difference between 5/8 & 3/4 clutch master cylinder

vector1 · #1 (**permalink**) 09-27-2008, 06:07 PM

i was bleeding the clutch today to check operation and after we finished i could not get the clutch to release. i know (at least i think i know) all the clearances were correct and everything was put together properly.

we could not get the pedal to release the clutch no matter how hard we pushed, and the bottom of the bellhousing is cut out so we could look at things, but couldn't see where the throwout bearing contacts the pressure plate.

there is a 5/8 cylinder installed now, tilton setup, and jbl specs a 3/4. the 5/8 worked fine on the previous clutch setup which looked to be otc, and this setup is an 8", so it should take more pressure.

my question, is there that much difference in cylinders to make one inoperable? i could have broke the pedal before the clutch would have moved, or it felt that way.

anyway, i don't want to pull the drivetrain so someone give me the straight scoop, or maybe i will have to pull it. i will be replacing the cylinder before i go any further though.

thanks for sharing.

Tom Kirkham · #2 (**permalink**) 09-27-2008, 06:51 PM

Yes there is a big difference. It is proportional to the surface area of the piston. So A=pi x dim x dim /4 Thus .75^2/.625^2 (note pi is in both the top and bottom and thus cancels out as well as the constant 4). So with the 3/4 in master cylinder you will have 1.44 times the travel over the 5/8 master cylinder. Note that the clutch pedal effort will go up by the same amount.

flipscobra · #3 (**permalink**) 09-27-2008, 11:45 PM

sounds like to me you still have air in the line, since you were using the same setup.

vector1 · #4 (**permalink**) 09-28-2008, 04:00 AM

though the clutch pedal will move 1.44 times more with the 3/4 for the same effect the effort will be less , correct?

Tony Radford · #5 (**permalink**) 09-28-2008, 04:59 AM

As you increase the size of the MS, the pedal pressure goes up exponentially.

vector1 · #6 (**permalink**) 09-28-2008, 12:06 PM

thanks for straightening me out. not what i wanted to hear though, looks like i have some work to do.

found a similiar explanation my brain could understand.

a larger dia. master cylinder delivers more fluid to the slave cylinders, yet requires more pedal force and less pedal stroke to achieve a similiar reaction.

so the 5/8 should be easier to release the clutch with a longer stroke. not happening here.

vector1 · #7 (**permalink**) 09-28-2008, 04:03 PM

drilled a sight hole in the bellhousing so i could look at the offending assembly and the bearing was extended without touching the diaphram. on further investigation it seems the instructions i was using do not match the bearing so everything is coming apart. another day wasted.

SSSammy · #8 (**permalink**) 09-28-2008, 05:50 PM

We have a local club member with a JBL and I have wrenched with him, so I know you have to pull the motor to get the tranny and bell housing apart.

Changing to a 3/4" master is easy. Tilton makes one with the same bolt holes as the 5/8". You should do that, it does not require taking anything else apart. Even so, you will probably need to adjust the throwout bearing clearance. The concentric slave is on a threaded collar. You can possibly rotate it by hand to close the gap to the diaphram. You have to undo the hydraulic lines first, then poke them back into the bell housing. If this is the case then:

Use a long screwdriver or other lever and collapse the bearing piston as far as it will go.

Reach your hand into the fork hole in the bell housing and rotate the slave cylinder on the threaded collar to the left (lefty loosey, righty tighty) until it closes the gap to about .1-.2"

Pull the hydraulic lines out, making sure the bleeder is on top. Reconnect, bleed, go play ..

Sam

Richard Hudgins · #9 (**permalink**) 09-28-2008, 08:17 PM

Mark,

The .625 (5/8) MC is for the rear brake circuit.

The front circuit and the clutch are .750 (3/4).

DO NOT MIX THEM UP!!!!!!!!!!!!!!!!!!!

Seriously !!!!!!!!!!

Make sure to look at the the pedal box and figure out what is what here.

The two MC's closest to the center of the car are for the brakes.

The outside unit is the Clutch.

The .75 MC gives the correct displacement for both the Tilton and the McLeod internal release mechanisms when using the standard Ford diaphragm pressure plate release requirements that all aftermarket manufacturers follow.

It could be that the PO did not put the bits in the right place.

If you continue to have issues you know my number.

It should be quite easy to sort this out as the systems are well proven and virtually idiot proof.

vector1 · #10 (**permalink**) 09-29-2008, 04:47 AM

my first thought when things were not working was the master cylinder was not the right size, and it is a 5/8.

there is a 5/8 for the rear brake circuit and a 3/4 for the front. i don't know how the 5/8 ended up on the clutch circuit, but i don't think it is going to make a difference in this situation. i'll change it to a 3/4 or 7/8 which is what is recommended for the throwout bearing that is in there, i think i have a 3/4 i can take off another project.

i had ordered the throwout bearing and retainer for the t5 i was going to install and then chickened out and used the tko600. i can't use the tb that was in there which has the threads as sammy referes to as it is flat faced and i needed round for this clutch.

after investigation the instructions don't match the installed tb and even has a different picture which is not unusual but the specs are totally different. instructions say .700 of travel with .300 of clearance, while another website says .125 of clearance and is now listed as chevrolet tb. guess which clearance i used. one site recommends 3/4 cylinder, another 7/8. something got mixed up somewhere, wrong tb, instructions, kit, etc.

"idiot proof" is getting to be a rather small umbrella.

thanks for the encouragement, input, and information.

jon@harrison.ne · #11 (**permalink**) 09-29-2008, 06:00 PM

Tony Radford got it part right. The pedal pressure goes up as the square of the MC diameter not exponentially.

john chesnut · #12 (**permalink**) 09-29-2008, 06:15 PM

I must be missing something. I understand the difference in the amount of pedal effort and throw with the different sizes of M/C. I don't understand how having the wrong diameter would prevent the clutch from releasing. Is it possible that the rod from the M/C to the pedal is not set correctly and the M/C piston is not retracting fully? Just a thought. John

vector1 · #13 (**permalink**) 09-30-2008, 08:03 AM

john, you missed the "i don't think it is going to matter in this case" sentence.

the drivetrain is returned to service, just have to tie up a few loose ends.

john chesnut · #14 (**permalink**) 09-30-2008, 03:17 PM

Have fun! I have been building a Cobra of my own design from the ground up. I started in June and dropped it off to be painted today. Am planning to take it to Run n Gun next week and give it a spin. I haven't had a "car" to drive for almost a year and am anxious to be back on the road. Down time is a bummer. John

scootter · #15 (**permalink**) 10-20-2008, 03:01 PM

Figured I would jump in on this. I have a Superformance with a 514. The clutch pedal is hard to push in. There is three holes to set the pedal to the master cylinder rod. Middle is normal setting where mine is. Bottom give less leverage and more throw, but way to much of a work out. Top hole is easier to push in, but have to raise the clutch pedal way up to get enough throw. The car comes stock with a 7/8" master, and has a 1" slave. Will changing to a smaller slave make the clutch pedal easier to push in?? I didn't take note of the type of pressure plate etc when I rebuilt the engine. DUH! It has the Ram friction plate, but not sure of the pressure plate.

Scott