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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 21474 Location: Oklahoma City
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Posted: Thu Nov 19, 2015 11:35 am Post subject: 004 clutch slave musings and solutions thread |
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So....one of the details I have been working on for YEARS....is a suitable replacement for the late 004 four speed clutch slave cylinder. Its a big issue. The lifespan of these is not especially long when compared to other designs. Its fairly unique in shape and size among hydraulic slaves. Never fit anything else. It also had several "defects" along with some nice features.
1. The diameter was kind of a miscalculation. This is not a major defect but does give it some odd characteristics. It is easily the largest diameter clutch slave piston (for a passenger car) that I have ever seen at 44mm.
Along with the nicely sized 19mm x 30mm stroke master cylinder this gives a huge pressure boost. It makes for such soft effortless pedal pressure...one of the softest I have ever driven...that it makes it a little ambiguous as to where and when engagement happens....and kind of guesswork for bleeding.
Solution: leave it alone for now
2. This is a HUGE defect and part of why the wear level is faster on this cylinder...is that the piston skirt behind the seal is exactly the same diameter as the piston body forward of the seal (see my crude diagram). This makes for poor fluid feeding and inflation of the seal cup.
Solution: This works quite well. Take a round or triangular file and carefully make five equally spaced grooves across the piston skirt to connect with the seal groove. Make them about .015" deep and about .030" wide. I will post pictures of this later.
3. The seal groove is located far back on the piston. It has to be because at full stroke the end of the piston comes out of the bore slightly. But....because the seal is the only thing centering the piston in the bore it cocks slightly from clutch pressure on stroke and retraction.
I have noted fret marks on the end of the piston and wear marks on the seal flanks at 6 and 12 o'clock positions. Also in the crappy diagram is the cross section of the other piston (I think is was an ATE or Schaffer branded rebuild kit that was late)...which shows a double seal.
I the two seal slaves I got long ago...the outermost seal was flipped the other direction...facing towards the clutch. This seals nothing....but tended to act as a centering device farther down the piston. I do know from my notes that the two seal piston lasted longer before wearing out.
With the same issue as the single seal piston...same size skirt diameter I can only surmise that VW figured out some of this and used the second seal as a centering ring.
The nice thing about these slaves is that with the bleed hole high up at 12 o'clock and a residual pressure valve in the master cylinder....these units were easy to completely bleed.
Solutions: There are several rebuild options I have been working at.
A. Taking pistons into a machine shop and having a shallow groove turned onto the piston where a second seal would be and having a proper nylon or teflon centering packing installed. Between machine work and having a packing made....its not dirt cheap.
B. The second idea I can do on a drill press at home....or even with a dremel if I have a good day....and that is to cut/mill five small slots about 10-12mm long, 5mm wide and about 3mm deep in the piston...longitudinally and equally spaced and then snap in small slices of nylon that have been sanded/clearance d to exactly touch the cylinder wall as centering pads. This will easily keep the piston centered in the bore.
C. The other issue is the seal itself. It is an inner shaft seal style U-cup. Its size is unique and it must fit the inner shaft tightly to prevent bypass.
At one point in time I had them made CNC back in about 1997. That company was absorbed and and there are now several that can do this. The material is EPDM.
But.... the trick to having these "turned" CNC is that the company generally has to use high durometer rubber material at about 70-80 durometer as compared to the 50-60 durometer of the stock molded seal. This harder seal does not present much of a sealing issue I found...but does make it nearly impossible to install the seal without tearing it. Also some companies can refrigerate or freeze the EPDM to make it hard enough to mill. Looking into that.
So.....take a look at the diagram and note that I propose to slice off the piston skirt with a razor saw carefully....after I drill a locating dowel pin hole and then drill and thread q 6mm locking screw through skirt and into piston. After then deburring and flattening both surfaces...assemble and measure to get accurate seal dimensions and then have the seals machined.
So....I am also looking into a place locally that could triple chrome plate the inner bores of the slave cylinders in a batch. I have about a dozen cores....all with usable pistons.
I can already recast the boots from pour-able silicone. They would only be good to about 250-275F but that should be sufficient.
Thoughts? Ray |
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Lars S Samba Member
Joined: October 04, 2007 Posts: 780 Location: Sweden
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Posted: Mon Nov 23, 2015 11:00 am Post subject: Re: 004 clutch slave musings and solutions thread |
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Thanks Ray for another great writeup!
I have never had a problem with a clutch slave cylinder myself (yet, my 411/412s have about 100.000 mile each) but I have a cylinder core which I would like to prepare as a spare so all ideas aroud this are of great value to me.
/Lars S _________________ Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold |
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 21474 Location: Oklahoma City
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Posted: Mon Nov 23, 2015 3:33 pm Post subject: Re: 004 clutch slave musings and solutions thread |
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Lars S wrote: |
Thanks Ray for another great writeup!
I have never had a problem with a clutch slave cylinder myself (yet, my 411/412s have about 100.000 mile each) but I have a cylinder core which I would like to prepare as a spare so all ideas aroud this are of great value to me.
/Lars S |
Having driven so many and for so many miles.....I have had lots of problems with them.
In general its a very reliable system. But....if you drive them in LOTS of stop and go traffic...big cities....huge amount of clutch usage and in very hot and/or humid climates....I typically got between 50-70k miles from them.
The issue in humid climates primarily comes from rust specking from humidity when the outer boot starts to die....and from small amounts of bleed past of the seal...which is normal but in high humidity it causes a rust ring. This normal "weeping" past the seal....is made worse on single seal piston due to the slight cocking in the bore. The twin seal piston did not have this problem.
The rebuilt cylinders typically were very short life after installing a rebuild kit ...about 30-50k max....because you just could not get the cylinder smooth enough again if it had rust pits.
Those that had shorter life in the high temperature areas had less rust but typically if the boot fails with high mileage....clutch dust tends to wear at the seal area and with the slight cocking of the piston...causes a noticeable wear mark in the bore.
So its not like they dont get good mileage. If I could just readily by replacements over the counter...i would...and just not worry about finding an answer. But spares and rebuild kits that are not super old NOS (and we know what issues those have)....are very hard to come by....and when you need one...you dont drive.
On a side note....I have found no end to the amount of modern clutch slaves that could be adapted to this quite easily with a mounting plate. One from my 2012 Golf would bolt up with little trouble and with the right adjustment and a new type hose connector...would probably do quite well for $50-75. I am looking at those options as well.
Also...a huge number of slave cylinders can be used by making a bracket with an adjustable 90* pivot and linkage...like the Vanagon used. Ray |
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Lars S Samba Member
Joined: October 04, 2007 Posts: 780 Location: Sweden
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Posted: Mon Nov 23, 2015 10:52 pm Post subject: Re: 004 clutch slave musings and solutions thread |
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That explains it Ray, we dont have big cities up here with heavy traffic and definitely no hot climate!
Also, when not using my cars for some years, I used to move them with some clutching and braking say 3 to 4 times a year which also might have helped the clutch slave cylinder to survive longer.
/Lars S _________________ Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold |
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 21474 Location: Oklahoma City
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Posted: Tue Nov 24, 2015 7:40 am Post subject: Re: 004 clutch slave musings and solutions thread |
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Lars S wrote: |
That explains it Ray, we dont have big cities up here with heavy traffic and definitely no hot climate!
Also, when not using my cars for some years, I used to move them with some clutching and braking say 3 to 4 times a year which also might have helped the clutch slave cylinder to survive longer.
/Lars S |
Oh definitely that helps. Just like any brake cylinder or shock absorber....if theycare not excercised.....the seals get set as they age.
As much as I would like to keep the simple design of the original slave cylinder.....its enough work to make it simpler to replace seals.....reconditioning the bore....having seals made....making boots.....solving the rocking/wear issue with the piston. ......that I could simply make a bracket and adjuster......and probably use any number of easily available slave cylinders that would work perfectly.
It may not be original.....but its not a major change. Ray |
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Lars S Samba Member
Joined: October 04, 2007 Posts: 780 Location: Sweden
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Posted: Tue Nov 24, 2015 9:43 am Post subject: Re: 004 clutch slave musings and solutions thread |
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raygreenwood wrote: |
......that I could simply make a bracket and adjuster......and probably use any number of easily available slave cylinders that would work perfectly.
It may not be original.....but its not a major change. Ray |
Agree to that and also you could choose a cylinder with slightly less diameter for a better pedal feel.
/Lars S _________________ Porsche 914 -72, Bahia Red daily driver
VW411 2-d -70, White, sold
VW412 4-d, -73, Gold Metallic, daily driver
Suzuki T500, -69, Candy Gold, sold
Suzuki K50, -77, Black, daily driver
BMW R69S -69, White, sold
Husqvarna 118cc, -47, Black, Sold |
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raygreenwood Samba Member
Joined: November 24, 2008 Posts: 21474 Location: Oklahoma City
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Posted: Tue Nov 24, 2015 11:20 pm Post subject: Re: 004 clutch slave musings and solutions thread |
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Lars S wrote: |
raygreenwood wrote: |
......that I could simply make a bracket and adjuster......and probably use any number of easily available slave cylinders that would work perfectly.
It may not be original.....but its not a major change. Ray |
Agree to that and also you could choose a cylinder with slightly less diameter for a better pedal feel.
/Lars S |
Oh I totally agree. I mean...if I am going to change anything...and its not going to be dead stock...I am going to improve it not just produce status quo (really that go's for rebuilding a lot of stock parts as well with me ).
The clutch pedal feel is really poor on the 411 and 412. The early ones had much better pedal feel (only tried two)...with the smaller diameter clutch slave on the early trans. Ray |
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