Brakes - Fitting discs and required ancillaries
What you actually need for the disc/drum conversion is pretty straightforward. Disconnect the steering arms, top and bottom swivel-pins, CV joint, and flexible brake pipes at the subframe - that’s it. There are, however, a number of ancillaries to consider.
Fitting discs designed for the Mini is easy enough. Just make sure you use CV gaiters for the disc-brake set-up - these have a different bellows shape to stop the gaiter rubbing the inside of the hub. The drum type will rub, then split, shedding grease all over the place. Moly grease is mighty mucky stuff to deal with and doesn't assist braking at all! A tip for racers - to stop the gaiters over-expanding when getting very hot and imitating the aforementioned, put either a decent sized split pin or (my favorite) a piece of very small bore pipe - as in the type supplied with WD40 aerosols, etc. - under the retaining strap on the drive shaft. This allows air to escape and return, as the gaiter gets hot then cools down.
Racks and steering arms
If the disc-brake set-up you’re applying has steering arms on it, remove them and fit those on your car already. This avoids the problem of mis-matching steering arms and steering racks. Mk1 and Mk2 racks and associated arms are different; mixing them up causes all sorts of grief. The only real way of telling one from the other is by looking at the end where the track rod end fits. Mk1’s were plain and round here, Mk2’s have what can best be described as ‘ears’ sticking out at 180 degrees to each other. These are basically to give some purchase for rod-end splitter tools. Identifying the rack fitted is also quite easy. Remove the rubber floor bung half way up the passenger side toe-board next to the rack retaining U-bolts. If there is a plastic plug visible in the rack body with a hex key fitting in it - this is a Mk2 rack. The Mk1 doesn’t have this. There are some rack reconditioners who have managed to cobble up Mk1 racks using Mk2 bodies. In which case a quick check is to calculate the ‘lock to lock’ capability of the rack. This is rather in-specific and difficult to accurately assess given that the Mk1 has 2.333 turns lock to lock, and the Mk2 2.7!
Some folk swear Cooper S ones are a must, particularly for competition, as they are quite a bit beefier than the standard arms. Consider. Standard arms are quite strong enough unless you spend all your time banging into immovable objects, or your opposition. The S ones will take more punishment before they bend. BUT.... the rack track-rods will bend first 99% of the time. It's far easier to change a steering arm than a rack on a Mini is it not? And standard steering arms are two-a-penny (dime-a-dozen?) in breakers yards - S ones are like hens teeth, costing lots, lots more. For competition use, ALWAYS lock wire the steering arm bolts as they have a habit of coming un-done - makes for interesting handling, believe me! If you can get hold of some suitable replacement cap-head bolts (Allen bolts) use these as they’re much higher grade material and more resistant to stretching. DO NOT use thread-loc gunge though. This stuff’s OK on bolts that’re non-torque critical, but reduce torque accuracy by up to 25% if you're using the wrong stuff (and there's LOADS of choices).
Flexible brake pipes from the drum set-up should be replaced with correct disc-brake items. Disc-brake ones are shorter to stop them rubbing on the tie-rods. New hoses always improve pedal feel too, particularly if they're steel-braided jobs.
Metro disc brakes
All the foregoing applies except for a couple of differences. Don’t use Metro steering arms - it’ll confuse your steering geometry. Some racers use them unwittingly - believing they give more leverage in the steering department. They do, but also screw up the Ackerman angles (no, no, don’t ask - a VERY difficult subject to explain in a couple of lines. Just trust me here OK?). You’ll also need to make sure you’ve got the right swivel-pins. Pre 1984 are the ones you want. The difference between these and later ones is the taper size. It’s impossible for the untrained eye to identify without comparison, so just buy four new ones (part no. GSJ268) and be done with it. They are not expensive. And I have to say a sight easier to deal with than Mini ones. They just screw into the hub as a complete unit. No mucking about with shims, expensive sockets to torque the nut up accurately, and heavy discussions on how much ‘slop’ the pins should have. One major hassle - you can’t easily fit ten-inch wheels with Metro brakes. I say ‘can’t easily’ because anything's possible with engineering, application and money, but very very difficult to do at home in the average enthusiasts garage. Metro hubs also use ball bearing type wheel bearings. You can fit taper-roller bearings (as in Mini) with no problems. Ball bearings were used to reduce rolling resistance to help with economy - taper-roller types have a considerably higher drag co-efficient but are very much stronger/longer lasting.
There’s always a bad egg in there some where and the Metro hub/disc set-up has one…it does weird things to Mini suspension geometry. Mainly manifesting it’s self as horrendous bump-steer and huge positive camber gains - even with standard bottom arms; worsening with a much lowered ride height. Bump-steer is so bad that sudden bumps caused by anything including mastic joints in tarmac can fire the car across to the other side of the road. This is accentuated by bigger and bigger wheel/tyre combinations. The worst inevitably being the ‘vogue’ thirteen-inch rim diameter and low-profile rubber.
The reason is relatively simple. The Metro has a significantly different king-pin inclination angle to allow for a track increase of some 1.75”, and very deeply inset wheels as fitted to Metros. This is juggled in an effort to reduce bump steer to tolerable levels and give some ‘feel’ to the steering but retaining a 'lightness' without resorting to a bus-sized steering wheel. When applied to a Mini, this goes completely hay-wire. Funnily enough, there seems to be two types of Metro hub. I have fitted several sets and noticed this was worse with some than others. The king-pin angles are different, but there are no distinguishing features at all. All casting numbers are the same, and indeed have been sold under the same part number since inception of the Metro. Weird, huh? I believe it was something to do with the vented/non-vented discs set-ups on earlier Metros.
So what’s the answer? None that’ll completely sort the problem. Using negative camber bottom arms (adjustable ones on racers) will help sort camber problems, as will maintaining a reasonable ride-height. Using wheel rims with the greatest inset you can find will help too. If wanting to run a real low ride-height, run a race car, use sticky-out wheel rims (coz you’ve got them) or retain ten-inch wheels, I STRONGLY advise using the pukka Mini set-up.
The last point to make is about connecting flexible brake lines. Metros use two where Minis have one. This is because the standard Metro system has a fail safe facility where if part of the system fails, the calipers are split so two of each four in both calipers will still work. This isn’t a problem as Mini Spares/Mania sell a conversion pipe kit to feed both ports on the caliper from one pipe on the subframe. Saves a lot of mucking about.
And that’s about it. I have not covered the basics - like how to rebuild the brake set-up prior to use/installation - this is dealt with in workshop manuals thoroughly. One thing I would like to add though. For racers, use of high-quality bearing grease is very important to avoid failure when they get very very hot - more because of heat transferred in by brakes than anything else. From personal experience I can unreservedly recommend Redline Oils ‘CV2’ grease. This can be used not only in the CV joints, but in wheel bearings too. Never had a failure when using this.
A quick word about these. Contrary to popular belief, replacing all the brake lines with stainless steel braided brake hose will not endow your Mini with amazing braking power. In fact it can seriously affect it. Because it’s flexible it’ll ‘move’ when standing on the pedal unless it’s tried down tight every inch or less along it’s entire length. If not it'll give a soggy pedal. It may look pretty, but is far less efficient, heavier, and far, far more expensive than solid metal tubing every where except where absolutely necessary - i.e. at each corner. Even the British Touring Car and major race teams have cottoned on to this. In fact if you want the ultimate, use what they are now using - aluminium-alloy based piping. Very light, stiff, and affordable. Other than that I would highly recommend the Scandinavian copper-based piping with brass unions. Easy to install and cheap.