E46 SMG Gearbox Clutch Slave Cylinder Repair

The clutch slave cylinder on the SMG gearboxes tend to be notorious for leaking. Since the parts are not sold separately, we’ve found a way of repairing the leak at a fraction of the cost of a new clutch slave cylinder. Upon removal of the slave cylinder, one can see that it is wet as the hydraulic fluid has leaked out. Usually the client would’ve noticed that there is hydraulic fluid on the ground and if driven long enough, it wouldn’t engage gears anymore.

The clutch slave cylinder is removed from the gearbox and then taken apart, starting with the locking plate. This is done using a sharp, small screwdriver, being very careful not to damage or break the tabs keeping it in place. The tabs can break very easily, as it was originally manufactured to be assembled only once.

Once the locking plate has been removed, the pin can be pulled out of the cylinder. An exploded view of all the parts can be seen above. Looking into the cylinder, the damage is clear. Score marks or a wear-pattern on the sides of the cylinder wall, is visible and can also be felt when running a finger across it. Because the cylinder is made of aluminium, it cannot be resleeved. This is because sleeves are usually made of metals which would interfere with the magnet’s function. The magnet is part of the piston which slides inside the cylinder. If you hold a screwdriver against the side of the piston, it will attach to the magnet.

Using a honing tool (which is actually meant for rear brake slave cylinder repairs), the inside of the cylinder wall is carefully honed. It is of great importance to ensure the cylinder is sufficiently lubricated. The hydraulic fluid (CHF 11S) used for the clutch slave cylinder, works well as lubricant. The aluminium is very soft and the honing tool can bite easily, causing more damage. It is vital to only take off the slightest bits at a time and feel regularly that the surface is being smoothed out without being compromised.

On the loose piston, the damaged seal can be seen on the right. The seal has score marks on it and it allows the fluid to bypass on these areas. This is the cause of the leak, but the seal is not sold separately by BMW. So, we’ve sourced an appropriate matching seal which can be used in hydraulic fluid. A standard brake seal cannot be used in hydraulic fluid, as it swells.

Upon reassembly, a tube socket is used to refit the locking plate in position. The complete clutch slave cylinder is then fitted to the SMG gearbox. The bleeding procedure as per BMW instructions has to be followed, using BMW diagnostic equipment. There is no manual bleeding procedure for this repair.