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MG MGB Technical - Larger Brake Callipers

Hi,

I have seemingly larger than normal (well larger than the ones on my '72 car) front callipers fitted to some mgb drums/kingpins. How can I tell if they are V8 ones or not.

Thanks in advance.

Iwan
Iwan Jones

The main difference between the two is their width. The V8 calipers were designed to be used with vented rotors which were considerably thicker than the stock B rotors. RAY
rjm RAY

Ray, V8's used plain discs, just a bit thicker, the RV8 used vented. Pads are a bit bigger on the V8. Iwan, Assume the callipers are still 2 pot?
Allan Reeling

Yes everything is a bit bigger/wider etc. Pads are bigger than my normal ones. Does anyone have info re serial numbers etc?
Iwan Jones

others will remember better than I but if (big IF) I remember correctly some V8 conversions used Rover SD1 and I've got something in my head about BL Princess or Maxi - but I could totally wrong about last two
Nigel Atkins

Can you post close up photo of your calliper, Iwan? Then we might be able to see what you have. All 1800 MGBs used the same calliper from the start to the end of production, and as others have said, the V8 calliper was similar but wider to allow for a thicker disc.
Mike Howlett

Yes, Princess is an easy 4 pot conversion. Use late TR6 caliper bolts: 12mm metric shank, but still 7/16" UNF thread, and the holes line up directly.
Paul Walbran

Iwan,
Are the callipers 4 pot?
I have a feeling that the slightly larger V8 pads will fit the 1800 callipers. It was often done as an up-grade, but still using the standard disc. The callipers are basically the same as the standard with the addition of a spacer to accommodate the thicker disc.
Nigel you are correct the 4 pot conversion used Princess parts but because BL had gone metric they required a stepped calliper mounting bolt which was a Triumph part.
Allan Reeling

Many years ago, I remember hearing about a home-made V8 calliper, which consisted of one half of an MGB calliper and the corresponding half of a Triumph 2000 calliper.
Dave O'Neill 2

"I have a feeling that the slightly larger V8 pads will fit the 1800 callipers"

This is often mentioned as a slight upgrade. V8 pads are handed in V8 calipers, i.e. there is an inner pad and an outer on each (unless you supplier has put three of one and one of the other in the box ...), but I think they fit either way in the 1800 caliper.

However retardation is limited by the grip between the tyre and the road. Any upgrade to the calipers that results in the same front retardation for a lower pedal pressure will be reducing the pressure to the rear brakes, hence reducing the retardation of the vehicle as a whole.
Paul Hunt

Paul, One limiting factor regarding V8 pads in an 1800 is the heat dissipation possible with the standard discs. You probably get better initial "bite" but the fade characteristics will be worse, i.e., more friction surface = more heat but not enough disc to "exchange" the heat with the atmosphere!! Hence the use of thicker V8 discs and vented RV8's.
Allan Reeling

Alan, quite the reverse. Friction is dependent only on the load applied and the coefficient of friction - area has no direct effect. It does have an indirect effect though, in that if the same energy is dissipated over a greater area the result is a lower temperature rise and the coefficient of friction not reducing as much in response. This is not to say that the thicker V8 discs are not also a gain - they are, as they lower temperatures further.

As Paul notes, ultimately it's the grip of the tyres on the road which limit braking and if you improve the brakes at one end only then overall braking is reduced as the other never meets potential.
However, after repeated stops temperature rise reduces the coefficient of friction ("fade") and the grip available from the brakes can be less than that available from the tyres. In this case, larger pads which reduce temperature rise are useful.

The best example fo all this is in fact the comparison between ex-factory V8 spec and the 4 cylinder spec.
Both shared the same rear brakes and the same piston size in the front caliper. If the greater pad area of the V8 actually increased froction then the braking would have become unbalanced - which it didn't. The objective was to reduce fade during longer or repeated hard stops - which they do (in comparison to 4 cyl discs and pads)
Paul Walbran

Not entirely convinced Paul, braking surely is a process of energy exchange, i.e., kinetic into heat. Increasing the size of friction area requires more pressure to get same retardation (load per unit area). Hence the V8's greater servo assistance. Can't imagine the temp would be less, it's just that since the contact area is greater the conduction path is greater to the disc and the pad should run cooler, but only if the disc can lose the heat to the atmosphere at a sufficient rate. If it can't temp will rise exponentially and coefficient of friction will decline. What also happens is the development of "gas" at the disc/pad interface…both these conditions produce fade. Cross drilling, and grooving not only help dissipate heat but also the gas.
So going back to the original point, which we both agree with, sticking V8 pads in an 1800 calliper with 1800 discs won't improve braking!
Pail Hunt's point about tyres, is well made, but when these B systems were developed tyres were nowhere near as good. Cross ply didn't put as much rubber on the road and probably the compounds had a lower CF too. So some improvement in braking efficiency aren't going to result in Bamby on ice!
Allan Reeling

"Both shared the same rear brakes"

Not as clear-cut as that, as the V8 had the later smaller roadster rear cylinders and not the larger GT cylinders. However roadsters had the larger cylinders to begin with, they were reduced as there was a tendency to lock up, which didn't happen with the greater weight of the GT. I assume the V8 had them reduced as with the harder springs any bumps might well allow them to lock.
Paul Hunt

This thread was discussed between 09/08/2014 and 14/08/2014

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