280mm brakes (again)

Considering the investment of having the discs redrilled and adapter bracket created, is there any reason why the specific VW caliper is not used?

There are many reasons why a particular calliper is used or not, sometimes it's down to the angle that  the brake pipes attach to the calliper which causes the hose to foul on the suspension. Other times the mounting lugs are simply in a position where it isn't viable to mount the calliper to a particular hub. That may be down to the lugs being too close or too far apart, they may be too close radially to the hub or insufficient clearance between hub and disc. There isn't anything magical between different brake callipers, an Alfred Teves calliper is no better or worse than a Lockheed, they are all very simple designs made by the million with an incredibly low rate of failure. This does not apply to some cheap aftermarket callipers that although they look good do have problems, The Mini R53 calliper works simply due to the fact that the mounting lugs are in an acceptable position and the brake (banjo) hose exits in the perfect position so you can fit ordinary MG hoses from your local supplier. 
Going back to fitting the VW calliper the reason it isn't fitted is because it would be a dangerous conversion or one that needs bigger callipers at the back as well. The MGF/TF calliper has a bore of 48mm as does the Mini R53, the VW calliper uses a 54mm calliper as does the Mini R56, this would give an increase of over 26% in braking effort on the front needing a similar increase at the back to keep a balanced braking system. This is why it is so important that people modifying their car brakes understand the consequences of what they are doing.

Thanks Philip. 
Having a 54mm caliper bore, and therefore significantly increased area, would mean the pedal stroke was longer for a set master cylinder bore?

Unless the rear callipers are changed the increase in pedal travel will be much lower than the 26% increase in piston area, at a guess it would be about 15% further. However it doesn’t alter the fact that the braking will be unbalanced and stopping distance will be greater, the front wheels will have locked up and be skidding long before the rear brakes have reached their optimum. The normal brake balance for most road cars is around 65% front 35% rear , those which are significantly heavier at the rear will have perhaps a 60/40 balance but even they have a very significant transfer of weight to the front under heavy braking. 
The Mini R53 and R54 are good examples of how the manufacturer altered the brakes with both larger diameter discs and calliper bores. With the R53 the front calliper bores were 48mm and the rears 34mm with a 22mm bore master cylinder. 
The R56 got 54mm bore front callipers and 36mm bore rear ones, the master cylinder was increased in diameter to 24mm. 
All of the big brake conversions that I have seen on a road going MGF or TF have kept to the same sized callipers bored, nearly all have kept the original callipers as well. The exception is the AP four pot brakes that have tiny pistons and are equivalent to a normal 44mm bore calliper. This is the reason the only increase in performance is due to a 26% increase in disc diameter counteracted by a 16% decrease in piston area, all in all a 10% gain, but only at the front.  Going to a 280mm disc gives a 16% increase in performance and that can be both front and rear. For simplicity I have used full disc diameters for the illustration but you should use the diameters at half brake pad height, the gains by increasing diameter are therefore greater than shown. 

That sounds a very reasoned explanation Philip. I think anyone looking to upgrade their brakes should take that advice. Over egging the bore size on the front brake callipers would require a similar change at the back or at the very least a change of proportioning valve. Beyond a certain point a large increase in front calliper capacity could also potentially dictate a change of master cylinder and/ or a review of the pedal fulcrum point.
Id go with your choices for the reasons explained.

If you do incresse piston size front and rear you may need to replace the master cylinder otherwise you will have a soft pedal because you are using a lot of stroke to move more fluid.
I changed my 1976 22mm Celica MC to a 1994 25mm Pajero one.
It fitted the servo exactly and gave me my pedal back.
You have to search forums for those who have done it before you and file it away for the time you come to travel that path.

I was going to buy 280mm kits for both front and rear from a guy on Ebay who sells under the name of Kezzermuldooney, I had a long chat with him and he told me about the possibility of using Mini R53 callipers due to them offering a better fit on the bigger discs but he couldn't make it work. I bought a pair of the Mini callipers and rebuilt them, they are as cheap as chips. The problem he had was trying to make them work with the 280mm discs, the placement of the mounting lugs was all wrong, looking at it for a few minutes the obvious answer was a bigger disc, low and behold the Mini R56 supplied the 294mm disc. There is a common upgrade on Mini R53's to fit the R56 calliper and disc combination. So there you have it, it wasn't my idea but I found the way to make it work. Even in their poor condition the sale of the four pots paid for the complete conversion front and rear including the pads and braided hoses

That is interesting. When yo fitted the discs how was the fit on the centre support that locates the disc to run without eccentricity?
Did it need a ring similar to those sold by Mike Satur in his kits?

MGB281 wrote:

I was going to buy 280mm kits for both front and rear from a guy on Ebay who sells under the name of Kezzermuldooney, I had a long chat with him and he told me about the possibility of using Mini R53 callipers due to them offering a better fit on the bigger discs but he couldn't make it work. I bought a pair of the Mini callipers and rebuilt them, they are as cheap as chips. The problem he had was trying to make them work with the 280mm discs, the placement of the mounting lugs was all wrong, looking at it for a few minutes the obvious answer was a bigger disc, low and behold the Mini R56 supplied the 294mm disc. There is a common upgrade on Mini R53's to fit the R56 calliper and disc combination. So there you have it, it wasn't my idea but I found the way to make it work. Even in their poor condition the sale of the four pots paid for the complete conversion front and rear including the pads and braided hoses

Very well done! When you do some digging it is amazing what you can find.
I fitted Corona struts and discs to my old Celica because they came with large two pot callipers. I like two big pots..they just work.
Searching for competition pads for a Corona yielded nothing. I found a website that listed pics and dimensions of every car pad. I chose to look at pads from cars with a motorsport heritage and low and behold the Group4 Escort rear pads were identical to the Corona front pads. Suddenly the world became my lobster and every major manufacturer made competition pads for them.
I chose Ferodo DS 2500's which were amazing.
A little digging can prove worthy.

Yes I had a local machine shop make them up for me, surprising how cheaply family owned businesses can make parts if you give them a few days to make it and you pay cash. I think all of the kits use either a spacer ring or machine out the centre of the disc. The spacer ring is the simplest solution because once made you can buy discs where ever you want.

I'm still playing with them Philip.  The block at the moment is I cannot move the 100mm wheel studs in the front hubs.  3 came out OK (ish), others rock solid.  Did you use some sort of locking fluid under the heads?  The one's that came out seemed to have something there. Is heat required?

They were just pulled in with a a spacer then an ordinary M12 nut, they should tap out without any problems.