Throw Out Bearing Question

[1968C20]

Hello,

Was looking for some help I was having with a throw out bearing. I have a 1968 C20 that I am restoring. This is my first project so things are going along generally well, but of course with a few mistakes along the way. The drivetrain is a 383 with a 4 spd. stock tranny GM4 3901131. The 383 is a rebuild from a 1971 donor C20. Unfortunately when I put the motor in, I used the wrong throw out bearing- the bearing was too long and the clutch didn't work properly. With the pedal in, it seemed like everything was still spinning, the gears would just grind when I tried to engage it into first gear. I used the same size throw out bearing as the one from the donor truck. Thought that was the right thing to do, but come to find out, I guess I should have checked the bell housings as well as probably some other things....live and learn- the hard/exensive way
At any rate, the crank now has about 18 thousandths of end play. Took it back to the guy who did the machine work and the bottom end of the motor and he said the thrust bearing had about 10-12 thousandths worn off of it. Now for the question. Could this longer bearing have put extra pressure on the crank therefore causing that thrust bearing to wear? I can't quite wrap my brain around whether a too long throw out bearing would put more or less forward pressure on the crank. Thanks for your help!

[haysonj]

I would find that hard to believe because if it was exerting that mush pressure the clutch would be partially disengaged and it would slip so bad you couldn't drive it. The longer throwout bearing should make the clutch disengage up higher in the pedal not lower so I don't know why the gears were grinding unless it wasn't adjusted right.

[1968C20]

Yeah, that's the part I can't understand either. No doubt about it, I am the one who put the wrong throw out bearing in, but I just don't quite see how that would put more forward pressure on the crank. However, the gears were grinding like things were "jammed" together and the clutch couldn't disengage. I know I'm missing something here, but just can't seem to put my finger on it.

[raycow]

Just to make sure no clues have been missed, what did the following come out of?

1. Transmission
2. Bellhousing
3. Flywheel
4. Clutch

Engine - I know you said it came out of a 1971 C20, but you also said it was a 383. So do you know the actual origin of the engine?

Can too long a throwout bearing wipe out the crankshaft thrust bearing? Yes, but it would have to be so long that it is jammed up against the front face of the transmission bearing retainer. As already pointed out, this would cause the clutch to slip continuously.

Ray

[1968C20]

1. Transmission- The transmission is the one that came in my truck- (recipient). The # is GM4 3901131. Not sure if this was the original tranny or not, but it was the one in the truck when I bought it.
2. Bellhousing- The Bellhousing was from the donor truck- 1971 C20, the truck where I got the engine I am currently dealing with.
3. Flywheel- Same donor truck- 1971 C20. The flywheel was machined as well.
4. Clutch- The clutch and pressure plate were given to me by the previous owner of the donor truck. He had purchased them and then decided to junk the truck rather than do any further repairs.
5. Engine - The engine was from the 1971 C20. I took it to a engine shop and and had it stroked out to a 383. Eagle rotating assembly.

The engine I used from the donor truck was paired up with an identical 4 speed manual transmission.

Could the clutch have been frozen before I started the motor? Had a faulty electronic ignition, so it was a few weekends before it was started. Would that explain the failure to disengage even with the too long throw out bearing?...Really puzzled- thank you for your help.


Joel

[raycow]

Thanks for your answers. It was a longshot, but I was looking for a possible bellhousing/transmission mismatch - wrong center hole size. This could cause a misalignment problem that would feel like a dragging clutch, but from what you said, you don't have it.

There is one other potential problem you might want to check on though. Unless your 3.75" crank is "internally balanced", it requires a 400 flywheel. I can't see how this would have anything to do with your clutch issues, but the imbalance could cause noticeable vibration.

Ray

[haysonj]

Originally Posted by 1968C20

Yeah, that's the part I can't understand either. No doubt about it, I am the one who put the wrong throw out bearing in, but I just don't quite see how that would put more forward pressure on the crank. However, the gears were grinding like things were "jammed" together and the clutch couldn't disengage. I know I'm missing something here, but just can't seem to put my finger on it.

Is it possible that the clutch disc is in backward, I did that once back when I was a kid but if I remember right the clutch wouldn't disengage at all. If the thrust bearing is not completely gone you can just get a new rear main bearing and replace it.

[1968C20]

Originally Posted by haysonj

Is it possible that the clutch disc is in backward, I did that once back when I was a kid but if I remember right the clutch wouldn't disengage at all. If the thrust bearing is not completely gone you can just get a new rear main bearing and replace it.

Trying to figure out this reply stuff-

I ended up taking it to a knowlegeable old chevy guy once I got it started and couldn't get it in gear. Figured I had reached my limits in my gravel driveway The only mistake he could find was the too long throw out bearing. He made no mention of any other problems. Once he put in the new bearing, everything worked fine.

Joel

[faribran]

my most current experience with excess thrust bearing clearance...... the thrust bearing may have been worn some.. but the throws of the crank did most of the wearing...
i know this doesnt address the CAUSE here... but just food for thought.

[1968C20]

Originally Posted by faribran

my most current experience with excess thrust bearing clearance...... the thrust bearing may have been worn some.. but the throws of the crank did most of the wearing...
i know this doesnt address the CAUSE here... but just food for thought.

According to the machinest, the crank was fine. The bearing wasn't completely gone, but worn enough for .018 endplay.

[1968C20]

Let me clarify my last statement- when I said everything worked fine, I mean the engine ran, and the clutch/tranny worked as they should. The crank still had the .018 endplay after only running for a total of 2 hours tops. Just whatever time it took to set the timing, carb, break in the cam etc.

[haysonj]

Originally Posted by 1968C20

Let me clarify my last statement- when I said everything worked fine, I mean the engine ran, and the clutch/tranny worked as they should. The crank still had the .018 endplay after only running for a total of 2 hours tops. Just whatever time it took to set the timing, carb, break in the cam etc.

It is possible the piece on the crank that rides in the thrust bearing was out of round but I would have thought it would have been caught during assembly. Also there was a GM service bulletin about champers on the bearing so more oil would get to the thrust bearing. I have looked in my garage but can't seem to find it.

[1968C20]

Another piece of information- the side of the thrust bearing that was worn was the side facing the the rear of the motor. That seems to make sense with pressure coming from the tranny side.

Just can't shake the idea that the throw out bearing had something to do with it-I remember the throw out bearings being quite a bit different in length. Is Ray on to something with the t.o. bearing being so long that it had everything mashed together?

[1968C20]

Thanks for checking. Trying to get to the bottom of this to avoid it happening again. Also, if it was the throw out bearing, this comes out of my pocket. If it was an assembly issue, then the machinest bears the responsibility. I have no problem paying if it was indeed my fault, but don't want to dish out the cash if there is no possible way the throw out bearing could have done the damage.