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MG MGB GT V8 Factory Originals Technical - Front Brakes

2 questions:_
I've got Rover twin pot callipers and Princess ones. Ive heard some people say the bigger Rover ones are best. Anyone got a view on this? And what I also need to know is what VENTED discs to use and with what pads. I'm sure vented is the way to - crossed drilled original V8 ones seem a bit agricultural these days!
But....I don't intend spending a fortune on special racing type parts. It seems a waste of money when standard parts (low price) from various cars can do a good job on the MGB. (Are Peugeot 505 Turbo ones the closest fit?)
Pete Hawkins

Is it my imagination or has exactly this same posting appeared here within the last month.

When you go with 4-pots, you need to change out the m/c to keep pedal travel and feel constant ... or at least I did on my last B. My newest conversion just uses twin-loop non-servoed stock MGB brakes, with drilled rotors, greenstuff pads, and SS hoses, but otherwise bone stock, and the stopping is just absolutely perfect. All I'm saying is, you might want to consider saving the hassle and money, when a stock system in perfect shape will stop the car amazingly quickly.

A good post and a couple of pointers from my own experience not however on MGB's may be worth mentioning.
If possible get standard (as in non modified) parts as you are suggesting. The quality of material is essential in these applications where safety is concerned and all the main makers will err on the side of caution here.
Vented is by far the better method. Four heat radiating surfaces in place of the standard two and add a suitable piece of ducting to get air to the inner faces.
Cross drilling is banned on some race tracks these days because there is no way of telling how well or otherwise it has been engineered until it fails and regretably the failure rate is rather high, hence the ban. On a raod car it is more trouble than it is worth.
Slotting may be worth a look if you are looking to get the absolute max stopping power from the discs. Again on a road car you may find it simply leads to cracks and more rapid pad wear.
As to pads...there seems to be more drivel touted on this subject than on air filters, if that is possible, and I have always stayed with the main suppliers like Bendix, Ferodo and so on. All make a good range of sintered metal pads and again the quality is there from the start.
Just a few thoughts and no doubt others will add theirs!
Peter Thomas


I recollect a similar question.

Have you discussed the disc issue with Rally Design, Demon Tweeks, Cambridge Motorsport, Hi Spec. If so what was the outcome?

For road use Greenstuff work from cold and do not squeal and in comparison to other similar spec pads equal or exceed in performance in virtually all areas, and very little black brake dust.


Ted has raised an interesting point.

As I see it there are 4 main points of interest

1) Stopping from High speed - large front and rear brakes.
2) Stopping at low speed - avoiding lock up
3) Long term braking - Heat dissipation
4) Unsprung weight

1) easy - front rear discs as large as possible, as many pots as possible with brake bias valve - in your dreams?

2) Factory cars increase front braking but reduce rear?. Increasing front more should mean a further reduction in rear. Weight transfer needs to be considered (lowering car increasing spring rate). Add into pot Bhp well in excess of Factory cars.

3) Add vented discs

4) Expensive Alloy callipers - Is it worth it?

All the above are possible with the right budget but what is the optimum solution.

Also there is the question of drilling grooved or plain.

Pads are a simple choice IMO Greenstuff.


My solution was to use established and effective technology and go radically oversized, accepting the increase in unsprung weight as the penalty for totally reliable brakes. To do this I used the rotors and calipers from a Mach 1 Mustang, counting on conventional technology to eliminate surprises and counting on the massive size increase to improve performance. Use of standard non-metalic pads gave completely predictable performance characteristics, massive size increase gives long pad life. Better choices of parts can be made, but the principle is valid. Use proven technology and upsize as needed for the desired results, keep it simple and cheap, and work within your acceptable limitations.
Jim Blackwood

Jim B.,
What is involved with the MkI caliper/disc 'bolt up'? What (if) "8"" Ford rear works with this setup and the the stock master cyl? Please explain your setup. Regards, Angus

I remember this subject from last year and there is a significant amount of info in the archives. I have copied the "attached" from Roger Parker's response but all of the archived info is worth reading. For info I have used the Princess callipers with V8 discs and still have a pair fo SD1 four pot callipers for sale (which will need a rebuild). What pads are people using in their Princess four pot callipers (for road use)?
The archived material:-

Roger Parker, Tamworth,
Standard MGB calipers use two 54mm dia pistons whilst the Princess/Ambassador caliper and RV8 use four 38mm dia pistons. The SD1 in two pot application uses two 57mm dia pistons or in 4 pot applications there are four 41mm dia pistons. Those wishing to do the maths will see the proximity of the surface area between the MGB 54,, standard and the 38mm Princess type.

Add to this the standard single line MGB system uses a 19mm master cyl bore and the later dual circuit uses a 20.6mm bore. Of note is the fact that the Princess/Ambassador used a 20.6mm bore as does the RV8. By way of comparison the SD1 uses 22.2mm bore master cylinders.

The point is that the ratio between the surface area of the master cylinder and the surface area of the slave cylinder (caliper) has to be close to original otherwise you end up with a seriously increased pedal travel as more of the master cylinder strokje is needed to transfer the required volume of fluid for the same piston movement at the caliper. Yes rear wheel cylinder diameter and stroke affects things but the main displacement is at the front.

It follows that it would be unwise to match the single circuit MGB 19mm master cylinder with either of the SD1 calipers, although I know it has been done. The reserve of travel on the master when the brakes are in good condition and all possible free play is removed from the rear brakes is reduced to a level that I personally wouldn't be happy with. In fact I have actually converted my bulkhead to take the late pedal box along with the dual circuit direct servo set up with the 20.6mm master, to match the Princess/Peugeuot brake set up.

I have now a much wider option of alternative master cylinders off the shelf with a larger bore should I wish to go further with the brakes. I decided to do the change of master not just for the safety of dual circuit brakes, but to keep pedal travel at a minimum. One benefit of the smaller master cyl against a bigger one if you can live with the longer travel is that the actual work ratio is increased. This means that for every pound of pressure you input the smaller cylinder with the bigger calipers see increased clamping pressures, or lighter effort for the same braking.

For interests sake the Peugeot discs are the ones from the 604Ti and 505Turbo. They are the same 273mm diamter as the MGB and V8. The RV8 disc is 270mm diameter and a not unsubstantial 25mm thick.


For the benefit of the metrically challenged (me) I'm going to use inch measurements. It's not a matter of needing larger pistons or more piston area so much as it's a need for more cooling and slower pad wear. When cool, the stock brakes have plenty of excess capacity in terms of being able to stop wheel rotation under any condition. So clamping force is not the issue. Heat and wear are. Changing to a ventilated rotor takes care of the heat issue, but requires a different caliper. Larger pads reduce wear. With the stock caliper near a 2-1/8" piston size, area is about 3-1/2sq in. The Mach 1 piston is 2-3/8 and has an area of about 4-1/2sq in. This is almost a 30% increase in piston area, an increase which should be about 30% more than the change to the SD1. Even though it's not needed, it was the smallest US factory caliper available in 1980 according to my research at the time, bearing in mind my plans to use an 8" Ford rear axle. The Ford is a single piston while the MG is twin opposed, but looking at the dynamics in terms of applied pressure there is no difference. Why? Because the center position floats. Say the piston applies a ton of pressure just for simplicity. With the dual piston it applies a ton to each side of the rotor, against a fixed object. With the single piston it applies a ton to one side, but the parts are solid and the caliper floats, so it also applies a ton to the other side as well. So what advantage is the dual piston? It does not have to have a sliding mount. It also has disadvantages as well such as the ability of 1 piston to stick and the need for more parts and therefore weight and complexity.

The diameter of the rotor increases from 10-3/4 to 11.3" which puts a fair amount of additional swept area out where it does the most good, and the brake pads approximately double in size, dividing that mythical ton of pressure over a larger area and reducing unit force to about half. Less force means less friction, less localized heat, and less need for cutting edge materials. With the Mach 1 weighing at least 1000lbs more than the MGB that means the brakes are now greatly understressed. That in turn means that organic brake compounds are perfectly adequate, which in turn leads to finer control and predictability overall, and good linear friction properties from initial pad contact through full lock-up. No cold brake problems.

Hydraulically this does require changes but it does not make the system inoperable. It does cause a significant braking imbalance and requires more fluid from the master cylinder. In my case that meant installing an adjustable dual 3/4" MC setup. These days a better match in component size should achieve the same thing, without changing the MC, simply by finding a caliper with a 2-1/8" bore which will accommodate a ventilated rotor. Again, in my case increasing the bore size of the rear cylinders would be appropriate, but a 30% increase may not be practical. I need the stock bore size, which increased 30% should give good balance. I'll probably try the GT cylinders unless I can find something larger.

Although I had new 5 lug flanges welded to the MG hubs,and made custom caliper mounts there is a combination that will simply bolt on. A mid 70's Dodge caliper will match the MG spindle, and a 4 lug disc completes the package, or the Ford hub can be used if the outer bearing bore is deepened about 1/4". I don't have the part numbers or applications though, since I never did the swap this way myself.
Jim Blackwood

Lots has been said on this subject over the past couple of years! My personal preference is for practical "tried and tested" solutions rather than theoretical debate. For those interested in a straight forward
"inexpensive" vented disk arrangement (with whatever caliper), I have just bought Pug 505 turbo disks as recommended by Roger Parker (66 the pair from Neat Autos... and have produced a drawing showing all dimensions V's MGB. Drop me an email if you'd like a copy. Pete
Pete Green

I am looking for a set of Princess callipers, but they seem to be scarce. Could anybody point me in the direction of a supplier or a scrapyard where you can find these callipers?



I went down the 4 pot/Peugeot route, the brakes are rather better than the original, but pedal travel is a little longer, I just like to have the reassurance of the 4 pots. I had to use larger offset wheels (15" minilite replicas) to clear the bleed valves.

Michael barnfather

To those that are using the Greenstuff brake pads:

Are you using the V8 pads? If so, do you have the part #? I have found a U.S. supplier that has a good price on them, but no listing for MG on their website. They told me that they do have the MGB pads. I thought I would try the V8 pads if available.


for many years we have supplied kevlar pads and shoes we tested these on our own pro endurace road race car. these sell fro $50 to $90 per set/ we also modify the mgb caliper to acept pads that are 25% larger in areaaa than the stock pads. excellent braking and much less thant he ulti piston conversions $275 for rebuilt, modified, calipers wih pads. kevar is a drum/rotor friendly compund with no warm-up needed and a .45 CoF. thanks for the space. ted
ted schumacher

Thanks for all your comments.
I needed answers rather than theoretical debate and I've now got a pretty good idea where I am going. Princess callipers - because they work with my master cylinder bore and will fit over solid V8 discs. V8 Conversions supply sleeves (at 1 each)to press into the mounting holes so that the original bolts can be used. If someone can tell me where to get bridging pieces I will use the peugeot discs which cost the same as the solids.
I could also do with a supplier of new pistons for the calipers.
AND Steve's idea of looking in the archive ( once I'd managed to find it was great. There is enough information in there to write a book!
Thanks guys
Pete Hawkins

This thread was discussed between 02/05/2002 and 14/05/2002

MG MGB GT V8 Factory Originals Technical index

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