Post-Mortem on one 4A0721261 Clutch Slave Cylinder Failure

[UrS4boy]

I have been trying to keep track of 4A0721261 clutch slave failure modes since mine failed before Christmas 2013. Finally got to see what was what Jan. 2014. (See link at the bottom of this post for the failure modes so far.)

Here is mine from Jan. 2014



Having recently posted questions on another forum about Mushasho's "naked" clutch cylinder actuation rod, I realized that if I still had my failed slave cylinder then I could take it apart and answer my question on how the rod was attached to the slave cylinder.

Mushasho's naked rod (insert rude joke here):



Okay here is my failed slave cylinder from this afternoon:

One side:



The other side:



With the boot off, exposing the actuation rod:



Better angle showing the end of the rod sitting in a socket in the slave cylinder piston:



The rod is NOT attached to the slave cylinder. It merely has a ball end that sits in the socket end of the slave cylinder piston. Held there, in part by the rubber boot (assuming the rod has not broken through the tip of the boot like mine did)



This photo shows how the actuation rod can be captured and held by a broken tip at the end of slave cylinder rubber boot:



Conclusion: For reasons not clear, the actuation rod of my slave cylinder caused the fatigue cracking of the plastic tip of the rubber boot allowing the rod to pass through the broken tip. Once out, the rod was disconnected from the slave cylinder piston and (very likely) twisted such that it would no longer push on the clutch throwout bearing activation arm.

Bottomline: Replacing the clutch slave cylinder might be a good idea after several years, even if it is not leaking. Either that or someone should develop a design for a brass or aluminum slave cylinder boot tip that would not fail.

REF: Clutch Slave Cylinder Failure Modes - http://forums.quattroworld.com/s4s6/msgs/316182.phtml

[LHD_Kid]

Either that or someone should develop a design for a brass or aluminum slave cylinder boot tip that would not fail.

REF: Clutch Slave Cylinder Failure Modes - http://forums.quattroworld.com/s4s6/msgs/316182.phtml

Or, do as audi did on the type 85 and have the rod made in one piece of metal with no plastic bits at all. (There's a plastic 'nipple' at each end of the rod...)

I just had mine made out of stainless, works a treat and will not fail (doing the same for the damper section near the pedal too...

Edit: I took a photo for comparison, will post it up when I download the pics from my camera...

[UrS4boy]

Or, do as audi did on the type 85 and have the rod made in one piece of metal with no plastic bits at all. (There's a plastic 'nipple' at each end of the rod...) I just had mine made out of stainless, works a treat and will not fail (doing the same for the damper section near the pedal too... Edit: I took a photo for comparison, will post it up when I download the pics from my camera...

Love to see the photo.

How do you attach the boot to the forward end of the rod?

[LHD_Kid]

Love to see the photo.

How do you attach the boot to the forward end of the rod?

You make the rod as an exact match of the plastic ended version...

(Apologies for the grubby paws, I wasn't taking photos for publication, only for my own records - and definitely not up to your standard Dave!!!)

Voila:

[diesel des]

What size is the pistons in these?

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[UrS4boy]

You make the rod as an exact match of the plastic ended version... (Apologies for the grubby paws, I wasn't taking photos for publication, only for my own records - and definitely not up to your standard Dave!!!) Voila:

Thanks for posting. I think there *might* be a market for more of these.

[LHD_Kid]

Thanks for posting. I think there *might* be a market for more of these.

I really agree, not difficult to make and it is a fit and forget - especially for those with uprated clutches, even more pressure on the little plastic bits

There is also the damper rod fixed the the clutch pedal, the rubber gets soft and the rod breaks exactly at that point. A similar action can be taken to remake in stainless, just without the rubber bit. keep the shape, the thread and the ridge for locating the rubber boot (my mark 1 version - the machinist didn't understand exactly what I was after and he forgot the boot ridge ! mark 2 was made but I didn't have a chance to photograph it... but you get the idea!)

The below may be of use (hope I'm not cluttering your thread!)

The steel version (mark 1 version remember, without the boot ridge):

[IMG][/IMG]


So, the part which fails (this one bent just by the thread)

[IMG][/IMG]

[diesel des]

Did anyone measure the bore dia of these or the master cylinder. I'm trying to work out the master cylinder size required in a custom pedal box.

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[Tom_]

LHD_Kid Still everything ok with these custom rods? I am thinking of making them, as I have Sachs SRE clutch and OEM BOGE cylinders are both done. I bought Sachs Slave and TRW main clutch cylinder.
I was thinking as the slave cylinder rod is freely put there, is there any chance that rod will fall off whilst you hit the clutch? I know that there were two versions of slave cylinder, one with Circle clip and other with that star washer.

[SteveH]

The rod shouldn't detach as it's trapped between the slave cylinder and the clutch release bearing arm/carrier. The plastic end on my slave split in two using the SRE clutch.

[Tom_]

Frankly I disassembly the old stock ones (to see what is going there), but I am struggling to keep these parts inside as the force pushing against my hand is really strong I can't put that circlip inside to lock that rod

[LHD_Kid]

My homebuilt ones are still going fine - interesting, I don't remember it being difficult to fit though

[Tom_]

I will check the inside , if there isn't something what shouldn't be there, but yeah, really hard to push it

[newsh]

Frankly I disassembly the old stock ones (to see what is going there), but I am struggling to keep these parts inside as the force pushing against my hand is really strong I can't put that circlip inside to lock that rod

Put the cylinder in the vice, find a small socket that fits over the rod, use a ball of rag and extension to push the rod in.
If you can get it set up at the right angle you can even push it with your chest which leaves two hands free to fiddle the circlip in.

[Tom_]

Thanks. I Will try it.

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