Throwout Bearing Clearance question?
 

Apologies if this question has been answered or discussed in another thread. If so just direct me there.

I recently replaced my TOB, Clutch fork, Pressure Plate, etc.., and now I am wanting to ensure that my throwout bearing does not wear prematurely by contacting the pressure plate when it's not in use.

I understand that on other manual transmission vehicles with cable operated clutch systems it is very important to adjust the cable to make sure there is a small clearance/gap between the throwout bearing and the pressure plate. That way the bearing isn't spinning when it is not in use.

I tried wiggling the end of the clutch fork with the car put back together and it feels like the clutch slave cylinder is applying a constant pressure onto the fork which is pushing the TOB into the pressure plate fingers....

Is this by design? Is this okay? I guess I could go to a dealership and pop the hood on a new car and see if that slave cylinder / clutch fork feels the same.

Let me know your thoughts, any feedback is appreciated! Thanks

There's definitely some pre load on the clutch fork from the slave cylinder.

This is not a cable driven system or manual linkage. I don't think there is a spec to set a gap between tob and pressure plate. I have not read or seen anything for this in changing clutch.

If you go by the repair manual, I can't recall any specific call out for the TOB.
When I did my clutch job, I didn't think twice about it.

Hi ya, Kyle2 and welcome to the tribe - :clap:

The TOB on these cars is "active", in other words it is in contact with the pressure plate all the time and spins whenever the engine is running. It's just "working" (under load) when the clutch is pushed in.

So.... the bearing is always spinning, no matter whether the clutch is pressed in or not...??

Sent from a potato

Yep, on this car at least, that's what ol @Ultramaroon learnt me.

As long as the engine is running the TOB is spinning, however, it's not under load till you push in on the clutch.

Actually, that makes more sense than the way they used to be, where the TOB was slammed up against the spinning fingers of the pressure plate every time the clutch was pushed in.

Where you been, chipmunk, I haven't seen much of you lately - :iono:

Spending a lot of time with family and my new daughter during lockdown.... and spending more time on 2 wheels instead of 4.

Have an usual noise around 5-6k rpm,but goes away as soon as I press the clutch in even a little bit. I don't wanna hijack this thread. I detailed it more in another thread. Dealer says it TOB, but it doesn't make any intuitive sense to me.

Sent from a potato

Time with family - that's good - :thumbsup:

New daughter - even gooder - ;)

Even though the TOB is active, when pressure is applied it becomes under a load. It's my understanding that a bad TOB can make one noise not under load then a different noise when loaded.

OP, your observation is correct. The self-adjusting, hydraulic clutch operating system is puzzling to a lot of people who are used to a cable.

There's a thin spring inside the slave cylinder that takes up the slack in everything between the piston down to the pressure plate fingers.

But we know that, as the clutch wears, the fingers lift away from the flywheel. This pushes the piston further into the slave cylinder. Where does that hydraulic fluid go?

When everything is at rest, with the piston in the master cylinder fully extended, a port opens to the reservoir the same way it does in a brake master cylinder. The excess fluid escapes to the reservoir and everything comes to a gentle equilibrium. That's why, with all the pedal adjusting going on in this forum, I occasionally pop in to mention the importance of proper master cylinder adjustment. The piston must be allowed to fully extend when the pedal is released. Otherwise, the system "pumps up," leading to clutch slippage and premature TOB failure.

Thank you for warm welcome! I've been a long time reader of this forum. But I could not find the answer to my question anywhere! So had to make account. Thank you for explaining that, I didn't realize that "active TOB's" was an option, but it makes sense now. I'd give you a "thank you" but I haven't made 10 posts yet.

That^ is an answer to a question I hadn't even thought of. So in order for the clutch hydraulic system to self adjust as friction disc wears, the master cylinder needs to be able to fully extend to release fluid. So I just want to confirm the best way to check that your master cylinder is able to fully extend is to make sure that there is a little play in the clutch pedal in the released/not-pressed position.

You're welcome.

I'll defer to ol @Ultramaroon to address your further question.

:)

Ok, humfrz, I remember the first time you referred to it as an active TOB or clutch. I don't think that's a thing. I just call it a self adjusting clutch. My 240z wasn't self adjusting. There was no spring inside the clutch slave. The rod between it and the fork was adjustable. Still, I learned the hard way what happens when that port to the reservoir isn't allowed to open. Soon after I replaced my first clutch master cylinder the clutch began to slip. Actually, I think that was when I went to the dealership and bought my first factory service manual.

Anyhow, I digress. The whole self adjusting thing isn't new. Someone figured out that a free-wheeling TOB was no big deal and little spring inside the slave cylinder makes life a whole lot easier. Nissan started doing it around 75.

A bit of play at the top of pedal travel is expected but a functional test is educational. Grab the top of the fork and pull it forward. the pin should sink into the slave. It takes a while for it to slip back out on its own. Don't freak out.