Don't flame: clutch adjustment revisited
 

When you all adjusted your clutch according to Fresh Alloys instructions, did everyone's clutch pedal physically get lower?

I first went 4 turns and the clutch pedal height became equal to that of the brake pedal. However, it still felt like the clutch engaged at the same height from the floor--just less total travel.

It seems like some have mentioned that the pedal itself lowers and some say the clutch engagement is closer to the firewall.

Is there any way that some of us are doing it one way and others another way? I really need to get this thing to catch closer.

I'm getting together with my mechanic later this week to discuss the possibility of swapping out the clutch master cylinder to fix the problem.

Thanks in advance.

I actually just made this adjustment this weekend and it has made the car much easier to drive. I haven't noticed that the pedal is any lower, but it does engage closer to the firewall. I think I only made about a 1/2 turn to get it where I like it. If you can make this adjustment first, I would recommend trying it, as it should be cheaper than swapping out the master cylinder.

lolo300zx

The travel distance of the clutch master cylinder cannot be changed. The way the system works is as the pedal is depressed, it pushes on the clutch master cylinder piston which displaces a certain amount of fluid. This displaced fluid compresses the clutch slave cylinder piston, which in turn pushes the throw out bearing onto the clutch assembly relieving the pressure of the clutch plate from the clutch disc, thus dissengaging the transmission drive shaft from the engine crank shaft. The travel of the clutch master cylinder and clutch slave cylinder is a ratio of the cross sectional areas of the respective cylinder pistons. Adjustment of the fork connecting the cylinder shaft to the pedal assembly only may change the position of the pedal. Those who have managed to shorten the travel of the clutch pedal are partially compressing the clutch master cylinder, which is like keeping your foot on the clutch and will cause premature throw out bearing and clutch disk failure.
Sorry for the long post, but it's the only way I could explain it.

From your description, I actually understand it the opposite way. It's a known fact that when you depress the pedal the disks are separated. Therefore, "slightly keeping your foot on the clutch at all times" (accomplished by this adjustment as you claim), will actually cause the two disks to be separated a bit rather than getting closer. I can see how a potential loss of traction can be accomplished here, but not premature wear as there is no over-friction due to too much pressure between the disks.
Am I wrong?

Mr Gee: I agree with you on your notions. As for the master cylinder idea, I was actually talking about SWAPPING master cylinders with a different one that had a larger piston in it to displace more fluid.

While I do not have a full understanding of this at this time, I am operating under the assumption that a different master cylinder displacing a different amount of fluid will alter the characteristics of the clutch. As I continue to reserach this I will keep you informed.

LOLO300zx: you actually felt a difference at 1/2 turn?

Speedaholic: premature wear would be a result of the slippage caused by reduced friction between the discs if there is pressure on the master cylinder. Mr Gee explained it correctly if this in indeed what is going on.

JZ,

You may want to re-read your earlier thread on this subject: https://g35driver.com/forums/g35-coupe-v35-2003-07/45963-changing-clutch-master-cylinder-alter-clutch-catchpoint.html

The explanations there coupled with the good answers you've received here should make it clear that to adjust the height of the pedal which causes the clutch to just start to disengage is as described in the freshalloy thread. For ALL manual clutches for over half a century the pedal height for clutch engagement/disengagement is a mechanical linkage adjustment.

I agree about the reduced friction between the disks, but I would think I would have felt it if after engaging a gear... - the result would probably be similar to riding the clutch - i.e. no car movement. Am I wrong?

Your clutch is probably still fairly grippy at this point, so you will still make it in gear fine since it is only a small amount of friction reduced, which would be a small amount of increased movement between the pads. As your clutch gets older and wears out more and more it would start to slip going into gears if the master cylinder was never completely uncompressed. When this starts the happen the wear is going to increase much more rapidly than usual.

Makes sense.

Thanks.

Question #1 based on your response: I think it has been difficult to decipher the testimonies of members who have performed this adjustment. Some claim the clutch catches closer to the floor, some say it just moves the pedal lower and some say it makes the clutch catch sooner and lowers the pedal. In my experience, it seemed like it lowered the pedal but did not lower the catch point of the clutch.

So, if my theory is correct (which it very well may not prove to be) if you swap in a different master cylinder with different characterisitcs then you could use the pedal adjustment technique to match that with the new characteristics of the master cylinder. See what I'm saying.

For example--say you swap in a different master cylinder that pushes a lot more fluid. For the sake of argument lets say that now, in the absence of changing the pedal height, the clutch grabs closer to the floor (and assuming that it is a correct fit that allows for both full engagement/disengagement). Now, you can change the pedal height with the aforementioned techniques to match the new performance of the system and have a different catch point with an optimal pedal height.

Presumably the whole rationale for having an adjustment mechanism is so that small differences in the clutch/transmission can be managed by Nissan technicians to put the clutch into spec. without throwing out the entire car!

by moving the disks apart by depressing the the master cylinder you are doing two things that can and will destroy the clutch disk/ the pressure plate and the throw out bearing. by doing this you are in essence riding the clutch and putting pressure on the throw out bearing (not a good thing) also if you have it depressed enough the clutch will slip and cause a lot of heat and wear

His account on the function on the master cylinder and slave is 100% on the money.

No doubt, I agree 100%
I was playing with the idea of using this adjustment on the account of the free play.

For the sake of argument, say the clutch will move from full engaged to full disengaged if one cubic inch of fluid is moved from the Master Cylinder to the Clutch Slave Cylinder. Now replace the Master cylinder with one of twice the original diameter. Still takes 1 cubic inch to fully disengage the clutch. However, the amount of motion of the Master cylinder that is required is now ½ of what it was originally. The mechanical linkage from the pedal to the master cylinder will now reflect the fact that half the pedal motion compared to the original will be required to go from clutch engaged to clutch disengaged. You will still be able to adjust the point at where the clutch pedal being depressed starts to disengage the clutch as it will not have been changed by the change in Master cylinder diameter!

In other words, with your new "fat" master piston, the pedal location where the clutch just barely starts to disengage remains precisely the same however the throw or amount of continued clutch pedal depression required to fully disengage the clutch will be half for the "fat" piston versus the stock. Again, the above is true for G-35's, 1950 Fords, 1956 TR-3's or any other car with a hydraulic clutch operating system. The point where the pedal first starts to disengage the clutch is adjusted by mechanical linkage changes.

You indicated that you intend to talk to youir mechanic about this - I'd be interested in what he tells you.

Best of luck to you.

Extra Credit Or Bonus Question
 

As an afterthought, in the theoretical change of Master Cylinder in that 1950 Ford or the 2005 G-35, while the pedal location of the start of clutch disengagement will not change and the pedal motion required to fully disengage the clutch will half, the amount of force required on the pedal will half. :cool:

Edited to clarify that the amount of force to move the master piston will half however that does not neccesarily translate into a halving of pedal force. Sorry.

EDIT: As Mr. Gee graciously later on points out, I have this Bass Ackwards - the force required at the master cylinder indeed increases not decreases. I am profusely sorry.