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Clutch slave blowout


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I’ve seen that this topic is associated with the wrong throwout housing... I’m just wondering if a longer slave rod could buy me some time? The odd thing is that I drove the car for about 60 miles and it was doing fine. Then I took it out to fine tune it (new build) and I felt the slave piston hang up... pumped it a couple times to see if it was going to cause me trouble and SPLAT! No clutch. Got home and replaced the slave boot, filled and blead the system and pump, pump, pump, SPLAT!

So I can see that in order for the slave rod to contact the fork, the piston in the slave is already very near the end of the cylinder. Also, the fork has a lot of room to move before the throwout firmly makes contact. So I know I had no idea what combination of clutch plate and throw out to use, quite frankly the numbers and years and sizes were hurting my simple mind. I’d try and measure height of the assembly and I was getting all F’d up. With no other option than a small and multiple med height throwout housings, I went with the med. Then....I found a tall throwout housing. Figures I’d find it after I put the engine in. !!!

So, is this classic throwout height issue?

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The slave piston only moves 1.18" so the push rod can only do the same.

The clutch arm on the outside also can only move through a precise range of motion or it hits the front or rear of the hole in the side of the transmission.

Likewise the inside end of the clutch arm with the release bearing can only travel through the same distance and position every time.

 

Clutches come in a variety of diaphragm heights that do not fit the fixed clutch arm motion. If the diaphragm height is small the clutch arm will run out of travel and the clutch will not fully disengage. To get around this the release bearing holder, or collar, varies in length. A longer collar allows the clutch arm to sit farther back. Just imagine you buy your child a tricycle but their feet do not reach the pedals. You can't lengthen the feet but you can get two blocks of 2x4 and bolt onto the pedal. Now their feed can reach. Same with the release collar.

 

Lengthening the push rod will force the clutch arm farther back and it may now bottom out against the transmission hole at full extension.

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Roger that. Is it hopefull that it all worked well prior to the demise? It shifted well, it seemed to feel like the pedal was working the clutch in the right range. The clutch action was near the top of the pedal travel, enough so that I might assemble it all again and make sure the master is returning all the way. The thing that concerns me is that the slave piston has to be so far near the end of the cylinder for the rod to make full contact with the fork and the fork to push the throwout into the PP... but it worked so well... for a while.

I guess I’ll dbl check the slack on the pedal to make sure the master is returning as you’ve stated before Mike.

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Check pedal free play same as the brake. If too tight the master may not be returning properly and pressure is trapped in the line to the slave. Next time it just keeps adding and adding till the slave pops.

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Yes there is a rubber seal around the piston between it and the cylinder wall of the slave.

 

 

The transmission holds 2 liters of GL4 gear oil. If low it's probably ok but it needs topping up. Better yet replaced, if you have no idea when it was changed last. Should be done every 30K. Destroying a transmission for lack of about $10 of oil isn't a good thing.

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Ok, working theory has changed. I melted the seal on my slave cylinder.

Everything worked well, the next day it all worked a little longer, but the seal was weakened/melted and gave up, I’m sure.

But in the back of my mind is this throwout housing concern. So, question: can there be functionality with the wrong throwout set up and if so, does the working pedal travel really give the missmatch away or is it one or more issues that speak of the problem?

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You can't melt the seal, you can only wear it out or tear it. If the seal was 'broken' the hydraulic fluid would just piss out. You said the piston popped out so there has to be pressure pushing it out, the release collar is too short or something in the clutch has broken allowing the clutch arm to travel too far.

 

How long has the slave, the clutch, the release bearing and collar been together and working? What's the history.

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The combination is all new and I drove 60-65 miles on it so far.

A grabbed a Centerforce PP and disc from a planed swap for my 73 Z that has a 5-speed now. I’m assuming I picked the year 79 for the date. I’m assuming that’s the year of the tranny I now have in my 620 too. All I have are assumptions to go by. The throwout options I had were the small and the medium when I put it together, then I found a tall, so I have one of those now to work with if needed. I went with the medium throwout. The fork came with the tranny.

This morning, when it began to fail, I felt a hang up in the clutch feel, kinda like the pedal hung up on the floor mat. As it turns out it was likely the piston almost coming out the end of the cylinder and hanging for a sec. I say that cuz a few shifts later, when it really let go, the piston was lodged 70% out the end of the cylinder and the PP was engaged all the way in. So I power shifted home and did my re-bleed. When I blead it the pressure never build up after closing off the bleader. So I pumped a number of times like it was a Ford. SPLAT! it did it again. Thats where I’m at. I know I took out 97.9% of all the slack in the pedal to master feel. I like a solid feel, maaybee I did it too much.

If the seal in the slave is likely good, cuz it’s new and heat may not have much effect on it, I might try to make sure the pedal has enough slack, and make sure the master returns properly. Then the question is, do I have the correct throwout again?

-I’ll be out for a while, I’ll check back in the AM

Thanks for the efforts to work through this Mike!

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Well again, thanks Mike for the efforts. It’s clear that the slave gave it up. Could have been worse, having not added fluids in the trans. I think the only reason the piston could have poped out and for there to have been enough pressure for it to get to that point, would be that the master wasn’t returning all the way. Really, I guess I need to just start back at square one and have a functional slave to work with, that’s due in on Tuesday. On the throwout concerns, I did notice that the fork was in the middle of the working range in the opening when all contact was made. The piston in the slave was well inside the cylinder as well. Enough, ideal? We’ll find out.

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In my experience, having the wrong collar causes all kinds of trouble. I did a zx 5spd swap without knowing about the collar height issues. I put the taller zx collar with the truck clutch, I tried to fix the issue by adjusting the slave rod as short as it would go. It caused fluid return problems and the clutch would start slipping, I would crack the bleeder and fluid would shoot out, then the clutch would work ok for a while. When I switched to the shorter collar, everything worked just fine. Sounds to me like you may need that tall collar if adjusting freeplay doesn't work.

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The throwout bearing collar needs to match the clutch. In most cases, a clutch kit will come with a throwout bearing collar. They may not be of Nissan quality, but will work if you don't have the right original one.

 

Another commonly overlooked feature is the spring on the slave. If the slave cylinder body has an attachment hole for a spring, then you have the manual adjusting slave and need the adjustable rod and spring. If there is no hole for a spring, then you need the solid rod and no spring.

 

The fork for the manual adjusting slave is different too. It has a hole in it for the spring and one for the rod.

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