Article reproduced with kind permission of Mike Fishwick

A brake disc should last – under normal use – for about 100,000 miles, but it seems to be quite a common ploy for MoT testers to “advise” owners that their brake discs are warped, and that a new set of discs and pads is necessary. In most cases this is gem of wisdom is based on nothing more than a cursory examination of the discs, which – particularly on an infrequently-used car – may exhibit signs of incomplete contact with the pads, often in the form of a rusted area around the edge of the disc.

Perhaps these “experts” fail to realise that this is not a sign of warping, but of light use, for our calipers are mounted on their securing bolts by rubber grommets, in order to provide the necessary sliding action as the pads wear down. This means that under light braking loads the caliper and pads do not fully align with the disc, usually causing only the inner halves of the pads to make contact.

Firm use of the brakes will provide full contact and uniform marking across the full area of the disc, but unfortunately many owners pay for new pads and discs, sometimes every year! This is how the tale that BMW discs warp easily has got around.

Another common piece of “advice” is that the discs are rusted, which is quite common, as they are, after all, made of cast iron! A thick layer of flaking rust around the edge of a brake disc is nothing to worry about, and can be easily removed by a little work with a hammer - a welder’s chipping hammer is ideal, but anything will do. Likewise, a band of rust around the periphery of the working area is simply a sign that the pads to not wear on the entire disc, which is again quite common, and can easily be scraped away.

Even scoring around the working area of a disc is not a reason for replacement, and is usually a sign that some grit has become trapped in the pads. Removal of the pads, and light cleaning with abrasive paper will prevent the situation becoming any worse, and after another 20,000 miles or so the disc will usually wear smooth again.

Unless you indulge in determined track day use, the only real reason for disc replacement is that of normal wear, when the thickness is below the manufacturer’s minimum figure, and constitutes an MoT failure. For example, the minimum thickness of a vented E36 front disc is 20.4 mm, a reduction from new of 1.6 mm. This small difference will not affect brake performance, and is simply an arbitrary figure intended to prevent some owners from running a disc until it becomes flexible, and to make us buy new discs after a small amount of wear.

There is plenty of choice when it comes to choosing new discs, from Genuine BMW items, which are good value, to exotica at far higher prices, but braking performance is not related to cost! Although many people claim that drilled or slotted discs are prone to cracking, it must be remembered that rabid track day use will rapidly destroy any brake disc.

I have used Zimmermann drilled discs (from German & Swedish) on our various Volkswagens for years, including a few sprint events, and have found no problems. These are perhaps the best value around, in terms of price, overall life, and freedom from “grooving” in line with the drillings, which is a real problem on some drilled discs. My Golf GTI and Heather’s VW Corrado each covered 50,000 miles on Zimmermann discs without any signs of wear, while after 35,000 miles on her current Golf TDI they are as new.

A great advantage of drilled and vented discs is that a film of water, which reduces initial braking response, does not build up on their surface, the pads pushing it though the holes to be dispersed via the vents. The drillings also help to prevent the pads from becoming glazed after periods of light braking, but do not appear to reduce pad life.

My Z3 uses more expensive front discs, in the form of the Black Diamond drilled and slotted type, the radial slots ejecting water and deglazing the pads, while the drillings – as on all drilled and vented discs – provide an escape for the film of gas which is produced by the pads under hard braking, and which can reduce pad-to-disc contact on plain discs. Used in combination with EBC Greenstuff pads, these discs and pads have now covered 50,000 miles, and are virtually unworn, but such life depends on driving style.

Do not waste money on drilled solid discs, as the drillings have nowhere to transfer the water or gas – a disc with radial slots will dissipate both.

If you do have to replace your discs, you will often experience difficulty in removing the fixing bolts, which will invariably be rusted into position. Those securing the caliper carrier to the hub will usually succumb to plenty of leverage, but I find most combination spanners are inadequate, and will usually split before freeing the bolt.

The only answer is a high-quality socket (such as “Snap-On” or Halfords Professional) operated by a half-inch bar rather than a ratchet, the extension being supported by a suitable piece of wood, in order to resist its tendency to slip off the bolt head. With the hub ball joint supported on wooden blocks and a couple of feet of strong steel tube over the turning bar these bolts will usually yield, but otherwise be prepared for prolonged treatment with penetrating oil. A 50/50 mixture of diesel and engine oil is fine.

The other problem area concerns the special shouldered socket screw used to hold the disc against the hub flange, which is an almost-universally neglected item. I remove mine every couple of years and re-coat them with Copa-Slip, but few cars are so lucky! If you do not share my passion for greasing threads, buy a few spare bolts before you start.

In general, if these bolts will not surrender to a 6 mm hexagon key of the type found in a good socket set, the next step will be to find a suitable 12-pointed Torx-type key, and drive it into the hexagon. An odd-sized key is necessary – I use a Sykes-Pickavant no.018800, which is between T40 and T45 sizes, being sold for the CV joint securing bolts of a Mk.1 Golf GTI. This will be totally locked into the metal, with zero clearance, and if plenty of torque then fails to unfasten the bolt, nothing will.

In such a case, the answer is to remove the head from the bolt, using a 6 mm bit, which will locate itself between the sides of the hexagon socket. Follow this with a 8 mm bit, which will separate the head of the bolt from its threaded portion, after which the disc can be removed.

The remains of the bolt can then be filed flush with the flange. Do not try to drill out the remains of the old bolt, but drill and tap a new hole. Replace the disc in an alternative position, and secure it using the wheel bolts – their tapered seatings will accurately centralise the disc around its central spigot.

Due to the unique design of a BMW disc bolt, it is not a practical proposition to simply mark and drill the flange, as a small inaccuracy will prevent the shouldered bolt from fitting into its bore in the disc. What is needed is a suitably stepped steel ring, which will fit into the bore and provide a hole to centralise the tapping drill of 6.8 mm (7 mm will suffice). An alternative is to make a jig, using a spare bolt. Carefully drill a 2 mm pilot hole into the slightly tapered base of the bolt’s socket, checking that it is parallel with the treaded portion. Now cut away the threaded portion, and enlarge the hole, from the back, to 6.8 mm. If you have drilled accurately you will now have a jig which can be inserted onto the stepped hole of the disc to guide the tapping drill.

I find the best drills available are the Black and Decker “Bullet” range, which have a unique tip design and do not “wander”. Once the hole has been drilled, tap an M8 thread into it, and your problem will be over.

Make sure that the new disc is completely free of preservative grease – particularly if it is a drilled or slotted type, as any grease trapped in these areas will soon be melted out onto the working surfaces of the disc and pads.

Fitting a disc – whether old or new – requires that the mating surfaces of the disc and hub flange be absolutely clean and flat. Check carefully around the holes and threads, filing away any protrusions. A small error at the hub flange will result in a large error at the edge of the disc, this problem causing a rapid wobble of the disc, with an associated load on the pads and oscillation of the caliper piston. This will result in a “soft” pedal feel, and uneven disc wear, giving a – thick – thin effect, with rapid wear of the pads and caliper piston seals. The tolerance for this type of problem, the “Run-out” level, measured at the edge of the disc, is 0.2 mm. This type of wear is, of course, due to bad assembly, and is specifically excluded from the guarantee on new brake discs.

Another cause of asymmetric disc thickness is that of leaving the car stationary for a prolonged period with wet brakes, such as laying it up for the winter after a loving final wash. The combination of cast iron discs, pads with a high copper-based metallic content, and water produces electrolytic action, which over a winter can result in corrosion of the pads and the area of the disc in contact with them. This problem is exacerbated by the use of drilled or slotted discs, which will retain small amounts of water. The answer is obvious – give the car a decent run after washing, with plenty use of the brakes, and lightly sponge wash the bodywork afterwards if necessary.

Laying up, and care of any little-used car is another specialised field, and we will look at that in detail later.