Article on The Internet Mini Encyclopædia
Brake Disc history
"Early 1275GTs had 7.5in S discs and 10in wheels. Later 1275GT had 8.4in discs and 12in wheels. The 10in discs were only fitted for (I think) two years at the start. I believe, but I'm not at all sure, that all Aussie Clubman GT's had S discs.
You can tell the difference easily. Cooper S discs have a drive flange retaining 'disc' built in, so it's a pain to get the studs out. 8.4in discs have the whole flange separate, which is mounted by bolts into the disc. See one of the MW/MM articles on brakes.
If the calliper has a funny metal spring on top of the pads with the pins on top of the spring (as viewed from the outside of the disc) then it is 8.4in. These don't fit under 10in wheels as 8.4in callipers are much bigger; also the hose goes into the centre of the cylinder like S.
You can tell S from non-S callipers because S ones have
strengthening ribs at the sides of the cylinder."
The Mini 25 was the first 'roundnose' to go to the 8.4in discs. Mayfairs that were made from Jan '85 (late B reg) onwards had 8.4in discs fitted. By late '85 (C reg) all Minis had them.
"I have a '84 Mayfair (B reg), which had drums. I've seen many other B reg Mayfairs with sunroof, discs and rev counter."
"Servos were fitted as standard from Oct '88 onwards (early F reg)"
Ed's note: my Mayfair from Oct 88 doesn't have a servo…
Choices for 10in wheels
"For 10in wheels there are two main choices... the expensive route... and the VERY expensive route.
The expensive route is to fit 7.5in Cooper S disc assemblies. You should be able to pick up a second hand set in the UK for between 100 and 200 quid. Mine were 120 and have been pretty good (not as good a 8.4in discs, but that's what you get for running 10in wheels). I have fitted grooved discs and will fit fast road pads soon. 7.5in discs do require strong leg muscles, but they do work well if you push hard. You can use 8.4in assembly hubs, driveshafts and CV's. It is often cheaper to buy the S callipers second hand, new discs/flanges and an old set of cheap 8.4's.
The very expensive route is to machine down an 8.4in setup to 7.9in and fit alloy 4-pot callipers, or machined Metro 4-pot callipers. This will give superb braking, but you're looking at best part of 500 quid for it.
Personally, I'd go to shows and buy some S discs for 100-150 quid. They'll be in a state, but with a clean up and recondtion will be great."
Converting drums to discs
When you use the drums heavily, they heat up, with several effects:
All this is seen as less braking effort - brake fade.
To reduce fade, you need to remove the heat as quick as possible.
In a drum brake, the heat is contained inside the substantially thick drum, and is not dispersed to the surrounding air quick enough. Hence minifins and superfins designed to remove the heat quicker. They're good for rear drums, but still don't remove enough heat from the front brakes on a powerful car, as the front brakes provide far more braking effort, and hence heat, than the rears.
The disc brake is open to the surrounding air, so heat is dissipated far quicker. Ventilated discs allow even more air to the surface of the disc to cool it even quicker. The amount of space around the disc determines how much cooling air flow there is, so big discs enclosed in small chunky wheels don't get a lot of air - larger wheels, more 'open' alloys and even air 'scoops' help to increase the air flow.
Separate to the fade issue, the efficiency of the brake depends on the pressure applied to the pad by the 'pot' (ie the piston) on the calliper. More, or larger, callipers mean more pressure on the pad, hence more braking effort (but more need to cool the brakes).
There are three sizes of discs that can be fitted to Minis. 7in were fitted to early coopers, with 10in wheels, 7.5in fitted to later coopers, then 8.4in discs which were fitted to all Minis (with 12in/13in wheels?) since about '84(?).
To upgrade to Cooper S (7.5in) disc assemblies, you'll need the following:
You can keep your drum drive shafts, but possibly not the steering arms.
Late mini CV's and hubs are compatible with the S setup, even though the S disk CV's body diameter is slightly smaller. You'll need the late mini outer CV gaiter.
All mini disk hubs are the same. The metro 4 pot callipers will fit under 10in wheels if the 8.4in disks are machined down to 7.9in (7.5in is a little small for the pads), and the calliper mounting holes are moved inwards a little. MiniSport sell alloy 4 pot callipers, which are essentually modified Metro 4 pots. These do fit under some 10in wheels, but only deep off-set alloys. Check compatibility with the retailer.
You can only use 8.4in discs if you use 12 or 13in wheels. but they're cheaper and easier to get because they were used on Metros. The standard callipers have two pots acting on the disc, one on each side.
The ultimate in braking (for Minis at least) is the set-up used on the Metro MG Turbo. This has 8.4in (?) ventilated discs, with 4-pot callipers. The discs can be machined down to fit other wheels, but you'd then need different callipers such as those produced by Mini Sport (in a range of go faster anodised alloy colours!). Alternatively, I think you can buy various sizes of vented discs, without having to machine Metro ones.
Here's what I don't know, can anyone else help?
Fitting Metro disc assemblies
It is important to make sure that you use the correct parts.
Metro bits to use:
Mini parts to use:
The wheel mounting lugs also need cutting off.
If the Mini has discs on it already and you just want 4pot callipers just swap the callipers over on to the Mini hubs and mini disks. You also need a set of Metro-Mini conversion hoses or adaptor kit to convert the twin pipe callipers to the single pipe mini system.
One source advises "Buy the conversion kit from minispares (the Avonbar hoses made by Goodridge are too short) so long as they tell you what changes are needed to re-balance the braking system. I changed the rear cylinders to Mini 1000 ones from the better 1275gt ones but do not do anything without getting educated advice. MINISPARES used to have a sheet to help you change over."
If you want vented disks then you need a pair of vented discs with the Metro drive flange (with the wheel placement bits milled off); the vented/non-vented callipers are different.
If you are going to put 12in wheels on your Mini then I suggest you use Mini disk hubs (Cooper S, 1275GT and Late minis disks hubs are the same) As Metro hubs require a little more work to fit on a Mini as you would need to fit negative camber bottom arms and modified Mini steering arms to fit them on to a Mini.
Converting Mini 8.4 Discs to Metro 4pot Assemblies
(see also Metro 4-pot Brakes)
Standard 8.4 inch disks fade when stopping QUICKLY from 90mph, for example during an emergency stop. The last 10mph is VERY difficult to shed off. Fitting 4 pot metro brakes (solid disks) which have bigger pads so will take longer to fade.
The Metro turbo 4pot vented brake set up was designed for a heavier car (the MG Metro turbo) with 94 bhp at the flywheel compared with 56 for a '78 Mini 1275GT. This makes the set up ideal for tuned/hard driven minis so long as the effect of the better front brakes is allowed for when resetting the brake bias either by bias-valves or by using different rear slave cylinders and removing the pressure reducer in circuit.
Insurance implications have to be considered - insurers must be notified of these changes because in the UK at least they represent MATERIAL FACTS regarding the vehicle (It has been significantly modified).
Performance brake pads other than uprated road pads are overkill with these brakes and can lead to long stopping distances after long periods of unevenful driving on motorways/freeways, as they work best when hot.
Brake pipe modifications are needed as the callipers have 2 inlets, this can be achieved by twin hoses and a splitter block on the subframe or the use of a short hose and banjo fitting on the body of the calliper. Metro callipers should be combined with the existing Mini suspension arms and swivel hubs/steering arms, as the taper of later Metro swivels (the sealed ones) are incompatible with Mini suspension arms - Metro arms/hubs produce strange angles that need to be corrected with adjustable bottom ams and tie rods.
Remove grease and bleed nipples and replace as required as they can shear off being smaller than Mini ones. Do put the callipers nipples pointing up please!
The reservoirs need to be bled separately - 2 nipples on the top simultaneously, one on the bottom, one-man-bleed kits (such as EaziBleed) can be obtained to work on such 2 nipple callipers.
The better brakes put more stresss on ancillaries, so expect to feel "tramp" on the front suspension unless harder bushes are used there and on the subframe mounts. Heavy duty front tie rods and a fillet welded between the front tie rod mount and the subframe may be considered overkill until they break!
The brake bias problem is likely to be the most significant difficulty encountered and professional help is needed here unless you have an airstrip to yourself ;)
Keep the 12in wheel Mini hubs. If you are not fitting vented disks, keep the Mini ones too and just fit the callipers. If you are fitting Vented disks then use the Metro drive flanges discs and callipers as described above.
Upgrading Metro discs to Grooved Crossdrilled Discs
"If you do not machine off the little flanges sticking out of the front of the Metro drive flanges, your wheels might not fit. Also if you do not check when fitting the drive flange over the end of the driveshaft that the hole through the end of the shaft is pointing away from these flanges and the wheel studs, insertion of the split pin will be impossible/very difficult.
Unless you have someone standing on the brake pedal the only way to get the driveshaft nut done up or undone is by taking the centre cap out of the wheel - fit the wheel with one nut, lower it down and use a long socket to get at the nut through the hole in the wheel.
When using the above method it is difficult to see if the nut and the hole in the end of the driveshaft are lined up- mark a line on the end of the shaft to show this or cut a little notch across the end of the shaft so it will be there for next time.
Remember, when tightening the nut up, there is a chance the split collar will lock on the driveshaft before the bearing has been properly clamped up between CV and drive flange, follow the instructions in the supplement in the Haynes manual to make a spacer that you can use to compress the assembly before you fit the split collar- if you don't use this washer there is a chance the bearing might loosen/fail prematurely.
Ed's note - DSN Classics (www.dsnclassics.co.uk) sell a special washer for this purpose if you don't want to fabricate your own - it may also be available elsewhere
Metro drive shaft nuts are tightened to 160lb/ft unlike Mini GT ones at 150lb/ft BUT by the time you have strained and grunted to get the nut round so the hole is accessible you might have pulled 200lb/ft DON'T use your 150lb/ft torque wrench to pull the nut round to line up the hole USE A LONG ARM - it is obvious the torque will have been exceeded and to try to do this with your torque wrench might render it "knackered"! "
Steps for removal
Steps for replacement
NEVER have a caliper not on a disc when pressing the brake or the pistons will drop out!
Do one side at a time, and roadtest after both are done. If vibration is felt through the pedal on braking the drive flange nuts may be removed and the disc rotated by 90 degrees and the bolts replaced; this can improve disc run-out (side to side wobble). Repeat on the side in question up to 3 times (90-180-270) to try to reduce the run out .
Long pedal travel then a firm brake can be caused by a loose drive flange or
bearing pushing the calliper pistons in as the disc rotates.
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07 May 2003