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Extending clutch MC rod, band aid fix?

Jason G.

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Oct 7, 2003
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Anderson, SC
After replacing everything in the entire clutch system, I was still having disengagement issues. The best I could manage was maybe an inch off the floor before I ran out of MC adjustment. Today I welded a nut in front of the piece that threads into the MC rod to give me a little more adjustment room and now am able to adjust it how I want without any drag.

I replaced the following:
-ACT 2600 pp and disc
-RRE chromoly flywheel (double checked the step -in spec)
-OEM fork and pivot ball
-OEM throwout bearing
-replace rubber line with braided line
-new pedal assembly with Shep bushings (not welded)
-Nabco MC
-New Slave (Dorman)

The fork position favors towards the engine (like the RRE pic that gets posted a lot)and I am still able to push the slave in by hand, not blocking the MC.

I'm somewhat concerned that I had to do this though. I've never had to do this with other DSMs and GVR4s with similar clutches.
Is this or using the 2g MC rod considered a band aid fix like the extended slave rod?
 
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turbowop

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As long as you can still push the slave cylinder in by hand, I would think it's okay.

What slave did you use? Green piston or red piston?
 

prove_it

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Did you verify the aftermarket master and slave cylinders to have the proper bore sizes? Sure it's a red piston, but I found a red piston in a smaller bore cylinder before.
 

GSTwithPSI

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Not to thread jack, but can someone school me up on what the differences are between "green" and "red" slave cylinders? I've never even heard of this?
 

turbowop

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The red piston is smaller at 3/4" than the green. I don't remember the exact size of the green, something like 13/16". The smaller piston will extend farther for the same amount of fluid pushed.

For some strange reason, all '91 GVR4's have the green piston, and 92's are mixed. All 1g DSMs came with the red piston. When using a heavy clutch, the red is preferred for the longer throw.

I've never seen a larger bore red piston before, but I wouldn't doubt that there are some out there.
 

GSTwithPSI

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Thanks, Mark. You'd think the green piston would be preferred with a heavy clutch, due to the larger mechanical advantage it would provide. Is there an actual colored mark on the piston itself then?
 

prove_it

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Never know with aftermarket, always measure to be sure.

Op, are you sure you bled the system well? It's weird that you've done all parts to correct low pedal, yet it's still there. Seems odd. I wonder if the flywheel has the right thickness. IMHO, extended rods are a bandaid, as they correct worn parts, that you've replaced.
 

iceman69510

Turn Right Racing
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Quoting GSTwithPSI:
Thanks, Mark. You'd think the green piston would be preferred with a heavy clutch, due to the larger mechanical advantage it would provide. Is there an actual colored mark on the piston itself then?



The piston itself is anodized a red or green color. Assuming stock parts. Who knows what aftermarket may have.
 

turbowop

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Quoting GSTwithPSI:
Thanks, Mark. You'd think the green piston would be preferred with a heavy clutch, due to the larger mechanical advantage it would provide. Is there an actual colored mark on the piston itself then?



You're thinking of it backwards. The fluid is pushing on the piston, so a smaller diameter piston will move farther for the same amount of fluid compared to a larger diameter one. It's opposite at the master cylinder, where a larger piston would push more fluid and in turn push the slave cylinder piston farther.

This is why people use the Isuzu slave cylinder for twin disks. It's larger than any standard DSM bore so it won't overthrow the pressure plate fingers and it widens the engagement point. But with heavy single disks, we need the smaller diameter piston to get more throw for proper disengagement.

As has been said, you can tell by the anodized color. All of the Dorman brand slaves I've seen follow the same color coding as OEM. I suppose some other brand may not.
 
Last edited:

tektic

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Quote:
Today I welded a nut in front of the piece that threads into the MC rod to give me a little more adjustment room



I considered this too. I ended up cutting and welding my pedal for the 4th and 5th time. I think I have it where it needs to be now. If the pin for the MC is aligned with the hole in the pedal assembly slightly biased to the fire wall than you should be good.
I just made this quick.
 

GSTwithPSI

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Quoting turbowop:
You're thinking of it backwards. The fluid is pushing on the piston, so a smaller diameter piston will move farther for the same amount of fluid compared to a larger diameter one. It's opposite at the master cylinder, where a larger piston would push more fluid and in turn push the slave cylinder piston farther.

This is why people use the Isuzu slave cylinder for twin disks. It's larger than any standard DSM bore so it won't overthrow the pressure plate fingers and it widens the engagement point. But with heavy single disks, we need the smaller diameter piston to get more throw for proper disengagement.

As has been said, you can tell by the anodized color. All of the Dorman brand slaves I've seen follow the same color coding as OEM. I suppose some other brand may not.



You're right, but I think you misunderstood what I was trying to say. The larger (green) slave cylinder would provide a greater mechanical advantage at the clutch than the smaller (red) slave cylinder would. In both scenarios, the master cylinder is the largest piston in the system. However, with a larger piston slave cylinder (green), you would have more output force on the clutch fork, which I would think would be more favorable when using a super heavy pressure plate. The red slave would travel further, but exert less force on the clutch fork. With the larger green piston, as you said, travel would be shorter; But at the same time, more force would be exerted by the larger piston.

In theory, it would seem to me you would want more mechanical advantage for a heavy pressure plate, i.e. the larger green slave. I'd think the larger slave would make for a pedal that's a bit lighter. In practice however, it makes sense as you explained, needing the extra travel for the heavy pressure plate to function optimally. This is just my observation though, as I didn't even know 2 different slave cylinders existed until today.
 

Jason G.

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Quoting tektic:
Quote:
Today I welded a nut in front of the piece that threads into the MC rod to give me a little more adjustment room



I considered this too. I ended up cutting and welding my pedal for the 4th and 5th time. I think I have it where it needs to be now. If the pin for the MC is aligned with the hole in the pedal assembly slightly biased to the fire wall than you should be good.
I just made this quick.



I'll have to check that out. Thanks for the pic. 5th time welding it? /ubbthreads/images/graemlins/ooo.gif
 

prove_it

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Quoting GSTwithPSI:
Quoting turbowop:
You're thinking of it backwards. The fluid is pushing on the piston, so a smaller diameter piston will move farther for the same amount of fluid compared to a larger diameter one. It's opposite at the master cylinder, where a larger piston would push more fluid and in turn push the slave cylinder piston farther.

This is why people use the Isuzu slave cylinder for twin disks. It's larger than any standard DSM bore so it won't overthrow the pressure plate fingers and it widens the engagement point. But with heavy single disks, we need the smaller diameter piston to get more throw for proper disengagement.

As has been said, you can tell by the anodized color. All of the Dorman brand slaves I've seen follow the same color coding as OEM. I suppose some other brand may not.



You're right, but I think you misunderstood what I was trying to say. The larger (green) slave cylinder would provide a greater mechanical advantage at the clutch than the smaller (red) slave cylinder would. In both scenarios, the master cylinder is the largest piston in the system. However, with a larger piston slave cylinder (green), you would have more output force on the clutch fork, which I would think would be more favorable when using a super heavy pressure plate. The red slave would travel further, but exert less force on the clutch fork. With the larger green piston, as you said, travel would be shorter; But at the same time, more force would be exerted by the larger piston.

In theory, it would seem to me you would want more mechanical advantage for a heavy pressure plate, i.e. the larger green slave. I'd think the larger slave would make for a pedal that's a bit lighter. In practice however, it makes sense as you explained, needing the extra travel for the heavy pressure plate to function optimally. This is just my observation though, as I didn't even know 2 different slave cylinders existed until today.




You have the same hydraulic pressure behind both slaves. Torque doesn't really exist in a hydraulic system. Both slaves will generate the same pushing force, the smaller the bore though the longer the travel as there is less fluid to move. Hydraulics are not a mechanical system which you seem to be comparing the two.
 

tektic

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5th time Yeah. I have my pin past this hole towards the fire wall now.
I have my clutch adjustment all the in instead of all the way out.
My pedal is still not in the right spot because of the return spring
not being able to close all the way. My clutch releases and I can
still press my slave in by hand so I'm ok with it. 6 times is too many.

edit: 3 -4 and 5 were too many. but it does work finally.
 
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GSTwithPSI

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Quoting prove_it:

You have the same hydraulic pressure behind both slaves. Torque doesn't really exist in a hydraulic system. Both slaves will generate the same pushing force, the smaller the bore though the longer the travel as there is less fluid to move. Hydraulics are not a mechanical system which you seem to be comparing the two.



Hydraulics are absolutely a mechanical system in the sense that they create mechanical advantage, just as pulleys or gear sets do. Yes, you have the same hydraulic pressure behind both slaves, but the output force will vary depending on the area of the cylinder's piston, which is clearly different between "red" and "green" slave cylinders. As you said, the constant here is the piston of the master cylinder, which is the largest piston in this particular clutch hydraulic system. Hydraulic pressure is created by the master cylinder as the clutch pedal is pressed, and is then transferred to the clutch fork by the slave cylinder. That much I'm sure you know. However, the amount of force exerted on the fork will vary depending on the area of the piston of the slave cylinder (if all other things in the system remain constant i.e input pressure at the master and the size of the master's piston). If you want a mathematical explanation, I can do that for you, but suffice it to say if you have two slave cylinders operating in the same system, the one with the larger piston will exert more force, and travel less distance (again, everything else being equal).
 
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prove_it

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Please, explain.
 

GSTwithPSI

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Piston 1 is the master cylinder:
Lets say the master cylinder piston is 16in² in area (we'll use a square piston for ease of understanding, normally you would be using the area of a circle). Now lets say you apply 32lbs of force upon the master cylinder's piston. That would produce 2psi of pressure. This is a constant in our particular scenario. 32lbs/in² divided by 16in² = 2lbs/in² of output force created by the master cylinder. Now, take the force created by the master cylinder, and multiply it by the area of piston 2 (the slave cylinder in this case)
.
Piston 2 is the slave cylinder:
Apply the 2psi created by the master cylinder to the slave cylinder's piston. We'll calculate for the smaller "red" slave, which we will say for example is 4in². Multiply the pressure applied to the slave's piston by the area of the slave's piston, and you get 8lbs of output force. 2lbs/in² * 4in² = 8lbs/in² of output force from the red slave.

Now, apply the same thing to the larger "green" slave. We will take the same 2psi on input pressure created by the master cylinder, and multiply it by the area of the larger green slave cylinder's piston, which we will say is 6in². 2lbs/in² * 6in² = 12lbs/in² of output force from the red slave.

This simple mathematical explanation outlines how 2 different sized hydraulic pistons would exert different amounts of force when the same input pressure is applied to both. The only thing that changed in the scenario, was the actual size of the slave cylinder's piston. Also, as you said, the smaller slave cylinder piston would travel a greater distance than the larger piston, yet exert less force on the clutch fork.

I'm not a mechanical engineer, and certainly no math whiz, but I believe this to be true. Maybe Barnes could chime in and explain what I'm trying to say better. I've related the numbers to the attached image, which is the best visual representation I could find.
 

prove_it

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I see what you mean now. I stand corrected.
 
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