipers.

Having converted to single feed you have the issue of a metric thread in the caliper connecting to an imperial pipe union on the car. Dual thread braded flexibel hoses are available from the likes of Clive Wheatley and can be made up by any of the specialists who deal in this sort of gear.

Now the unit needs to be fitted to the hub. If you use the standard MGV8 solid disc then the standard Princess (or Ambasador) caliper will fit over this with no mods sicne the Princess uses a 270 x 12.8mm against the MG 273 x 12.8mm disc.

You do need to trim the backplate somewhat if you want to keep it, but most junk it to aid the flow of cooling air. Finally bolting the caliper to the stub axle needs a shouldered bolt since later calipers have a large diameter bolt hole in the caliper, although the hole centres are the same as MGB. Fortunately Girling went this way earlier and Triumph had to adapt for the models that used the Girling calipers. In this case GT6 and TR6 use a shouldered bolt with the same imperial thread as the MGB stub axle.

Unless you drill out the holes in a new tab washer to the bigger bore it won't fit. Alternatively use a thread lock to ensure caliper bolts don't loosen in use.

Quite whether you stay with a standard V8 solid disc or go ventilated depends on both the power of your engine and the type of wheel fitted. Power is obvious, but wheel design dictates how much air gets round the brakes and so how well it is able to get heat away. A wire wheel is the best and is more effective than adding 25mm to the brake diameter and uprated pads. If the wheel is an enclosed design like the Minilite of standard Rostyle then with a warmed V8 the vented disc is almost a requirement.

To vent the brakes is quite straight forward and involves not just the disc but also the spacing of the caliper. My route has been to use a Peugeot 505 Turbo?605 Ti 273mm x 20mm vented disc. Redrill the centre to match the MGB hub and bolt up as normal. The caliper is spaced by way of one of a number of kits available as it is a common caliper to be used in many competition car conversions.

The venting operation doesn't improve the brakes beyond the standard performance as found with the solid disc, but what it does provide is greater cooling capacity so that as the heat generation increases the capacity of the brake to disperse the rising heat energy remains above that increased generation of heat.

Roger Parker


You say you want to go to Princess 4 pot calipers, but unless you already have a set ready to fit you may wish to consider something else. The Rover SD1 Vitesse was fitted with 4 pot calipers and vented discs in the later years of it's life, and these offer a considerable increase in pad area and piston area above the princess unit.

I have fitted a pair of the almost identical (but difficult to find) Metropolitan Police spec calipers to my GTV8 some time ago. The only difference between the two sets of calipers is that the Vitesse versions are for a vented disc and the Police one's for a solid disc.

Fitting is exactly as per Roger's guide above and you can use the same parts, i.e. Peugeot disc for the vented version and V8 for the solid. Clive Wheatley is again the man to speak to for flexible hoses and the link pipe is required again for the Vitesse calipers as per Roger's guidance.

These calipers will offer you even better braking performance than the Princess versions and you will not need anything other than standard Mintex road pads unless you want to go racing. The bigger piston area also reduces the amount of pedal pressure required, and although in theory the pedal should travel slightly further I have not noticed this at all.

I have V8's with both the Princess and the Rover setup fitted and would recommend the Rover option anyday the performance is SUPERB!

Dave White
MG Northumbria
Dave White

In addition to the SD1 and Vitesse calipers it might be worth looking at Sherpa V8 and some Ford Transit calipers.

Also look at the cost of pads, RV8 pads fit in the RV8 caliper (obviously!) and probably also the princess caliper while Land Rover Discovery pads fit the 4 pot vitesse caliper at considerably less cost.

Dave Brooke

Beware that there are many detail differences to the same basic caliper casting. For example the Met Police spec is not just one spec, as braking on patrol SD1's was an on going problem even after the vented disc/4 pot route that formed the basis for the Vitesse system as the Police versions predated the Vitesse. There were both solid disc and vented disc 4 pot fittings and usually the solid disc fitting had a single feed which is an advantage. These calipers can be used with vented discs by using a proprietry spacer kit should you come upon some. Sherpa and Transit vans used these solid disc 4 pots.

Range Rover and Discovery also used a variation of the same 4 pot caliper, BUT do be careful to check the mounting to stub axle arrangement as there are some with a wider spacing and so this would need some engineering work to enable it to corrctly fit the mGB upright.

Finally note that all Princess/Ambassador/Morgan/RV8 calipers have a 38mm diameter piston against the 41mm diameter size of the Rover based ones. The standard MGB uses a 54mm diameter piston that equates very closely to the 38mm diameter 4 pots. Conversely the 41mm diameter REover units relate to the base SD1 two pot caliper piston diameter of 57mm.

This means that whilst the work achgieved at the wheel is increased due to this difference in size so is the amount of fluid displacement from the master cylinder for the same piston movement. The end result is a brake pedal that goes down further, especially where the single circuit 19mm master cyl bore is retained. There is also a need to ensure that slack is kept to a minimum in the rear brakes to reduce the extra pedal travel to a minimum.

I do know many who have fitted the larger calipers and not felt that the pedal travel is excessive, yet there are some who have felt that it is. The point here is to be aware of the situation.

Roger Parker

This thread was discussed between 21/01/2001 and 28/01/2001

MG MGB GT V8 Factory Originals Technical index

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