Hydraulic clutch question

[pcoghlan]

I have been having a little trouble with my clutch. I only got the car on the road last week but the shifts became progressively more difficult over the duration of a test 20 mile drive so I had a look and decided to bleed the clutch. Long story short the bleeder valve thread was shot and so I ended up replacing the slave cylinder. This might have been responsible for allowing air to gradually leak in.

I am now bleeding the new slave cylinder and adjusting the actuating rod etc but have a simple question.

I have seen multiple references to the fact that the rod should be adjusted with the fork fully forward, the slave piston fully back and a 5mm gap between the hemispherical ball and the forks at rest. Here is my issue. The rod seems barely long enough and so I need to have it right up against the fork such that ANY pedal movement results in fork movement. Then, with the rod adjusted to its extreme I get a working clutch and the wheels don't turn (rear end up in the air) in gear. This is with the pedal touching the floor. If I let the pedal rise even half an inch they start turning.

I am thinking of making up a new rod (seems easy enough!) that is maybe half an inch longer. This would allow me to run the clutch without having to have the rod at its extreme adjustment.

So, after all that the question once again is this. I need the rod to touch the fork without slack at rest, is this an issue? Others seem to indicate a 5mm gap is needed.


Thanks,
Paul

[trularin]

Try this:

With your slave bled and ready, adjust the "slave to fork" rod until it just ( and I mean just ) touches between the pocket of the slave and the pocket of the fork.

Now, get your wife, honey or friend to press the clutch all the way to the stop ( yes, put in a stop ). Measure the travel five times and get the average.

If the average is around 3/8 of an inch, you should be okay ( just my opinion here ), if you are too short, you should look at volume versus pressure. In other words, for the amount of travel the pedal does, is enough volume displaced to move the slave 3/8 of an inch.

And, when you change the volume, the amount of pressure will change. If the pedal is very light, you may want to change the volume to increase the pedal feel.

Then you can adjust the pedal stop for length of travel so you get 3/8 of an inch.

Start out with the rod just touching.

BTW, try to make one end captive if you can.

Hope this helps.

[pcoghlan]

Many thanks for the reply Trularin

I think my travel at the cylinder is in excess of the 3/8" you mention. I would estimate (will measure later) it closer to an inch or even a little more.

I do have the rod touching the actuating forks with the clutch pedal 'at rest'. It appears then the the 5mm gap isn't a requirement then.

Extending the rod may not help me. If the rod at its present length touching the forks results in a decent shift only when the pedal is touching the floor then extending the rod would only help if a) the floor was lower(!) or b) the rod was slightly depressing the fork at rest.

This brings me to the whole pressure/volume question. I have a .875 master cylinder and a .75 slave. The pedal is certainly not light but bearable.

Paul

[trularin]

Might want to cut that diameter back. I do not have all of the math with me, but if you were to reduce the size of the cylinder, the travel would be less.

You might want do that and then see what you have. Are you using Tilton?

If everything starts to looks good and you are still over shooting, you can adjust the stop out to limit your travel.

You might want 1/2 inch for your clutch setup.

What you do not want is over shoot. That will have the fingers in a position that at high RPM it may fail to release.

Yes, I have a grade 8 pin at the fork creating a pivit point for the rod. Then I have the rod just touch the cup in the slave.

Hope this helps.

)

[pcoghlan]

Not sure about the make but think it is Wilwood.

I should probably have started all this by saying this is a UK Cobra with...wait for it..a 383 SBC in it (they are all the rage in the UK, honest! ). Running a Tremec TKO II.

So reducing the master cylinder diamter (say from .875 to .75) will reduce the slave cylinder throw. Hmmm...I think that would make things even worse.

Paul

[trularin]

It is a matter of volume.

Let us say you have 1 inch diameter and 3 inches of travel...you have a cylinder 3 X 1 of volume ( oil ). You are going to pack that into a slave. If the slave is say 1 inch, it is going to move 3 inches. If it is .5 inches, it is going to move 6 inches. The volumes need to balance.

So, since you have .75 and that is NOT going to change, you can work this backward to understand that by dropping the volume on the pedal side you will reduce the travel on the slave side.

Does this make sense or does this make it worse?

Try this; calculate the volume of the slave at .75 with 1/2 inch of travel. Now calculate the length of travel to match the volume using your .875. do it again with the .75.

Hey I could be completely nuts and all of this is wrong.

[pcoghlan]

Thanks Trularin

No, I do get the whole volume thingy, honest!

I am just saying that reducing the master to .75 would worsen the situation. i.e. give me less travel at the slave cylinder.

Just took it out for a drive and the shifts are 'OK'. I do need to bring the clutch pedal to the floor to get a smooth shift but it does fine. I will keep an eye on it and maybe bleed the master too as that has a bleeder valve.

Side note, I went into NAPA to look for bleeder valves last week and had a near 'situation'. We (Brits) call them bleed nipples. Haven't a clue what he thought I was looking for but judging by the look he gave me he must have heard 'give me a bleeding nipple'!

Paul

[Speedysnake]

Paul,

Take a run down to Boynton Beach & ask the guys at Custom Cars (Backdraft dealer) to give you a hand. I bought my car from them & they're pretty good folks to work with.

Paul Neeld

[trularin]

I think I completely misunderstood what you were doing there.

I thought you were having too much travel and were looking to move the rod in to help the disconnect of the clutch because it was giving you problems.

Did the nipple work?

[pcoghlan]

Thanks guys. I will play it by ear over the next week or so to see if she gives me problems again. Might also run down to Boynton Beach if need be. Thanks Paul.

Last time I ask for a bleeding nipple in NAPA I tell ya!

Paul

[Ronbo]

In the US they're typically called "bleeder screws".

Don't feel bad though, parts counter people are always a crap shoot. NAPA, generally has one or two old guys in the store. If nobody's over 40 I don't even bother trying.

[pcoghlan]

I cant think of them right now but believe me in the last three years I have been over here I have had at least a dozen similar experiences.

Paul