Throwout bearing issues?

[Mike F.]

I have a 70 L-46/M-21 that has been eating throwout bearings every 2k miles or so.

Bought the car from the previous owner, drove it about 8k-10k miles with no issues. Decided to pull the engine for rebuild, with engine and trans. out I discovered numerous cracks in the interior of the original bell housing. Sourced another original used OEM bell housing.

Installed rebuilt engine, new Luk clutch kit, new "used" bell housing and trans.. (The Luk clutch kit uses a throwout bearing that looks like it has a Kevlar (or non-metal) collar on it.) About 2k miles later, the throwout bearing starts making noise. I pull the trans apart and find the throwout bearing has failed. The collar was fine, the bearing/race part had failed.

Go to the local NAPA and get a new throwout bearing, this time an all metal one that looks like it should last 100 years. Get the car back together, 2k miles later the throwout bearing is making noise again.

The clutch is adjusted per the AIM, the clutch pedal has 1 1/2" or so free play at the top of the pedal. The adjustment on the clutch rod are within specs as well. The noise starts as you depress the clutch to engage, so I presume the bearing is off the pressure plate with clutch pedal out.

I'm tired of changing throwout bearings. What's the fix?

Pics are for explanation purposes only..........................................
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[Ed S.]

Re: Throwout bearing issues?

My gut reaction guess is that something is out of alignment or out of balance. When the parts that are designed to rotate are doing their thing they are transferring excessive pressure or forces on adjacent stationary components like the bell housing.

[Terry D.]

Re: Throwout bearing issues?

Mike
So how many throw out bearings have you gone through? If it is only one since the rebuild, maybe it was just a bad part. If the problem started after the rebuild then I would start looking at parts you replaced.
Did you make sure the new bell housing was centered? Just because it was an OEM doesn't mean it was not abused and out of whack.They make offset dowels for this very problem. Is your clutch linkage adjusted correctly? Here is a link that tells how to align a bell housing. http://www.catalograck.com/ImgVD/LAK...llhousings.pdf
Keep us informed of your progress.

[Duke W.]

Re: Throwout bearing issues?

Have you inspected the clutch fork and verified that it is an undamaged correct OE equivalent part?

Also, it's common to install the bearing incorrectly on the fork... difficult to explain, but you just slide it on... don't pry the leaf spring retainers up to install it. Some years ago, John Hinckley posted pictures showing incorrect and correct installation of the bearing on the fork.

It's definitely worth an archives search.

Duke

[Stephen L.]

Re: Throwout bearing issues?

Mike, attached are photos of a correct and INCORRECT throw out bearing installation on the clutch arm.....Hope this helps.

[Mark E.]

Re: Throwout bearing issues?

Mike,

Assuming the T/O bearing is installed properly, a likely cause is poor bell housing alignment, especially since the bell housing was replaced.

This is straight forward to check and adjust once the tranny is pulled, assuming you invest in (or borrow) a special dial gauge designed for this purpose.

[Timothy B.]

Re: Throwout bearing issues?

Sometimes the clutch fork will wear at the sides that contact the T/O bearing, when this happens the fork will push on the outside spinning part of the bearing causing noise and damage. Look closely at the fork for wear and I would also check the alignment as mentioned.

[Mike F.]

Re: Throwout bearing issues?

Thanks for the reply's. It'll be a few weeks until I get to it.

[Terry D.]

Re: Throwout bearing issues?

Mike
The only tool you need to align the bellhousing is a dial indicator with a magnetic base. Again if you read the article it is a pretty simple procedure.
If you have any questions feel free to contact me.

Terry

[Joe R.]

Re: Throwout bearing issues?

Mike,

The throwout bearing is only made to ride on the fingers of the pressure plate when the pedal is depressed. Your clutch linkage is not adjusted properly and the throwout bearing is turning all the time your running the motor.
It will wear out quickly if not adjusted properly. Read the service manual and adjust the clutch properly and ANY throwout bearing will work for 10 of thousands of miles.

JR

[Richard G.]

Re: Throwout bearing issues?

I'm not sure I agree with the issue being a bell-housing alignment issue. Although I agree it would be prudent to check it.
The bearing rides on the nose of the transmission and any alignment issues would likely show up as fretting corrosion on the fingers of the clutch or the face of the bearing. I would recommend cutting the bearing apart using a thin cut-off wheel to determine what type of failure occurred inside the bearing. Post pictures of the throw-out bearing before you cut it and the bearing internals after its cut. A lot can be ascertained from this style inquiry. Tim's idea of looking clutch fork wear is a good one. Although any uneven wear may just cock the fork on the pivot ball and still push correctly on both sides of the bearing. I like Duke's idea also, verified that it is an undamaged correct OE equivalent part. If you find nothing it is possible you have a case of bearing infant mortality, although two in a row is unlikely. Don't forget to inspect the bronze pilot bearing for excessive clearance. I have even seen them assembled without the pilot bearing installed! Of course, the most likely cause is the bearing is loaded all the time and the bearing is not designed for 100% duty cycle. You mentioned there was clearance in the pedal. Confirm the springs are unloading the bearing like they should. Sometimes we look too deep for the obvious issues.
Rick

[Joe L.]

Re: Throwout bearing issues?

Mike,

The throwout bearing is only made to ride on the fingers of the pressure plate when the pedal is depressed. Your clutch linkage is not adjusted properly and the throwout bearing is turning all the time your running the motor.
It will wear out quickly if not adjusted properly. Read the service manual and adjust the clutch properly and ANY throwout bearing will work for 10 of thousands of miles.

JR

JR and Mike-----

I agree. However, what I don't understand is the 1-1/2" of free play that Mike reported as having. With that much free play the release bearing should not be in continuous rotation with the clutch engaged (i.e. normal running, non-shifting condition).

The problem that I've always found with Corvette clutches is that with the specified free play (1-1/2"), I could never get full clutch disengagement with the pedal at full deflection. Consequently, hard shifting and "gear clash" would occur. I've always found that the best I could do to achieve full disengagement at full pedal deflection was to have "a little" free play with the clutch engaged. As a matter of fact, this problem is why I do not recommend machined flywheels or any non-stock type clutch.

[Richard M.]

Re: Throwout bearing issues?

For reference this is what I disassembled a few years ago on a running driving '66. The car had sidepipes so it likely was never obvious.. Incorrectly mounted bearing in the fork. It was a failure waiting to happen.