Clutch Master Cylinder Innards

[Aubs]

Right, this might be a bit difficult to explain without pictures, but here goes:

I have a problem with my clutch control system which I've traced to the master cylinder (M42 box btw). basically it seems to want to displace the fluid up the filler pipe rather than moving the slave.

Rather than just buying a master i decided to take it to bits, to see if I could figure out what was wrong with it. I've removed the piston, and all the seals there seem fine. I looked right down the bore, and saw a small metal 'cage' at the bottom (the best I can describe it), which has a hole in the middle aligned with the output port on the cylinder. Under this was a little rubber 'nipple', which seems to have shifted, and was obscuring the output port. I pulled both out and found that the cage was, rather than being circular, flattened on one 'side'. It looks like if this was circular, the base of the piston would fit neatly into it.

Question: Have I found my problem, or is this little metal bit supposed to be shaped in this way? And if so, can I get a replacement? RealOEM just lists the whole cylinder?

[Motorhole]

I think for the price of a new cylinder you might aswell just replace the unit...what is it, about £30?

[Brianmoooore]

BMW do a clutch master repair kit - 21 52 1 156 001
No idea if it contains the part you need though.
A new cylinder, of a quality worth fitting, is just shy of £80.

[Aubs]

Thing about this cylinder is, the last owner says it's only 10 months old. It looks pretty new as well

Thanks for the part number Brian, i'll be picking up one of those repair kits

[Brianmoooore]

Thing about this cylinder is, the last owner says it's only 10 months old. It looks pretty new as well

This is probably one of Motorholes £30 ones! The repair kit probably won't fit your Chinese monkey metal copy.

[Aubs]

So how do I tell if its genuine? The only marking on the casting is '3904', stamped on one of the machined faces.

[Aubs]

Any ideas? Anyone feel like taking a look at the markings on theirs (if its genuine)?

[Brianmoooore]

FAG - 1 - 85446K on one a genuine one I removed recently.
What material is yours made out of?