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Take Care of Your Clutch--Preventing or Curing Pedal Issues

Old 02-13-2009, 09:45 AM
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Default Take Care of Your Clutch--Preventing or Curing Pedal Issues

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[Note: For HD, depending on your network connection speed, you may need to press the pause button to let the video preload part way, before you start watching. But the image quality makes it worth the wait.]

Clutch pedal issues have bedeviled Corvette owners since the advent of the C5. I got on the case after buying my first Z06 in August 2000 and pedal woes bit my new car. Since then, I’ve learned a lot about the maintenance a clutch needs to keep performing normally under aggressive driving. And I’ve shared those insights in many threads, emails, private messages and phone calls over the years.

To get the word out more effectively, I’ve distilled that knowledge into a seven-minute video that lays out the symptoms and root cause, and demonstrate a protocol for prevention and cure of clutch pedal issues.

I particularly want to thank George Westby, Director of the Advanced Process Laboratory of Unovis Solutions and his lead engineer Martin Anselm. They collaborated in the analysis of clutch fluid and clutch dust samples, which I sent them from 2006 Z06s.

Click here to download a copy of the Unovis lab report

That report forms an important basis for concluding that clutch dust is infusing the clutch fluid of aggressively driven cars. And it is clutch dust that, if allowed to accumulate, constitutes an abrasive that damages integrity of seals in the hydraulics.

For clarity sake, driven “aggressively” means cars that are launched or see high-rpm up-shifts or down-shifts. And consequential clutch problems affect many brands of car besides GM and motorcycles too.

The good news is that we have figured out the root cause. That means the preventative maintenance (frequent swaps of the fluid via the master cylinder reservoir) definitely works. It also explains why, if the seals are already damaged by clutch dust, no amount of clean fluid will heal them. The crucial element is prevention, never allowing the clutch dust to accumulate in the fluid. The tell of its presence is murkiness or cloudiness. So don’t allow the fluid to stay murky.

I think water entering the fluid via heat-and-cool cycles plays a roll in clutch pedal issues. So frequent changes of the clutch fluid ought to be part of routine maintenance for owners who don’t do aggressive launch or shifts. Suspect there are a few owners like that around.

Hope the video helps more owners avoid the trouble and expense of clutch pedal issues…and helps GM further reduce warranty claims for clutch issues easily avoided by preventative maintenance. The routine I personally follow costs about $20 and two hours of my time per year. That’s a small investment with a big return in reliability and driving pleasure.

Best to all,


Comparison of various brake fluids

How the Clutch Hydraulics Work
It’s always a good idea to go to the source document for a definitive description of the operation of the clutch hydraulics. In this case the Corvette Service Manual 2006, Volume 3, page 7-360, and I quote:

The clutch hydraulic system consists of a master cylinder and an actuator cylinder.

When pressure is applied to the clutch pedal (pedal depressed), the pushrod contacts the plunger and pushes it down the bore of the master cylinder.

In the first 0.8 mm (0.031 in) of movement, the recuperation seal closes the port to the fluid reservoir tank, and as the plunger continues to move down the bore of the cylinder, the fluid is forced through the outlet line to the actuator cylinder mounted to the driveline support assembly.

As fluid is pushed down the pipe from the master cylinder, this in turn forces the piston in the actuator cylinder outward.

As the actuator cylinder moves forward, it forces the release bearing to disengage the clutch pressure plate from the clutch disc.

On the return stroke (pedal released), the plunger moves back as a result of the return pressure of the clutch.

Fluid returns to the master cylinder and the final movement of the plunger opens the port to the fluid reservoir, allowing an unrestricted flow between system and reservoir….

Close Quote. There you have if from the source.

Summary: Clutch fluid circulates between the master and actuator (slave).

Note: The volume of fluid in the entire system is a very few ounces. One ounce is in the master cylinder reservoir until diminished as you depress the pedal and create the flow described above.

How Clutch Dust Enter the Fluid
Update February 17, 2009
Today I spent time at the workbench with two Chevy master technicians (one who’s worked on Corvettes since the C3 was first released). We examined the operation of the Corvette clutch actuator and concluded following:

1. The bell housing is fouled with blown clutch dust during aggressive driving.
2. Clutch dust is penetrating the accordion shield on the actuator main shaft. That is obvious from visual inspection.
3. The piston slides along the shaft sealed by an O-ring. The shaft has a film of lubricant or clutch fluid on it. During aggressive driving, this film gets coated on each stroke with a fine layer of blow clutch dust. That is obvious from visual inspection.
4. The O-ring slides along the shaft and squeegees some of the clutch dust down the shaft where it contacts the clutch fluid and is infused.
5. The conclusions were unanimous and seemed obvious from a physical exam of the surfaces involved.

Plus, keep in mind that it is confirmed that clutch dust is getting into the fluid. The question is how. We believe the answer to that is in points 1-5 above.

[The following two photos are courtesy of Chuckster; they are of a C5 actuator, which is very similar to the C6 version]

Minimizing Water Infiltration to the Clutch Fluid
Update February 23, 2009
I've been following the protocol for eight years now and never had shift-related pedal issues. That's includes a lot of burnouts, launches, and red-line shifts. So what I'm doing is definitely effective.

Brake fluid is indeed hygroscopic. By my approach the cap is off the reservoir for very brief periods and, while that occurs, each time you are swapping about 35-50 percent of the used fluid with new. Do that one-to-three times and the impact of water absorption from the atmosphere is essentially nil. In fact the more times you make the swap in succession, the less the impact of transitory water infusion.

The industry standard for "wet" boiling point is the fluid containing 3% water. That's amount of water infusion is not going to happen in fluid that's kept clear and fresh via the reservoir. I suppose water infusion can be an issue if you remove the cap from the reservoir outdoors on a very humid day (or in the rain) and then take a lunch break. But few of us would do that. And a couple flushes by the protocol and the water is gone anyway.

Another aspect of risk is using brake/clutch fluid from a can that's been open for months or left with the cap off for an extended period. That's ill-advised. But I routinely keep a can going for 30-60 days by....
(1) cutting a slot in the membrane at the neck vice removing the entire seal
(2) replacing the cap when not pouring from it
(3) keeping the can in double layer of double-seal zip-lock bags between uses

Those steps minimize moisture infiltration to a can that's been opened.

Details on Remote Bleeder

Glazing the Clutch by Driver Error
Update June 4, 2009

On Launch
A driver can easily glaze the clutch on launch by inducing too much slip from too high a launch rpm or riding the pedal to keep the rpm up. Either way, the friction surfaces overheat and the clutch’s clamping power decreases dramatically. Once the friction surfaces cool down again (30-60 minutes), the clamping force is fully restored without lasting damage.

Here are some other ways to glaze the clutch.

During a Burnout
Overheating the clutch during a burnout can happen when:
(1) you don't have enough water on the tire tread surface
(2) you position the rear wheels too far forward in the tacky rubber and then start the burnout. In this situation the rear tires have too much grip to spin at the hit of the clutch; so the clutch slips instead. This quickly glazes the clutch.
(3) you do everything correctly except continue the burnout too long, allowing the engine speed to be drawn down by the growing traction at the rear tires. A crossing point is reached and the clutch begins to slip. That produces a spike in heat in the rotating surfaces and the pedal reacts negatively.

During High-rpm Shifts and Power-Shifting
On a high-rpm shift (using the LS7 as an example) the flywheel spins at say 7K (pre-shift) and 200-300 milliseconds later the post-shift rpm is drawn down to 4700 (1-2 shift), 5100 (2-3 shift), 5400 (3-4 shift). Those rpm deltas produce significant heat that must be dissipated from the rotating friction surface. A power-shift makes for an even higher pre-shift rpm and yields even more heat. That's one of at least three reasons that power-shifting is risky and can have unintended consequences.

Cumulative Heat Leads to Malfunction
The LS7 clutch disc friction material has strands of raw copper interwoven to help dissipate heat from the surface. And there's substantial air-flow within the bell housing to provide cooling and evacuate clutch dust. But these design features can't deal adequately with the accumulated heat from
(1) Improper burnout technique
(2) Too high a launch rpm or too slow a clutch release
(3) Power-shifting or improperly timed high-rpm shifts
(4) Any combination of (1), (2), (3)

Once the cumulative heat reaches a break-point, the clutch either glazes entirely, losing clamping force after the launch attempt, or no longer disengages properly during high-rpm shifts.

Here's what I do to keep within the limits of the my clutch:
(1) Keep the clutch fluid clean including changes between passes at the track (usually after 3-5 passes with my driving style).
(2) Keep the launch rpm at no more than 3600-3700 and make a fast, one-piece clutch engagement
(3) Refrain from power-shifting
(4) After every pass, I lift the hood and sniff the driver's-side firewall. If I smell even a slight "eau de clutch" aroma, I know I'm not getting the clutch pedal out fast enough. That rarely happens to me because I observe rules (1) (2) and (3) just above. The alternative is an aftermarket clutch with a different personality than the stocker.

A pointed warning to owners is to recognize the limits of your clutch. That means not adopting the all-out racing launches and power-shifting advocated by some drivers with stronger and more heat-tolerant after-market clutches. Such approaches give a dramatically truncated clutch life. My approach should give at least 200 drag strip passes at stock power-levels, before the clutch requires replacement.

Last edited by Ranger; 09-11-2009 at 11:00 AM. Reason: New Information
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Old 02-13-2009, 09:54 AM
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Old 02-13-2009, 10:01 AM
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Great instructional video.

Thanks for posting.
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Old 02-13-2009, 10:04 AM
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The Ranger Method is an absolute must if you want to keep your clutch operating smooth!

Thanks for the great video and information!

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Old 02-13-2009, 10:04 AM
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Where the hell have you been?
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Old 02-13-2009, 10:08 AM
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Great read and video, thanks Ranger. Something to do today for sure.
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Old 02-13-2009, 10:17 AM
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Thanks for sharing this with us Ranger.
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Old 02-13-2009, 10:28 AM
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Originally Posted by jschindler View Post
Where the hell have you been?
without the toe tap...
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Old 02-13-2009, 10:53 AM
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I would like to add that even if you do not shift that aggressively that the clutch fluid will go murky in no time at all. My guess at a good interval would be every 1K miles for mild use.
Thanks Ranger
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Old 02-13-2009, 10:56 AM
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This is why this is the BEST Corvette site in the world....Great Job and thanks for sharing....
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Old 02-13-2009, 11:00 AM
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Thank you...We were just thinking about changing ours. The video is perfect. Thanks again..
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Old 02-13-2009, 12:56 PM
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COOL! Where can I find one of those jumbo reverse hypodermic thingies that was used to suck out the dirty fluid? Last time I did mine I used a turkey baster. It worked but not as good as the one in the video.
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Old 02-13-2009, 01:30 PM
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I can attest to Rangers advice and thank him every day. Since my first "event" I have followed his advice religiously and change the fluid frequently. This problem doesn't sneak up on you, it happens abruptly when you least expect it, so prevention is the operative word. Check that juice every week and/or after every time you "wring her out"!

Thanks, Ranger!!!
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Old 02-13-2009, 02:35 PM
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Originally Posted by Vette junkie View Post
COOL! Where can I find one of those jumbo reverse hypodermic thingies that was used to suck out the dirty fluid? Last time I did mine I used a turkey baster. It worked but not as good as the one in the video.
Hi Vette junkie,

The MixMizer can be found at Walmart, typically on a pegboard near the gas cans in automotive. Costs about $3. I've used the same one for 8 years.

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Old 02-13-2009, 02:43 PM
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Mods, this is a stickie-worthy. May be better in tech section, if Ranger is OK with that?
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Old 02-13-2009, 02:45 PM
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Any M6 owner needs to implement Ranger's clutch fluid regiment as part of routine care and maintenance of their vehicles.

Just Do It!!!!!
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Old 02-13-2009, 03:52 PM
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Old 02-13-2009, 04:14 PM
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Very nicely done..
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Old 02-13-2009, 05:23 PM
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I do not do hard launches (yet) but have followed your advice since picking up my 2006 coupe at the NCM. I have seen dark fluid every time and crud on the inside cap once. I do the process at every oil change. Your new video is great, nice work.
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Old 02-13-2009, 05:36 PM
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Thanks again Ranger

I first read about clutch maintenance on your website & my 08 w/ less than 4000 miles has had its fluid changed twice since May 08. Both times the fluid was dark only after a month or two.

BTW: for those that don't want to go to a WallyWorld, I purchased a 60cc irrigation syringe on amazon dot com for $8.23 including shipping from KCK Medical & it was medically sterile . Turkey baster FTL
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