Can’t enter first or reverse at idle (ZF box)

[Brian C]

I have a ZF DS25/2 mated to a small block Chevy via a Kennedy bell housing. My clutch master cylinder is 3/4” and my slave cylinder is 7/8”. Plumbing is all hardline except for the last 2 feet to the slave which is braided stainless. The system is all new.

I cannot get the car to go into first or reverse when idling.

I’m only getting about 1/2” slave pushrod movement with full pedal depression. Is that normal for the ZF?

we tried bleeding the slave cylinder via the normal method like you’d do your brakes and no air came out.

I’m not sure what details folks need to help, so let me know what I can answer! Appreciate any help.

 

[Mike Pass]

Use mechanical means to move the slave until the clutch is free. Measure the distance that the slave has move to free the clutch. As you know the diameter of the slave cylinder you can calculate how much volume of fluid is needed to move the slave. This volume of fluid will have to be displaced by the master cylinder which will be the same volume. Knowing the volume of fluid needing to be moved and the available movement available from the pedal box you can calculate the required bore of the master cylinder needed to displace that volume of fluid.
Cheers
Mike

 

[Brian C]

Thanks Mike, I hear what you’re saying. I think that if I need more travel, I could either decrease the slave cylinder diameter or increase the master cylinder diameter, right?

I checked, and with my pedal setup, I’m probably getting just under an inch of master cylinder travel. I would think that if I used a 3/4” slave (same as the master), I’d get the same travel on the pushrod, right?

The slave rod goes to a lever which is connected to a splined shaft. what is the normal rotation needed on this box? 45 degrees? Or does it vary by clutch type?

is it possible that reclocking the lever might help?

thank you

 

[Julian]

The position of the lever on the spline shaft is somewhat critical, ideally positioned perpendicular to the slave at rest.
To setup, you should disconnect the push rod, feel for where the lever just starts to engage the forks on the throw out bearing, then separately measure full distance where the piston is backed into the slave cylinder fully (spring loaded so you'll have to overcome that resistance). Then set the push rod length such that the slave cylinder doesn't bottom and the lever has 1/4" free play prior to riding on the throw out. This should give you the maximum opportunity and ideally when depressed you want 40 thousands of disengagement between the friction plate and flywheel. If you cannot obtain consider a smaller bore (long throw) slave to help get there.

 

[Phil G]

PM Sent

 

[Brian C]

Thanks guys. Unfortunately I cannot see the flywheel to clutch through the bellhousing inspection holes or from the bottom of the car, so I’m riding on feel a bit.

I did get the car in the air, and with the trans in gear, when turning one rear wheel manually, the other wheel turns the opposite direction (LSD in action). I had my wife push the clutch in, and then only the one wheel spun with minimal effort. It spun as freely as in neutral gear lever position. So if I can trust that test, I should be getting disengagement…

Julian, I can follow about half of your steps mentally, just because I’m unfamiliar with some of the terminology. I will attempt your procedure next time I have a day to burn. Are you saying if I disconnect the lever from the slave rod, it should turn freely until it “hits something” inside (fork on the throw out), then set it there so my slave pushrod starts engaging at that point?

 

[Brian C]

Quick update: took the car for a drive. I found that by putting it into second (which it will go into easily), then quickly moving the lever into first or reverse, it will go in. So, at least I have a workaround for now. Is this maybe because first and reverse are not synchronized, so the gear teeth have to be perfect to engage? Wonder why it goes in so easily when the car is off…

 

[Walter Williams]

YOU INPUT SHAFT IS TURNING AT IDLE ?? CLUTCH IS NOT FULLY DISENGAGING ??? AT LEAST THAT WAS THE PROBLEM WITH MY 1966 CORVETTE . I AM NOT A MECHANIC .....SOME ONE CHIME IN PLEASW

 

[Bill Kearley]

Post # 6 suggests you have an open diff. The suggestion of quickly going into first and reverse while moving suggests a dragging clutch. put someone in the car with foot on the clutch and in gear, you, with some effort should be able to rotate the tires, in the same direction ! Make sure the pedal travel and volume are enough to disengage the clutch, other wise you have a linkage problem.

 

[JP Verweij]

On external slave you should see atleast 1"travel. With a common 3:1 ratio of the fork thats about 5/16th throw on the Clutch release bearing which is about minimum needed to disengage the clutch.
But it all depends on the fork ratio, if the ratio is less then 3:1 you'll get away with less travel of the slave as long as you see atleast 5/15 throw on the release bearing.
I have a .0813 master on a 0.875 slave..

 

[Brian C]

Post # 6 suggests you have an open diff. The suggestion of quickly going into first and reverse while moving suggests a dragging clutch. put someone in the car with foot on the clutch and in gear, you, with some effort should be able to rotate the tires, in the same direction ! Make sure the pedal travel and volume are enough to disengage the clutch, other wise you have a linkage problem.

Hmm, you’re right, wheels turning in opposite directions suggest an open diff. Is the diff in the ZF of a type where both wheels would spin the same direction if in gear and off the ground? I’ve read there are some LSD types that they’d spin opposite directions.

Here’s what happens: in gear, clutch in, only one wheel spins and it spins pretty freely. As the clutch is released while still manually turning the wheel, the other will “catch” and start turning as well, but in the opposite direction. I can feel the friction increasing as the other wheel starts to catch.

I will get a actual slave movement measurement a little later today when I can bother my wife to help push the clutch.
FYI I did look inside the bell-housing inspection hole and see some mechanical linear movement (this is behind the clutch to the trans side). It looks like it’s moving quite a bit, more than an inch, so maybe there is some good leverage inside with the fork? Ill try to get a video if my phone can focus.

thanks everyone!

 

[Brian C]

On external slave you should see atleast 1"travel. With a common 3:1 ratio of the fork thats about 5/16th throw on the Clutch release bearing which is about minimum needed to disengage the clutch.
But it all depends on the fork ratio, if the ratio is less then 3:1 you'll get away with less travel of the slave as long as you see atleast 5/15 throw on the release bearing.
I have a .0813 master on a 0.875 slave..

Thanks. I can see some movement via the inspection hole, will try to get a video. Not sure if it’s the throwout bearing or not though.

 

[Julian]

One other area to consider when purchasing components and bolting them together is indexing to ensure the ZF input shaft is crank center. It's not always easy for a home mechanic so often gets overlooked, but offset dowel pins are available. Also what type of crank pilot bushing are you using for the input shaft? A lot of the cheaper ones are bronze and not a true Oilite, after a bad personal experience years ago (the ZF came apart twice!) I stick with a needle pilot bearing now.

 

[Rune]

Does the gear moves freely ? There are a way to adjust the forks little if not. check this first. You can find how to adjust in the repair manual.