hydraulic slave cylinder problem

[Mike M.]

Iam building a 39 chevy sedan delivery( that's been in our barn for 40 years untouched)--figured if i don't get it running, nobody will. so i pulled it out, got a sbc crate engine and pulled an old muncie off the shelf. there is no room to run a mechanical Z-bar clutch set up so i got a slave cylinder(mounted external to the block/bellhousing). i installed a master cylinder out of a 63 chevy pass car(with 1" bore) to activate the slave cylinder. . the slave cylinder has a 3/4" bore. no way in hell will the clutch pedal hydraulically depress the throw out fork, although i can move the throw out fork rearward with a prying motion( long screw driver, and the effort to depress the new luck pressure plate is not unreasonable). So, i think the slave's 3/4" bore isn't comparable with a 1" bore master cylinder. since the slave and master are already fabricated to fit the abc and the firewall, i'm considering sleeving the bore of the master cylinder down to 3/4".. your thoughts will be appreciated. mike

[Carl N.]

Re: hydraulic slave cylinder problem

Mike, while I don't have answer you might look at the set up on C4 vettes, my 87 had a setup like you describe and it worked fine. I had to replace both slave and mc and they were inexpensive.

[Michael J.]

Re: hydraulic slave cylinder problem

IIRC, been a long time since hydraulics school, the smaller the bore the more pressure will need to be applied and the longer the distance of travel, compared to a larger bore. So if you want the master and slave to behave the same way, they need to be the same bore. Not sure that helps you.

[Mike M.]

Re: hydraulic slave cylinder problem

thanks much mike. will sleeve the MC to 3/4" bore like the slave cylinder's bore. happy holidays, mike

[Joe R.]

Re: hydraulic slave cylinder problem

Before you sleeve the master cylinder to 3/4", try to make sure that this is the diameter that you really need for the setup you have.


SOME GENERAL INFO

In a stock C2 (the only geometry I am familiar with), the full travel of your foot on the clutch pedal is several inches, but the actual travel of the throwout bearing is about 0.5".

For discussion purposes, let's assume that the foot travel is 5". That means that the end-to-end mechanical reduction of the entire arrangement would be (5.0/0.5) = 10. If we also assume that the foot pressure to push the pedal to the floor is 50 pounds, the mechanical conversion performed by the linkage converts a 5" travel at 50 pounds (at the clutch pedal) to a 0.5" travel at 250 pounds (at the throwout bearing).

In the C2 linkage, this estimated 10:1 mechanical conversion is made up of three separate dimensions in the linkage:

* The ratio of the foot travel to the travel of the input lever on the Z-bar
* The ratio of the Z-bar's output lever length to the input lever length (these lengths are not identical)
* The ratio created by the clutch fork pivoting on the clutch fork ball


A SHORTCUT METHOD

Fortunately, all you really care about is the end-to-end ratio. If you are using a clutch, clutch fork and bell housing from a C2, try measuring the the actual travel of the clutch fork pushrod on a stock C2. This travel could also be calculated by carefully measuring the ratio of a C2 clutch fork, and assuming that the throwout bearing end moves 0.5". For discussion purposes, let's assume that the actual travel of the pushrod on the C2 clutch fork for is 1.0 inch (ratio of two times the throwout bearing travel).

Then, measure the actual travel of the pushrod on your clutch master cylinder (clutch pedal pressed all the way to the floor). Using the example numbers above, what you want is for the full travel of the master cylinder pushrod to translate into the full travel of the clutch fork pushrod.

So, if the master cylinder pushrod travels 2.0" and the desired clutch fork pushrod travel is 1.0", the diameter of the master cylinder has to be one-half the diameter of the slave cylinder.


COMMENTS

The simplified method described above works because some aspects of the clutch pedal geometry are fixed. Pushing the clutch pedal all the way to the floor moves the master cylinder pushrod a specific distance. Your task is to translate that distance into the necessary travel of the slave cylinder pushrod.

If the two travel distances were identical, the diameters of both the master and slave cylinder would be identical. However, if the required slave cylinder pushrod travel is less than the fixed travel of the master cylinder pushrod, the master cylinder diameter must be less than than the diameter of the slave cylinder. The ratio of the two diameters should be the ratio of the two travel distances.

The overall goal is to use the entire travel of the clutch pedal to move the throwout bearing 0.5" (assuming you are using a C2 clutch). If you get this right, you will have the lowest possible foot pressure required to properly release the clutch.

If the throwout bearing is not moving at least 0.5", the pedal pressure will be lower but the clutch won't fully disengage. If the throwout bearing is moving more than 0.5", the pedal pressure will higher and the clutch will open more than necessary. In extreme cases, this excess travel can damage the clutch or possibly cause it to stick open at high rpm.

I should note that most of the dimensions mentioned above are estimates to be used for illustrative purposes. The only dimension that I know with good accuracy is the 0.5" travel for the fingers on a C2 clutch (throwout bearing travel). I have read about this particular dimension and I confirmed it with McLeod when I recently purchased a clutch from them. I even used an arbor press setup to verify that the clutch opens fully when the clutch fingers are pressed in 0.5"

I did this after reading that some clutches sold for C2 applications require more than 0.5" travel of the clutch fingers. This results in a clutch that will not fully release when used with the stock C2 linkage.

I'm not sure what clutch you are using, but perhaps you could call the manufacturer to confirm that the required throwout travel (clutch finger travel) is what you expect. In your case it's okay if it's different from 0.5", but that will affect the calculated diameter for your master cylinder.

I realize that this post has gotten a bit long, but my intent is to help you make sure that you only have to change the master cylinder diameter once, rather than going through a trial-and-error procedure and/or ending up with improper clutch linkage geometry.

I hope you get that '39 on the road soon!

[Patrick H.]

Re: hydraulic slave cylinder problem

You might try and use a system from a new car (Corvette, Cameron) built in the last 20+ years that uses a hydraulic system. Then, you know that the parts will work with each other. I believe this is commonly done for street rods and similar cars.

[Mike M.]

Re: hydraulic slave cylinder problem

joe: exactly what info i was looking for. i guess i must slept thru the hydraulics lecture 50+ years ago in Physics 101. thanks again. mike

[Mike M.]

Re: hydraulic slave cylinder problem

thanks pat. after i got the 64 pass cr master cal and aftermarket slave cylinder, it occurred to me i should have tried the 90 era corvette slave/master cal but since i already had the firewallfab;d for the master cal and the bracketry fab'd for the slave, i decided against the 90 corvette set-up. i shoulda thought of that option before i did all the fab work. thanks anyway. regards, mike

[Joe R.]

Re: hydraulic slave cylinder problem

I'm not sure how you will sleeve the master cylinder to be the exact diameter you calculate. You will need a very smooth bore, a suitable rubber seal, and a piston of the correct diameter that works with the rubber seal and your existing pushrod.

However, I recall that White Post Restorations in Virginia seems to be able to sleeve just about anything. They could probably do the entire conversion for you, using components from their inventory. The key is to be certain that you know the exact bore diameter you need.

[Dan D.]

Re: hydraulic slave cylinder problem

Hi Joe,

Excellent write-up! And I agree with you completely if the bore sizes are the same. But if the bores are different, then I think it would be the cross sectional area and not the diameter that would determine the travel distance and the force exerted.

Your thoughts?

-Dan-

[Joe R.]

Re: hydraulic slave cylinder problem

Hi Dan:

You are absolutely right! Thanks for catching that. I'll see if I can edit the post.

When I wrote the post, I was speaking from memory (a dangerous thing to do at my age). I knew that what mattered was the ratio of the two areas, but I mentally used the equation for circumference of a circle (pi x diameter) instead of area (pi x radius squared). While circumference is linearly related to diameter, area is not.

I'll go see if I can edit the post accordingly.


Thanks,

Joe

[Joe R.]

Re: hydraulic slave cylinder problem

Before you sleeve the master cylinder to 3/4", try to make sure that this is the diameter that you really need for the setup you have.


SOME GENERAL INFO

In a stock C2 (the only geometry I am familiar with), the full travel of your foot on the clutch pedal is several inches, but the actual travel of the throwout bearing is about 0.5".

For discussion purposes, let's assume that the foot travel is 5". That means that the end-to-end mechanical reduction of the entire arrangement would be (5.0/0.5) = 10. If we also assume that the foot pressure to push the pedal to the floor is 50 pounds, the mechanical conversion performed by the linkage converts a 5" travel at 50 pounds (at the clutch pedal) to a 0.5" travel at 250 pounds (at the throwout bearing).

In the C2 linkage, this estimated 10:1 mechanical conversion is made up of three separate dimensions in the linkage:

* The ratio of the foot travel to the travel of the input lever on the Z-bar
* The ratio of the Z-bar's output lever length to the input lever length (these lengths are not identical)
* The ratio created by the clutch fork pivoting on the clutch fork ball


A SHORTCUT METHOD

Fortunately, all you really care about is the end-to-end ratio. If you are using a clutch, clutch fork and bell housing from a C2, try measuring the the actual travel of the clutch fork pushrod on a stock C2. This travel could also be calculated by carefully measuring the ratio of a C2 clutch fork, and assuming that the throwout bearing end moves 0.5". For discussion purposes, let's assume that the actual travel of the pushrod on the C2 clutch fork is 1.0 inch (ratio of two times the throwout bearing travel).

Then, measure the actual travel of the pushrod on your clutch master cylinder (clutch pedal pressed all the way to the floor). Using the example numbers above, what you want is for the full travel of the master cylinder pushrod to translate into the full travel of the clutch fork pushrod.

So, if the master cylinder pushrod travels 2.0" and the desired clutch fork pushrod travel is 1.0", the surface area of the master cylinder piston has to have one-half the surface area of the slave cylinder piston.

The equation for the surface area of the piston is pi times the radius squared. Stated another way as a simple multiplication, we get:

PISTON SURFACE AREA = (3.14) x (diameter/2) x (diameter/2)

So, to make the surface area of the master cylinder piston equal to one half the surface area of the 1" slave cylinder piston, the diameter of the master cylinder piston should be .708".


COMMENTS

The simplified method described above works because some aspects of the clutch pedal geometry are fixed. Pushing the clutch pedal all the way to the floor moves the master cylinder pushrod a specific distance. Your task is to translate that distance into the necessary travel of the slave cylinder pushrod.

If the two travel distances were identical, the diameters of both the master and slave cylinder would be identical. However, if the required slave cylinder pushrod travel is less than the fixed travel of the master cylinder pushrod, the master cylinder diameter must be less than than the diameter of the slave cylinder. The ratio of the two piston surface areas should be the ratio of the two travel distances.

The overall goal is to use the entire travel of the clutch pedal to move the throwout bearing 0.5" (assuming you are using a C2 clutch). If you get this right, you will have the lowest possible foot pressure required to properly release the clutch.

If the throwout bearing is not moving at least 0.5", the pedal pressure will be lower but the clutch won't fully disengage. If the throwout bearing is moving more than 0.5", the pedal pressure will higher and the clutch will open more than necessary. In extreme cases, this excess travel can damage the clutch or possibly cause it to stick open at high rpm.

I should note that most of the dimensions mentioned above are estimates to be used for illustrative purposes. The only dimension that I know with good accuracy is the 0.5" travel for the fingers on a C2 clutch (throwout bearing travel). I have read about this particular dimension and I confirmed it with McLeod when I recently purchased a clutch from them. I even used an arbor press setup to verify that the clutch opens fully when the clutch fingers are pressed in 0.5"

I did this after reading that some clutches sold for C2 applications require more than 0.5" travel of the clutch fingers. This results in a clutch that will not fully release when used with the stock C2 linkage.

I'm not sure what clutch you are using, but perhaps you could call the manufacturer to confirm that the required throwout travel (clutch finger travel) is what you expect. In your case it's okay if it's different from 0.5", but that will affect the calculated diameter for your master cylinder.

I realize that this post has gotten a bit long, but my intent is to help you make sure that you only have to change the master cylinder diameter once, rather than going through a trial-and-error procedure and/or ending up with improper clutch linkage geometry.

I hope you get that '39 on the road soon!

Dan Dillingham has pointed out a key error in my post, but apparently I can no longer edit the original version.

So, I have edited the copy above to correct the error. The changes are highlighted in Bold.

Sorry for the confusion, especially since my original intent was to help Mike get the correct diameter for the master cylinder on the very first attempt.