BMW Clutch Kit Guide
BMW Clutch & Flywheel Tech Info
Choosing a BMW clutch used to be straightforward: a stock OEM Sachs BMW clutch was sufficient for 90% of BMWs. But as BMW engines evolved with turbochargers and performance has increased substantially, the want for BMW performance clutches has similarly risen. This page will guide you through the many BMW clutch options you might find not just on our site but on others as well. We have a slight bend towards the brands we offer (OEM, Clutch Masters, and Tilton) but our info will educate anyone looking for BMW clutch upgrades.
The Fundamentals of Clutch Operation:
The video below does a really great job of explaining how a clutch operates and the relationship between each component. It even uses a BMW! Now if only a similar video existed demonstrating the various differences in clutch material.
Contents of a Clutch Kit
Throw Out Bearing
|The Throw Out Bearing is also known as a Release Bearing. The clutch pedal operates a hydraulic system that pushes on the clutch fork inside the transmission bellhousing. The clutch fork engages the throw out bearing (TOB) and pushes it down the transmission input shaft. The TOB then makes contact with the diaphragm springs of the pressure plate. Bearing and material wear will cause knocking and rattling sounds until the clutch is pressed.
|The Pressure Plate consists of an outer cover, diaphragm springs (sometimes called fingers), and a friction surface (ring). The pressure plate is attached to the flywheel so it's always spinning at engine speed. When the clutch is depressed, the TOB contacts the diaphragm springs, which uses leverage to pry the friction ring away from the clutch disc. The diaphragm springs are the flat steel levers in the middle of the pressure plate. Imagine using a pry bar to separate two pieces of wood - the pry bar is the diaphragm spring and the wood is the friction ring.
|The disc slides onto the transmission input shaft and spins with the transmission. It's two-sided with an engine side and a transmission side. The pressure plate clamps the disc to the friction surface on the flywheel. The disc material has to grab and hold onto both friction surfaces, similar to how a brake pad grabs hold of a brake disc. The disc is designed to be the normal wear item in the whole driveline. It's engineered to be user-friendly and absorb abuse so that something more critical doesn't fail.
|The alignment tool is used during installation. It keeps the disc centered within the pressure plate and flywheel assembly so everything remains lined up when the transmission is slid into place.
Additional Items (Some Required, Some Suggested)
Given the location of the clutch within the transmission a lot has to be removed to gain access. Here is a list of items that will usually need to be removed. Items with an asterisk (*) are probable replacement items that cannot be re-used and should be replaced during the job.
Exhaust hangers, gaskets*, and hardware.
- Driveshaft, hardware, flex disc (guibo), and center support bearing.
- Transmission mounts and hardware, and transmission seals (the rear output shaft is almost always leaking).
- Transmission fluid
- Shifter linkage rebuild and transmission detent spring replacement.
- Clutch slave cylinder, fork, pivot pin, and spring clip.
- Pilot bearing* and rear main seal (usually attached to the crankshaft and engine block and accessed with the flywheel off).
- Flywheel and mounting bolts*. BMWs since 1992 have used a dual-mass flywheel (DMF) which are almost always in need of replacement with the clutch.
Pressure Plate Technology and Design
The pressure plate's role is to apply pressure to the clutch disc, hold it, and then release it. In high torque applications where the disc needs a little extra help, additional clamping pressure can be built into the system. The stiff pedal that most performance clutches suffer from comes from this extra tension inside the pressure plate. Clutch Masters makes their own pressure plates with more leverage built into the design, thereby reducing pedal effort to near stock levels. The friction ring is connected to the outer cover with thin metal strips called drive straps. The drive straps provide additional tension for the friction ring and are a known weak point on OEM clutches, which is why Clutch Masters uses thicker and stronger straps and heavy-duty hardware.
Clutch Masters is one of the only manufacturers to design and produce their pressure plates in-house. Other performance clutch brands rely on an off-the-shelf pressure plate, sometimes modified with upgraded internals but sometimes not. Beware of cheap imitation HD clutches sold by "big box" retailers. They often lack the premium materials and attention to detail that comes from the dedicated clutch manufacturers. Clutch Masters re-engineered the basic pressure plate to give 70% more clamping force with drastically reduced pedal effort compared to other HD designs. They re-positioned the diaphragm spring closer to the fulcrum point - the diaphragm spring does not have to move as far, therefore less effort is required. To date, Clutch Masters is the only US manufacturer to have higher clamping force with near-stock pedal effort.
Clutch Disc Materials
The material used in the clutch disc is hugely important in deciding what clutch to use. Each material compound has its own characteristics and here is where your decision-making needs to be focused. As the need for more grip and holding power goes up the degree of user-friendliness and streetability goes down. The clutch is a compromise by design. On one hand you want a clutch that can handle your power and torque. But on the other you don't want something that will break your transmission or be hard to live with. Keep in mind that all BMW clutches use an organic material, even the F82 M4 with its 405ft-lbs of torque.
Refer to this basic chart from Clutch Masters that demonstrates the balance between streetability and torque holding:
And our own chart on when to use each Clutch Masters kit, broken out into horsepower output vs. type of use:
You have to decide what is most important to you: ease-of-use (such as around town) or Hoonigan-style burnouts (tons of grabbing power with little regard for smooth driving).
Rigid vs Sprung Hub Clutch Discs
Clutch discs have either a rigid (solid) hub or with multiple springs on the hub. At the moment of engagement with the friction surface there is some shock and jolt that is transmitted through the disc and hub and to the transmission input shaft and the rest of the driveline.
Sprung hub discs use a series of springs on the disc hub that absorb the shock as the clutch is engaged. Since the clutch is spinning at a different speed than the flywheel, the engagement can be abrupt especially with certain clutch materials and driving styles. Sprung-hub discs can make certain aggressive materials feel less like an on/off switch. A stock clutch is usually not sprung hub as the dual-mass flywheel absorbs the shock. However, switching to a single-mass flywheel almost always requires using a sprung hub disc since the flywheel can no longer absorb harsh engagement. The smoother engagement is easier which is preferred for street use or less-experienced drivers and helps prolong disc life. Springs also dampen vibrations and reduce clutch chatter on the lightest aluminum flywheels (up to a point).
Since most of our clutch recommendations will include a single-mass flywheel, a sprung-disc is the most common BMW clutch upgrade. Clutch Masters kits with a part number ending in "-D" use a sprung hub, or dampened, disc.
Rigid hub discs are used for either flywheel depending on application and disc material. All cars that come stock with a dual-mass flywheel will use a Rigid disc since having an extra sprung component is unnecessary. Rigid discs can be used with single-mass flywheels as well, however, we try to avoid that on street-driven cars. The lack of springs will make engagement more direct and require more concentration to operate smoothly. Rigid hubs are used more often with higher-capacity materials because they require quick and sharp engagement. The split-second "delay" from a sprung hub will not have the ideal “bite” to properly engage a Kevlar or ceramic disc. A Rigid disc may be slightly lighter than Sprung-Hub and more reliable. Broken springs is a very rare but possible failure and is usually indicative of poor driving habits.
Clutch Masters kits with a part number ending in "-R" use a Rigid disc.
Puck or Puch? Button or Segmented?
You may see mention of "6-puck", "8-puch", or "Button" clutches. Race and some HD clutches use a design with smaller individual clutch surfaces instead of a single larger "full-face" disc. The smaller clutch sections are known as pucks, puchs, pads, or buttons. All of these terms are interchangeable. A segmented clutch is slightly different where it has smaller contact patches but they are bonded to a full-face backing surface. In all cases the theory is the same - the smaller sections produce greater pressure for more clamping power while also reducing weight. The greater pressure is sometimes required with aggressive compounds like ceramic or Kevlar. The number of pucks varies based on the clutch material and intended use.
One of the latest trends in clutch technology is the Twin Disc or Double-Clutch design. This uses multiple small clutch discs instead of a single larger disc. Although smaller, the multiple discs have greater surface area available, which means more torque capacity. But their small size provides a drastic weight savings. These kits can be set up for street, mixed-use, or full track use by varying the clutch disc materials and internal components. Although more expensive these Twin Disc kits offer huge increases in torque capacity and reliability. We've tried them and abused them and are very impressed with the punishment they can absorb. It's no surprise that this design has its roots in muscle car drag racing and then caught on in the drift world too. There are street-friendly organic discs available as a ultra-HD street clutch and also ceramic materials for dedicated track use.
Perhaps their greatest advantage is in strength. The pressure plate gets a total redesign. Instead of using the traditional cast iron pressure plate design they use a machined aluminum or steel cover. For the track/race kits this also does away with the failure-prone drive straps to make it more reliable as well. This makes the Twin Disc clutches ideal for drifting and clutch kicks, since this type of driving most often leads to strap failure on a traditional clutch. Street kits keep the drive straps to make engagement smoother and quieter.
Another advantage is cost. Although the upfront cost is very expensive, the consumable costs are very low. Replacement clutch discs are less than what a OEM clutch kit costs. And the kit can be rebuilt in your home garage without special tools. When used correctly and under the right circumstances these clutches can last a very long time and hold up to whatever modifications you add.
The flywheel provides the friction surface for the clutch to grab onto (and also the ring gear for the starter). The flywheel is bolted directly to the crankshaft and spins with the engine. There are two flywheel designs: single-mass (SMF) and dual-mass (DMF).
|Before 1991 most models used a basic single-mass flywheel (SMF) - a steel plate with a machined friction surface and a ring gear for the starter. Although similar in design to aftermarket performance flywheels they are heavier and provide some dampening of vibrations. SMFs were known to last the life of the car. They could be refurbished during clutch overhauls. A SMF will have some vibration and driveline thrash that sounds and feels unrefined. It's normal behavior but in the desire for more refinement and smoothness, the SMF gave way to DMF.
|Nearly all BMWs since 1991 use a dual-mass flywheel (and a handful of pre-1991 models). The DMF is essentially two steel plates with a rubber mass, metal springs, or a fluid damper unit sandwiched between them. The DMF absorbs engine and transmission vibration and provides a cushioning effect to clutch engagement. Failures of the DMF are well-known and common. The damper unit inside breaks down, especially with heat and hard use. A failed DMF usually cannot be diagnosed until the transmission and clutch are already out of the car. Under the best case scenario the flywheel has to be replaced during the clutch job. Sachs recommends the DMF be replaced with every other clutch (or sooner). However, we have seen DMFs come apart completely with catastrophic results - a spinning flywheel acts like a circular saw, slicing into the bellhousing or even beyond. While good for street use thanks to its smoothness, for safety and reliability we recommend a single-mass flywheel for track and sport use.
|Steel "lightweight" flywheels bridge the gap between OEM single-mass and lightweight aluminum flywheels. This are a SMF that has been machined to reduce weight, typically 20-30% lighter than before. Because of its mass it still has some dampening features and the heavier weight makes it easier to drive than full aluminum flywheels (revs do not drop as much during shifts). Because it is a SMF it will not have the smoothness of a DMF so some noise and vibration should be expected (but less than aluminum). In turbo applications steel works better than aluminum because revs don't drop and the engine can stay on boost.
|Aluminum flywheels are at the extreme performance end of the market and offer substantial increases in performance due to their lower weight. However, they are so light that they lose almost all of their dampening effects. There is noticeably more noise and vibration, especially at low RPM. The engine revs quicker and more power is transferred to the wheels thanks to the lower weight but the downside is that revs drop faster during shifting, which requires a larger-than-usual throttle blip and can make rev matching difficult for some.
The "X" Factor
The enemy of all clutches is heat. Excess heat for the chosen clutch material will cause it to slip or fail completely. But where does that heat come from? Hard use and abuse. The stock BMW clutches are actually very very good (realize that a F82 M4 with 405ft-lbs uses an organic clutch). They are actually too good and cover up driver error/poor technique. The driver becomes the X factor. Driver > abuses clutch > excess heat > clutch slippage and wear or even component failure. Poor driving technique and bad habits reveal themselves more with performance clutches because they don't have this "safety net" margin of error.
Remember that the clutch is designed to be the weak link in the driveline. Upgrading the clutch means that something else may become the fail point (transmissions are expensive!). The clutch should sacrifice its life so something else doesn't have to. Is the multi-plate ceramic clutch worth it on your street car?
Putting It All Together.
What kind of clutch do you want? Answer these two questions honestly, -
- What type of driving will you be doing?
- How much horsepower (torque) are you making?
It's our opinion that street cars should be easy to drive and forgiving. We want smooth engagement and quiet operation. When not being abused there is nothing wrong with an OEM clutch on a naturally-aspirated or stock turbo BMW. Tuned turbo cars up to 475hp and are driven daily should be able to use a OEM or FX100 clutch. For a little more grip the FX250 is the next level up without sacrificing drivability. If you're making over 475hp we would use a FX250 that tolerates slippage or a FX350 if hard launches will be routine. At those power levels you are likely sacrificing some drivability or comfort anyway.
For a club racer or dedicated track car we like to use the FX400 with a sprung-hub disc. It's a ceramic disc so it can be tricky to use from a dead stop. The ceramic does not like to be slipped and can be grabby but with experience it becomes second nature. If tricky engagement bothers you the FX350 is still a good choice and is more than capable for any naturally-aspirated or a FBO turbo BMW. It's also more user-friendly for novice or amateur drivers. If the budget allows, and you're competing for lap times, the Twin Disc 725 and 850 are the top choices. Although the upfront cost is high, the return is greater with a supremely capable clutch and minimal weight. If the rules allow it, a Twin Disc clutch is our recommendation.
Have further questions or want to talk this over with a human? Contact us by phone, 877-639-9648, or by email, [email protected].
E46 M3 SMG Faults & Service
SMG in the E46 M3 is the second generation sequential shifting, often referred to as SMG-II. It consists of an electric motor supply hydraulic fluid to a pump, which then energizes solenoids, actuates a shift linkage, and changes gears. The system is heavily integrated into the rest of the car including the engine computer, EWS, and body electronics. There is a lot of moving parts and programming logic built into the system and when it's working correctly is quite advanced for automatic and paddle-shifting modes.
The motor and pump are located on the left side of the engine block just below the starter. A hydraulic fluid tank is bonded to the intake manifold. Hydraulic lines run from the pump to the shift actuator on the back of the transmission. The shift actuator takes the place of the usual manual transmission shift lever and linkage. The actual transmission, clutch, and flywheel are essentially the same as the manual Getrag 420G 6-speed.
Problems with the SMG system can be traced to one of only a few common trouble spots: a fuse or relay, the electric motor, the gas-charged accumulator tank, or the seals inside the pump. The entire system is dependent on electric power and hydraulic pressure. In many cases a bad fuse or relay is the only fix needed and we strongly recommend carrying spares of both. They are cheap enough that you should install new parts as part of any SMG service. The electric motor fails due to heat and being overworked trying to supply pressure to the pump. The pump fails due to a loss of pressure and also being over-worked creating the high pressures (45-90 bar, 661-1,323 psi).
In the recent past BMW did not sell individual components of the SMG system. Only a complete replacement SMG hydraulic pump with the accumulator and motor. And it was frighteningly expensive - just under $4,000 for the parts alone!!!! More recently, BMW stopped selling the complete pump altogether, leaving SMG owners literally stuck in neutral! Some enterprising independent shops can remove the SMG system and convert it to a traditional 3-pedal manual. This can cost almost as much as a new SMG pump but for current car owners that prefer a manual this is sometimes the preferable option anyway.
BimmerWorld has watched and evaluated the market closely and we've come up with a catalog of factory and aftermarket solutions to the SMG failures that we think will keep the SMG market alive for anyone who wants to save the sequentials. This catalog consists of individual replacement motors, accumulators, sensors, wiring, and completely rebuilt pumps that are both more affordable than factory BMW but also include improvements or serviceability that will help prolong their life.
Click here for all of our SMG service items.
2008 E9X M3 Clutch & Flywheel
When it was new in 2008 BMW built the M3 with a balanced flywheel and clutch assembly. For awhile early on you could only buy the two together. Starting in 9/2008 BMW stopped the balanced assemblies and reverted to sales of separate clutch and flywheel items. The flywheel and the pressure plate are still balanced - but separately from their respective factories.
BMW stopped selling the 2008-specific parts and superseded the parts to the 2009+ parts. When shopping for a clutch and/or flywheel for a 2008 model you can buy any of the 2009+ parts and they will be a direct fit. If you're still using your original 2008 flywheel - even after 15+ years - you will need to align markings on the pressure plate with markings on the flywheel.
BMW also recommends buying a new clutch return spring for the clutch pedal. The original spring had a weak spot so BMW updated the part with a reinforcement. All new and current M3 clutch return springs are the reinforced version.
BMW Clutch Kits, Related Parts, and Accessories
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