Clutch slave pre-engaged? Huh?

[moroza]

I just installed the trans to the engine, then the (brand new) clutch slave to that. Its rod came fully extended and had some resistance going in, but apparently it's enough to disengage the clutch - with it installed, I can put the trans into gear, turn the output yoke, and hear the (also brand new) clutch moving across the flywheel and pressureplate. The hydraulics aren't connected yet. Is this normal and will it go away once there's fluid in the system?

[h2oyo]

I dont remember that happening when I installed my new slave cylinder. In fact I dont remember there being and pressure at all until I removed the transmission to upgrade to a 6 speed. Then the slave cylinder had pressure when I was trying to get the bolts started. I think I would take it back off and make sure the slave moves and that the piston isnt stuck or something.
Johan

[strayts]

wait.. why did you install the slave on the trans without connecting and bleeding the hydraulics? (i saw in your other thread you don't have the brake/clutch hydraulic lines installed yet)

[moroza]

Hm.

I noticed it had resistance to being collapsed, like there was a return spring inside or something. It's definitely not stuck, though. It took a fair bit of effort to push it into the trans, and I decided it'd be much easier to do while everything was out than when it was in the car.

If it's not supposed to have any resistance at all without hydraulics attached, then something's not right.

[strayts]

is the bleeder screw closed? if so, of course there's pressure!

[attack eagle]

strayts: not with the hydraulic line off nd nothing but air in it there isn't!

[moroza]

It feels like there's *air* pressure in there. There's a short metal line attached to it that doesn't go anywhere else (once assembled, it goes to the rest of the pressure line). I'll check to see if it's clogged or something...

[h2oyo]

That is what I was thinking, nothing connected to it there should be no pressure. If there is and its enough to disengage the clutch that would seem to be a lot of pressure to me.
Johan

[strayts]

hmm yeah, does the line coming out of the cylinder have a cap or anything on it?

[moroza]

Welp, I just removed the clutch slave, and its bleeder screw and feed line. Still acts springy, like there's an air pocket trapped in there. Makes a slight slurping-kinda noise when pushed in.

[paanta]

I did the slave a couple months ago. It had quite a bit bouncy feeling resistance going in, even with no fluid and with it detached.

[TouringDan]

bench-bleed it first... that'll help a lot with the end-bleed.

[moroza]

Originally Posted by TouringDan

bench-bleed it first... that'll help a lot with the end-bleed.

How's that work?

[ross1]

No way is the pressure of a return spring disengaging the clutch. It takes a LOT of pressure to do so. The pressure plates generate thousands of pounds of clamping force and even with the mechanical advantage provided by the diaphram design it still requires several hundred pounds of force to disengage the clutch.
You aren't hearing the clutch disc but probably just the TO bearing.

[moroza]

Well, with the slave installed, I can put the trans in gear and turn the output shaft quite easily, and hear something moving... this is a brand-new 280mm 850Ci clutch so I'm not convinced my one hand can overpower it... Without the slave it's locked up just fine as far as I can tell.

[ross1]

Originally Posted by moroza

How's that work?

You don't bench bleed a slave cylinder, this is sometimes used for masters.
To gravity bleed it just fill the resevoir, open the bleed screw and let it drip. Once the flow is continuous without air bubbles close the bleeder. Simple and effective but requires a few minutes.

Originally Posted by moroza

Well, with the slave installed, I can put the trans in gear and turn the output shaft quite easily, and hear something moving... this is a brand-new 280mm 850Ci clutch so I'm not convinced my one hand can overpower it... Without the slave it's locked up just fine as far as I can tell.

If installing the slave requires superhuman effort to get it into place perhaps this is the case. You tightened the pressure plate bolts didn't you?

[BigM62]

I know alittle about clutch slaves, but even more about bitch slaves. Sorry, I could not help myself.

[moroza]

Used: trans, engine, flywheel, clutch release arm.
Brand new: flywheel bolts, friction disk, pressureplate, pressureplate bolts tightened to spec (on the high side if anything), throwoutbearing, pilotbearing, clutch pivot pin, clutch slave (850Ci), trans mounting bolts.

It took a good shove to get it just far enough onto the mounting studs to put a nut on. Then it gave roughly 20ft-lb resistance as I tightened the nuts to get the slave in.

So...are brand-new slaves supposed to be springy until filled and bled? (too many entendres here...)

Originally Posted by BigM62

I know alittle about clutch slaves, but even more about bitch slaves. Sorry, I could not help myself.

You poop on my thread. Give me a ride in your car sometime and we'll call it even.

[Binjammin]

Originally Posted by moroza

Hm.

I noticed it had resistance to being collapsed, like there was a return spring inside or something. It's definitely not stuck, though. It took a fair bit of effort to push it into the trans, and I decided it'd be much easier to do while everything was out than when it was in the car.

If it's not supposed to have any resistance at all without hydraulics attached, then something's not right.

The slave has a spring in it. You're experiencing normal behavior. The pin is normally fully extended when you buy it. When you install it, you push the pin against the clutch release fork, and it pushes the pin into the slave cylinder's housing. It's only a few pounds of pressure, and is not enough to actuate the clutch. When you bleed it and put pressure on it again, it will work as normal.

Enjoy your normal slave cylinder.

[moroza]

That helps, but why would I see a substantial force on the pin with nothing but air in the cylinder (and all screws/lines disconnected)? The clutch is definitely being disengaged by the slave alone. When it's all connected, does the fluid create a vacuum when the master disengages?

[moroza]

Problem found.

Turns out that the E31 slave cylinder has to be modified to use in the 420G trans - the pin has to be shortened. I did not know that when I was putting all this together, but that's what a call to UUC (whose flywheel I'm using, hence the E31 clutch kit and slave) confirmed.

Well, sheet.

I got my E31 slave for $110. UUC's modifed one is $325 and they can't/won't tell me how much it has to be shortened. Buying it seems absurd.

Do I:

1. shorten the pin myself, either at the business end or at the mounting end (will practice on another slave first)? Or
2. give it to a machine shop to do nicely? Or
3. stack a washer or two at the slave cylinder mounting studs, with an O-ring to keep dust out?

Underlying question to all of this is - how precise does this have to be? Keeping it too long could burn out the clutch by having it slightly disengaged all the time. How bad would it be to have it slightly too short? Is a slave cylinder's travel just enough for proper clutch disengagement, or is there a decent margin for overshooting it?