CNC hydraulic slave cylinder sunk cost goose chase

[jimmyhoffa]

So I'm a Midwest boy, and we have a lot of humidity. So much so, that in my case CNC slave cylinders with anodized bodies last one or two seasons before pitting irreversibly through the anodizing and leaking.

Myself, not anticipating that specific failure mode, snagged 3-4 CNC slave cylinder rebuild kits from KarTek before they went NLA. I know the Wildwood slave exists, but I'm ankle deep in rebuild rubber parts. I milled the seal head section of a pitted one in half, replicated the seal geometry, designed it with an internal shaft bushing, machined it and honed it with a Sunnen mandrel hone. (Standard plateau hydraulic cylinder two-stage finish. Heavier grit for oil retention so I don't smoke the seals, and fine plateau finish to keep from abrading the seal too much. The finish actually photographed pretty convincingly.) This may just seize up on me but I'll give it a go. Wow. What a huge amount of time. I re-used the CNC pull piston, end cap (just anodized black) and I used the Wildwood 1/8NPT bleeder because it's nicer.

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1974 Chenowth 2RL #1244 Street Legal
My other car isn't ridiculous.

[ps2375]

Seems you are a Midwest boy with time and some skills, nice job.

[EVfun]

Very nice! I would also check the angle of your slave cylinder when installed.

Often, if you use the stock VW mount on the transaxle there is a bit of side load on it as the rod passes through the release arm at quite an angle. I used a transaxle mount with a heim joint at the top, extending up from the bracket. This takes some of the angle off the other end of the cylinder.
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[jsturtlebuggy]

I stopped using CNC slaves awhile back and use Speedway Motors purple colored ones.
I am not having the problem you having.
As been said mounting it at correct angle so there is no binding in the travel.
I also modify the stock wing nut to the 5/16” thread to use as a pivot Point at clutch arm and a thin 5/16” nut to lock in place.

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Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[jimmyhoffa]

For sure, I agree with the careful mounting of the slave. I was unhappy with the flexing of the Saco / EMPI copy slave saver with the cantilevered spherical rod end, so I machined my own that uses a spherical joint inserted into a bore, and it does not flex, but articulates very freely. (also my car does not look like trash anymore, I grit-blasted and painted the frame )

Really, the rest of the equation is one of these. This is probably next. The little pivoty ball nut thing that all the VW slaves use is both too soft to work correctly anyway, and not an amazing idea from the get-go.

https://www.kartek.com/parts/heavy-duty-mendeola-o...nders.html
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1974 Chenowth 2RL #1244 Street Legal
My other car isn't ridiculous.

[jsturtlebuggy]

That the Heavy Duty cross shaft setup I used on the 094 5speed that is in Manxter Dual Sport I sold.
I bought mine from Weddle Industries at that time. Works well.
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Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[raygreenwood]

A couple of items.

Every car I have ever owned except for my type 3s and my one bus....were all hydraulic. I have worked on the hydraulic clutch systems for many, many different vehicles.

The CNC system....I explored back in the 90s when parts for my 412 clutch slave were unobtanium. Aside from having to fab a bracket for the axial mounting (poor design but it's what fits on most type 1 based trans)......I found that the CNC had a mountain of problems.

It's aluminum. That in itself is not the issue but it is a poor choice for everything else going on. The main issue is the combination of steel fittings and hardware mounted in aluminum....and incorrect anodizing.

The aluminum cylinder needs to be a class 3 hard anodized surface. And it needs to have very specific "tooth"/surface profile.

The shaft gland using a round profile o-ring instead of square or x is not a great design....and most of the CNC o-rings back in those days were made of the wrong rubber type. The life was short and they got debris into the cylinder. That created scoring.

When you score through the anodizing and also have metals of the wrong charge attached to the cylinder.....and moisture in the fluid creates RAPID corrosion. You can see that right at the opening of your cylinder. This corrosion UNDERMINES the anodized surface and spreads laterally.....very similar to what happens with nickel plating.

CNC had a decent piston and good internal seal cup design.......

As a final note.....your "hone", pattern will wreck the seals. Brake and clutch seals do not ride on roughly honed bores. These are NOT like engine cylinders that need a rough cross hatch for "oil retention".

They only need a very fine, even tooth.....preferably without lines of any kind. This is why most modern brake and clutch cylinders are LAPPED.....not honed.

Yes.....many people get away with a very soft and light hone on rebuild.......but If you want a number to work with.....it would be minimum 600 grit silicone carbide for cast-iron and a little finer for aluminum. About 800 works better. Wet dry abrasive paper on a lapping tool works better.

You only need a very fine tooth profile for just enough fluid lubricant in the valleys to make it slick. You do not want to "break in" or wear the edges of the molded EPDM cups.....these are not piston rings. If you create any abrasive wear to the seal cup lips.....the demise of the seal has begun. The outer skin of the seal cup is harder than the molded interior of the seal.

The seals COMPRESS to fit and seal. They do not WEAR to fit and seal like an engine cylinder.

The recommended surface profile for the piston speed our brake and clutch seal cups have (brake and clutch are pretty slow at maybe 2 feet per second max) ....is ~65 to 80 Ra in micro-inches or ~1.6 to 2.0 Ra in microns.

And no....these numbers to not relate/Convert to "Grit", conversion charts you find on line. Those are mainly for grit blasting which dies not cut lines.

You can hit those numbers I listed.....by first polishing smooth and then about 10 strokes with a two stone hone with 600 grit stones or with your stones wrapped in 600 to 800 grit wet/dry.

Nice looking work so far though!

The other HUGE flaw with this slave cylinder design that others are mentioning....is that it has too much axial misalignment. When the U-cup seal misaligns during the stroke....it leaks fluid past the seal in small amounts. Once that fluid hits air.... it absorbs moisture and becomes highly corrosive. Ray

[jimmyhoffa]

Definitely Ray. I left out some critical steps and details just in the name of being reasonably concise. I was figuring, as you hinted, that nobody would go through the trouble to replicate my work here, as we certainly agree a large percentage of the CNC product line in general was relatively poorly designed. (Particularly their master cylinders, which I am not a fan of) CNC stuff was usually moderately reliable and affordable and got people out into the deserts exploring and racing at a fair price point, and that I can get on board with!

-All the O-rings we're upgraded to Parker EPDM rubber, and the gland design was modified by me on the sliding shaft end to promote more seal rollover and hopefully slightly better small end seal life, at the cost of perhaps needing to bleed the slave occasionally and maybe slight seal weeping over time.

-The picture was of the rough hone, and the appearance now is like a matte-finish mirror after several minutes with a single-stone 800 grit silicon carbide Sunnen mandrel hone. The style with three contact points (2 glide shoes and 1 stone) have always produced the best results when used outside of a Sunnen honing center, like you'd use a AN-series port-a-hone. My Mahr gauge I borrowed showed ~4Ra so maybe not the best, but pretty close! I expect reasonable seal life.

The real answer here that a non-crazy person would have given in my shoes is to just buy the Wildwood one, which IS type 3 anodized, has a better seal design and has parts available, likely far into the future. I'll go with that if this experiment fails.
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1974 Chenowth 2RL #1244 Street Legal
My other car isn't ridiculous.

[raygreenwood]

[Clatter]

Now you just need to go ahead and whip up a replica cylinder in stainless or something..
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Bus Motor Build

What’s That Noise?!?

[jsturtlebuggy]

CNC did use a stainless sleeve on their large reservoir master cylinders.
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Joseph
Fair Oaks/Orangevale, CA
Elrod Motorsports
Motion Tire Motorsports
Having fun with Dune Buggies since 1970
Into Volkswagens since 1960

[jimmyhoffa]

[raygreenwood]

[rayjay]

The question is - How much more expensive would SS be versus the aluminum that has to be anodized ??? Or is this a "profit center" ?

[slalombuggy]

Have you considered switch to a DOT5 non hydroscopic brake fluid? I've been using it in my cars for 15 years and haven't had any issues with it at all and they all sit for 6 months every year in an unheated damp garage all winter. I've flushed the fluid a couple times and gave never found any races of moisture.