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Automotive disc brake systems have an air of mystery and some fear for many vehicle owners, BMW and MINI owners included. While the job of our vehicle braking systems is certainly important, critical even, the system itself is actually rather simple. With this said, we certainly don’t want to blindly go at a “servicing” of the system without an understanding of the system, what it does, how it can fail, how it wears and how to actually determine what and when to service.
In this article, we will address the brake rotors, or discs. Specifically; why, when and how they should be replaced. We see and hear DIYers, internet experts and claimed service experts offering (often while pounding on the table) their input on when disc brake rotors should, and should not, be replaced. Brake discs do wear and fail in other modes and do need to be replaced periodically, but lets take a look at the actual reasons and modes that should prompt replacement.
Expert #1 claims that the brake rotors need to be replaced whenever the pads are replaced, due to the pads and rotors having become a matched set and running new pads on previously worn rotors will cause symptoms and issues such as; braking vibrations, braking noise and accelerated pad wear or even braking failure. In fact, the worn pads and the rotors will indeed have some wear pattern and the rotors may need to be replaced. However, if there has been no noise or vibrations from the brakes and the pad/rotor wear surfaces are basically smooth and free of rust, the wear pattern will typically not be an issue that “requires” rotor replacement. This is assuming the pads are being replaced BEFORE they have fully worn out and the metal backing plates are contacting the rotors (which does indeed require that the rotors be replaced) and the minimum rotor thickness has not been reached.
Expert #2 claims that the brake rotors need to be replaced whenever the pads are replaced, due to the rotors wearing out with the pads. This claim may note that the rotors ares simply “worn out” or that they will be worn too thin for further use causing imminent brake failure if new pads are installed without rotor replacement. This claim has some truth to it as the rotors do indeed wear and become thinner. There is a specific minimum thickness specification for each rotor and this is stamped on the rotor. With this noted, similar to the notes in #1, above, if the rotors have not shown vibration or noise symptoms, the wear surfaces are smooth and not overly ridged and the rotors are not rusty on the wear surfaces, they may be usable with a new set of pads. The final test for rotor useability is to measure the rotors’ thickness and compare these measurements to the minimum thickness specs. The minimum thickness specification will be stamped on the rotor, commonly on the outer circumference or the rotor hat. If the rotors have not reached the minimum thickness and it is determined that there is enough material to run through another set of pads, the rotors can be reused.
On the other side of these two replacement claims, we have those who would re-use any brake rotor that does not exhibit warpage (causing vibrations while braking), to include overly rusty, worn past the minimum thickness, extreme ridges (scoring) in the wear pattern and heavy edge lips (due to heavy wear). All of these conditions can result in a minimum of noise or vibration, to a reduction of braking power to actual brake failure. In the comparison of these schools of thought on brake rotor service, we’ve gone from the extreme of replacing every brake rotor regardless of condition to attempting to never replace them (regardless of condition).
Examples of common brake rotor wear and faults:
When does a disc brake rotor NOT specifically need to be replaced? A brake rotor may be considered reusable if ALL of the points below are met:
NOTE: Even if all of the points below are satisfactory, reusing a brake rotor that has wear from a prior set of pads, increases the potential for brake noise when new pads are installed.
* The brake pads have not worn fully through the friction material, i.e., the brake pad backing plates have not made contact with the rotor.
* There is no vibration or pulsation in the suspension during brake application.
* There is no squeal or squeak noise generated during brake application.
* The pad wear surface is relatively smooth and without obvious scoring.
* The rotor is not at the minimum thickness specification (or less).
* The wear surface width is equal to the brake pad’s friction material width, i.e., the pad is making full contact with the rotor and wearing the surface evenly.
* There is no rust within the pad wear area on the rotor.
* There are no cracks in the rotor’s wear surface, or anywhere else.
* The external, non-wearing, surfaces of the rotor do not exhibit excessive flaky rust
What are the indicators that a disc brake rotor SHOULD be replaced? A brake rotor should (may we argue … MUST?) be replaced if ANY of the following conditions is evident:
* The brake pad friction material has worn fully away and the metal pad backing plates are contacting the rotor.
* The vehicle exhibits a pulsation or vibration when applying the brakes. While this could be due to worm control arm bushings (as the most common alternative cause), if the pulsation or vibration only occurs while applying the brakes, it may likely be due to a warpage, pad material transfer or hard spots on the rotor …. all of which, require rotor replacement. Actual warpage should be no more than 0.003″ of run-out in the rotor’s surface.
* The rotor’s pad wear surface exhibits noticeable scoring.
* The rotor is at or very near the minimum wear thickness.
* The rotor surface has noticeable lips at the edge of the wear surface transitioning to the unworn surface.
* The rotor’s wear surface shows uneven pad contact across it’s face.
* The rotor’s wear surface shows rusty areas where the pads should be making contact.
* Inspection shows micro-cracks in the rotor’s wear surface.
* The external non-wear surfaces of the rotor are heavily rusted and flaky, especially the cooling vent vanes between the inner and outer rotor wear surfaces.
Most BMW DOHC (Double OverHead Camshaft) engines (except some ///M engines) use hydraulic valve lifters (or tappets) in order to eliminate the service requirement of periodic valve clearance adjustments. The hydraulic lifters fill with oil and are pressurized by the oil pump. A hydraulic piston in the lifter is then able to make full contact with the camshaft and the valve stem and, in effect, vary the “thickness” of the lifter. As the parts (valves, lifters and camshaft) heat up and the distance between the valve stem and the camshaft decreases, the hydraulic lifter is able to keep full contact yet not bind and prevent the valve from fully closing against its seat, as a mechanical lifter or rocker arm would do if there were no clearance adjustment.
It’s not uncommon for the hydraulic lifters to begin to fail as they age. Oil varnish build-up in the oil feed holes, worn/weak pre-load springs and worn lifter pistons can contribute to lifters that will not stay “pumped-up” with oil pressure. This causes valve tapping, just like a mechanical valve lifter or rocker arm that is out of adjustment. The cure is to replace the lifters.
Prior to installation, the lifters should be pre-bled in order to remove air from the lifter and fill it with oil. This makes the final bleeding of the lifters, after installation, much quicker. Here are the basic steps to pre-bleeding the hydraulic lifters. In this case, we are using a Mityvac vacuum pump to pre-bleed the lifters. In this example we will be bleeding the flat tappet style lifters as used in the BMW M50, M52, M54, M56, S50 and S52 6-cylinder, M24 & M44 4-cylinder and the M60, M62 and S62 V8 engines. Other style hydraulic lifters, such as those used in the later BMW Valvetronic engines, can be bled using this method as well.
* Mityvac vacuum pump kit
* Container to hold the bled lifters, fully submerged in oil
* Small washer to place in Mityvac cup
* Paper towel or clean rags
1) Fill the lifter holding container with enough fresh motor oil to fully submerge the quantity of lifters that will be bled prior to installation. In the case shown, we will bleed 12 lifters at a time and install them in either the intake or exhaust lifter cages (cam carriers).
2) Place the washer in the bottom of the Mityvac vacuum cup. This will keep the lifter from “sticking” to the bottom of the cup after the vacuum process, making it easier to remove the lower lifter with the magnet tool.
3) Place two or three lifters in the Mityvac vacuum cup and cover with fresh oil.
4) Connect the vacuum pump and apply as much vacuum as it will generate. You will see air bubbles being drawn from the lifters’ oil feed holes.
5) Release the vacuum and repeat, until there are no more bubbles being pulled from the lifters’ feed holes or until only a few small bubbles continue to show. Note that the vacuum will actually cause the oil to aerate, generating air bubbles within the oil itself.
6) Remove the lifters, using a magnet pick-up tool, and place the lifters in an oil bath that fully covers the lifters.
7) Once the full set is pre-bled, they are ready to install.
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While this post is detailing a BMW with the M54 6-cylinder engine, This post and the scenarios outlined can be applicable to any later model BMW or MINI.
2002 BMW 325xit
* First start of the day. Engine initially starts (as usual), then dies after 2 seconds. Would not re-start.
* Engine would crank fast and smooth with no pulsations from cylinder compression.
* This was the second cold start in two days, with no appreciable run time after the first one, the day before.
* Attempt re-starting, sounds like the starter pinon is not extending to the flywheel ring gear (free spinning starter sound, not the normal pulsating sound of cranking the engine over). The starter cranking sounds like there is some load on the starter, but no pulsing from cylinder compression.
* Trying a few more times produced no change.
* The hood was opened the the engine cooling fan can be viewed. As the starter was cranked the fan could be be seen to be turning, meaning the starter is indeed engaging and turning the crankshaft.
The fast crank without any pulsations implies that there is no compression on most, if not all, of the cylinders. If this were an M20 6-cylinder, with a camshaft timing belt, the obvious initial conclusion would be that the timing belt was broken (or stripped). This would allow the crankshaft to spin but the camshaft would not be turning or would be out of phase. Either condition resulting in bent valves and no compression in the cylinders. Since this M54 engine uses a timing chain (as all other BMWs, besides the M20 and a few Euro only engines, do), this scenario is less likely or common. Yes, timing chains do break, but this is not all that common and this engine had absolutely no prior indication of potential timing chain issues.
While we could perform a compression test (rather superfluous since there is clearly not enough compression to cause any pulsations in the cranking) and remove the valve cover to verify the condition of the timing chain and if the cams are turning, the conditions and symptoms point to a possible “perfect storm” of minor issues that add up to result in the loss of compression.
* A cold-start with shut-down after just a few seconds, can cause the engine management to wash the cylinders with a rich fuel mixture. Add another cold start later in the day or the next day and another fuel wash and the oil that normally coats the cylinders and pistons (and the piston rings) can be washed off …… reducing the piston ring to cylinder sealing. This was indeed, what had been done on this car. It was started on Saturday (cold morning), moved up the driveway and then shut down. The next morning (cold again), the failed start was attempted.
* The vehicle has 200,000 miles on it and certainly has, at least, a moderate amount of carbon and varnish build-up in the piston ring lands (grooves), which prevents the rings from sealing cylinder compression properly. The piston rings ride in grooves in the circumference of the piston. The rings do not seal to the cylinder walls by their spring pressure alone. The real sealing happens as the piston is moving upward in the cylinder, on the compression stroke. The building cylinder pressure goes into the ring grove (called a ring land) in the piston and makes it’s way behind the ring and actually pushes the ring outward against the cylinder wall, increasing the rings’ sealing. Without this added pressure, the rings don’t have a lot of sealing power. As an engine runs and ages, the ring lands can (and mostly do) start to fill with varnishes and carbon, to the point where the added sealing from the cylinder compression pressure is greatly reduced or eliminated altogether.
We can make a case for a rich fuel wash rinsing the oil from the cylinder walls and piston rings (reducing compression to some degree) and varnished/carboned ring lands that have reduced cylinder wall sealing … all adding up to the near-full loss of compression that is great enough to allow the fast starter cranking (and of course, no start) that would be just like a broken timing belt or chain.
So what do we do about this? This situation can be overcome in one of two ways:
* Remove the spark plugs and add a small amount of oil to each cylinder. Wait a bit, then cover the spark plug holes with rags and crank the starter. Re-install the spark plugs and crank the engine until it starts (similar to the step below).
* Crank the starter for a few seconds, continuously. After about 10 seconds of cranking, the sound become a bit more rhythmic with some indication that cylinder compression is building. Take a break with the cranking, we don’t want t destroy the starter by just continuously cranking for minutes at a time as this will get the starter quite hot. Continue the cranking and there should begin to be some indication of some firing. Remember to give the starter (and the battery) a break occasionally. More cranking should eventually get the engine running, perhaps weakly, for a few seconds after cranking. Eventually, the engine will run on its own and clear out all the unburned fuel. After a few minutes of working with it, the engine should be running normally.
How can we help to prevent this from happening again?
* In cold weather, don’t start the engine and then shut it off after just a few seconds. However, without the loaded up ring lands, this likely would not commonly result in the scenario experienced here, but it may result in a no-start due to flooding.
* Deal with the common varnishing and carboning of the piston ring lands (grooves). See below.
Making regular use of the Liqui-Moly Engine Flush product, which is specifically engineered to help remove these deposits, will clean the piston ring grooves and work to keep them clean as the engine continues to be run. The particular car in this example was new to us and had received a fresh oil change just before our purchase, so we had not done any oil service yet….. nor done the Engine Flush treatment that we always do on “new to us” vehicles. We did do a treatment after getting the vehicle running again. We will also perform the treatment a few more times as the oil changes are performed.
Here’s a short video from Liqui-Moly, outlining the Engine Flush (shown in Euro packaging, This product has also been called Motor Clean):
While single, periodic treatments will keep a low mileage engine nice and clean, one Engine Flush treatment will typically not be the magic cure-all on a higher mileage engine. It will start the cleaning process and improvements, but a multiple treatment plan will result in full cleaning and full performance improvements. For example; On the car outlined in this article, we will use cheaper non-synthetic oil and make frequent oil changes at 2000 miles, or so, using the Engine Flush at each change. After three or four treatments, we’ll return to using the Liqui-Moly full synthetic oils with 5000 to 7000 mile change intervals.
The Liqui-Moly Engine Flush will clean the internal oiled surfaces of the engine, from baked on oil varnishes (normal, with age) and built-up carbon deposits (typically due to overly long oil change intervals and/or low quality oils), but its real function is to work on cleaning up the varnishes and carbon in the piston ring lands (grooves), allowing the rings to float and work properly with the added compression pressures from the cylinder. The Engine Flush is added to the existing oil before changing. Add to the hot oil and engine, run at idle for 15 minutes, drain and perform the rest of the oil change.