Crank Play

[airemdwn]

Just pulled a 350 4 bolt out of a 79 Blazer. Seemed like a strong running motor with no knocks or tapping. Now that its on the stand and getting cleaned up i pushed on the balancer to move it and noticed alot of endplay back and forth on the crank. almost 1/8 of an inch.
Is that normal for the 2 peice main blocks? What should i look for when i pull the pan off?

 

[76zimmer]

No, thats way outta line....should be .002-.006 per GM service manual 1971.

Sounds like major thrust wear from a clutch. Check the rear main bearing. It will be severely worn on the thrust surface.

 

[4X4HIGH]

That is BAD, the typical thrust bearing endplay is .005"-.008". With .125" of endplay i'll just about guarantee that the thrust surface of the crank is damaged as well as the bearing itself.

 

[diesel4me]

I've seen quite a few engines with excessive end play,that still ran great and made no unusual noises--it wasn'tdiscovered until the starter gave greif engauging the ring gear or disengaging,or during routine work on the engine..one 1963 T-bird with a 390 my friend owned had so much end play,I think the only thing holding the crank in alignment was the v-belts!--you could watch it slide in and out a good 3/8" while idling...eventually the oil pressure light started coming on while going around corners,and you'd hear a loud clunking rattle..weird thing is it never seized or blew up!..but in most cases too much end play is a bad omen and means you'll need a new crank & bearings..

When I worked at the parts stores with machine shops,we had a few of the motors we rebuilt come back rapping,and it was excessive end play--the engines were rebuilt to strict specs and it was correct when the engines left the shop--we found many guys were just pounding the harmonic balancers on with a 5 lb sledge instead of drawing it on with the right tool,that can bash the thrust bearing surfaces easily--they areonly soft babbit...also,the machinist said GM had service bulletins warning owners of motor homes and heavy duty pickups automatic trannies that a clooged cooler or restricted cooler lines can cause the converter to "balloon" and put tremendous end thrust loads on the crankshaft--the big blocks with TH400's did it most often...manual tranny's put more strain on the thrust bearing also,due to the throwout bearing pushing the crank inwards and putting a heavy load on the crank thrust bearing surfaces..I had a old Mustang a friend owned with a 6 banger & 3 speed tranny that wore the thrust bearing so bad it stalled when you pushed the clutch in!..and you could see the crank push out of the timing cover a good 1/4" and hear the timing gears scraping on the cover!....

 

[airemdwn]

The bearings look brand new and the bottom end is super clean BUT the crank thrust surface is wasted badly! it doesnt even touch the bearing thrust surface! I dont know what was holding the crank in place!
It almost looks like someone changed the bearings and just reused the crankshaft.

 

[76zimmer]

how do the side surfaces of the rod and main bearings look?

 

[airemdwn]

Now i need some help!
Turns out this is a 383 and not a 350. i dug around all the paperwork for it and found most of the specs, unfortunately nothing specific about the crankshaft itself. theres abount 22k miles on and its almost 5 years old.
I didnt notice b4 but the flexplate and balancer are externally balanced, but the paperwork for the motor has a balance record. I have never seen a balance sheet for an externally balanced motor,, my 383 is internal and came with an almost identical sheet with all the weights and bob weights.

Could this motor have been run with the wrong flexplate and balancer wiping out the thrust bearing?

I really want to fix it now, should i just buy a new internally balanced crank and change the bearings? Or will i need a trip to the machine shop to match the bob weight to the original balance record?

 

[Fordum]

Can you get any pics? Also, its a long shot, but what about numbers on the crank? Sometimes they are useful.

Right off the top of my head, I don't think an out of balance condition would tend to wipe out the thrust bearing.

Either way, with only about 22K, it sounds like something was done wrong. Any luck with the name of the shop that did the work?
They might have some records.

 

[4X4HIGH]

There are several things that can cause a thrust bearing failure. First is a "balloning" torque convertor if it's an auto trans, or a plugged tranny line or plugged cooler. If it is a manual trans then it could be from a misadjusted clutch fork or the use of a centerforce clutch or from someone riding the clutch pedal. The last thing that can cause this failure is a bad ground.

 

[Chevy305]

There are several things that can cause a thrust bearing failure. First is a "balloning" torque convertor if it's an auto trans, or a plugged tranny line or plugged cooler. If it is a manual trans then it could be from a misadjusted clutch fork or the use of a centerforce clutch or from someone riding the clutch pedal. The last thing that can cause this failure is a bad ground.

What makes centerforce clutches wear out thrust bearings? Is it because of how hard it holds requiring more pressure from the clutch fork?

 

[diesel4me]

Never heard of a centerforce clutch causing excessive end play,but I suppose its possible--maybe the added centrifical forces made by the weights on the pressure plate exert forces on the thrust bearing?...

I'm not sure,but I think you might be able to get a thrust bearing seperately that is "oversized" on the thrusr surface,but if the crank is worn or scored you'll have to get another crank,or have that one reworked,they might be able to build it up with weld and machine in back to spec..

 

[4X4HIGH]

What makes centerforce clutches wear out thrust bearings? Is it because of how hard it holds requiring more pressure from the clutch fork?

The weights clamp the disc harder as the engine spins faster which pushes the crank forward, it has nothing to do with the TO/bearing.

 

[Fordum]

The weights clamp the disc harder as the engine spins faster which pushes the crank forward, it has nothing to do with the TO/bearing.

Man, you folks are so far beyond me when it comes to GM stuff, I hate to comment.
But maybe you would like to rethink this one.

I have installed a couple of Centerforce clutches in the past, and as I remember, the whole assembly bolts to the flywheel.

The weights are mounted on pivot arms and tend to fly outward when it spins.
This causes the arms to pivot and increase pressure on the springs.

Since the whole thing is bolted to the flywheel, and the weights are acting radially, there is no increase in the axial load.

It would be the same to say that stronger springs put more load on the thrust bearing all the time the engine is running.

All the stronger springs do is pull out on the mounting bolts to press harder on the disk.
All the force is limited to the structure of the flywheel.

There might be a slight amount of loading when the clutch is disengaged, but the Centerforce design is designed to minimize that.

That is why their system is usually better than increased spring tension. The extra clamping only occurs when the engine is spinning faster than idle.

 

[4X4HIGH]

Just as suspected I knew someone would argue my point. For me it's like pissing up a tree when I give out first hand knowledge of problems i've ran across in the past. I don't know why I keep giving advice really.

 

[Fordum]

Nope, not saying you are wrong. When facts contradict theory, theory has to change.

Something some so-called climatologists should learn.

If you have seen it, then it happened. I just don't see the mechanism that causes it.
That is why I am so interested. I love learning new things.

On the surface of it, there is no way a force acting radially can cause an axial load unless it acts against something.

When the throwout bearing starts to press on the springs all that changes.
Now when the weights increase the spring load, they are acting against the bearing and causing an axial load.
The design of the Centerforce is supposed to prevent that, but may not.

If the shape of the weights cause a wind load like a fan, that could cause it too.
But that would be an incredibly stupid design since it would suck horsepower like a sponge.

If you have seen worn thrust bearings with no other obvious cause than a Centerforce clutch, then there has to be a way its doing it.
But its not due to a weight bolted to the flywheel pressing harder on that flywheel.
I wish it was.
I could use that to build a reactionless space drive that actually works.

 

[4X4HIGH]

Nope, not saying you are wrong. When facts contradict theory, theory has to change.

Something some so-called climatologists should learn.

If you have seen it, then it happened. I just don't see the mechanism that causes it.
That is why I am so interested. I love learning new things.

On the surface of it, there is no way a force acting radially can cause an axial load unless it acts against something.

When the throwout bearing starts to press on the springs all that changes.
Now when the weights increase the spring load, they are acting against the bearing and causing an axial load.
The design of the Centerforce is supposed to prevent that, but may not.

If the shape of the weights cause a wind load like a fan, that could cause it too.
But that would be an incredibly stupid design since it would suck horsepower like a sponge.

If you have seen worn thrust bearings with no other obvious cause than a Centerforce clutch, then there has to be a way its doing it.
But its not due to a weight bolted to the flywheel pressing harder on that flywheel.
I wish it was.
I could use that to build a reactionless space drive that actually works.

The weghts have NOTHING to do with the TO/bearing. In fact the TO/bearing should NEVER even touch the pressure plate until you depress the clutch pedal (if it is riding on the pressure plate then you will prematurely wear out the TO/bearing AND also cause the thrust bearing to wear). The centerforce weights move outward as the RPM's increase which then adds pressure to the disk (pushes harder against the disk) THIS is the force that destroys a thrust bearing.

 

[Fordum]

Hang on, I think I got it.
I see the mechanism now.
I figured you were right, it just did not make sense. Now I see it.

Either you are right for the wrong reason, or I am misreading your post. I can't tell which.

I am going to assume for now, you are misunderstanding the way the CF is wearing the thrust bearing. If I am wrong, I apologize.

But just for a minute consider this:
You are standing on a horizontal disk that is floating on a pool of water. Your feet are in straps that are bolted to the disk holding you down.
In front of you is a shaft that is also bolted to the disk sticking straight up.
On top of the shaft is a hinge with a bar hooked to it sticking out toward you.
Attached to that bar is another bar going straight down to the disk with a wooden block between the end of the shaft and the disk.

You grab the bar in front of you and start to pull down.
The wooden block begins to crush.
You pull so hard that you completely crush the block, and your feet would come off the disk if they were not strapped down.

Does the disk you are standing on settle any deeper in the water?

No, because you are not pushing the disk deeper, you are just increasing the pressure the lever, attached to the disk, is putting on the surface of the disk.

And in just the same way, a weight mounted on the flywheel, acting through a pivot mounted on the flywheel, clamping a disk harder to the same flywheel, cannot move that flywheel axially.

BUT, and here is the part I cannot tell from your writing, the key is that the weights DO NOT move axially.

Edit:I was in a hurry when I wrote this, and typed the wrong thing.
I would use strike-through, but its not supported. So I put the mistake in red.
I meant to say radially instead of axially.

They PIVOT.
In other words, they swing up away from the flywheel when the rpm increases.

THAT is an axial motion!!

For any action, there is an equal and opposite reaction. Which means, when the weights move out, the flywheel tries to move in.

The extra clamping force is irrelevant, it acts only on the surface of the flywheel. But, when you punch the throttle, the weights fly out and that pounds the thrust bearing.

Not a lot, of course, but it does it every time you increase the rpm. Plus, when you back off, the weights move in and the flywheel tries to move back.

All this could be prevented if the weights slid along an inclined plane instead of pivoting, but the did not ask me when they designed it.

Oh, and I knew that the TO/bearing does not touch. The only reason I brought it up, was to say that stronger springs would cause a heavier load on the thrust bearing when the clutch was disengaged.

Whether the extra strength was due to actually stronger springs, or increased clamping due to the weights.

I gotta run. If you reply, I'm not dodging you, got an appointment. I will check back later.

 

[Chevy305]

Hang on, I think I got it.
I see the mechanism now.
I figured you were right, it just did not make sense. Now I see it.

Either you are right for the wrong reason, or I am misreading your post. I can't tell which.

I am going to assume for now, you are misunderstanding the way the CF is wearing the thrust bearing. If I am wrong, I apologize.

But just for a minute consider this:
You are standing on a horizontal disk that is floating on a pool of water. Your feet are in straps that are bolted to the disk holding you down.
In front of you is a shaft that is also bolted to the disk sticking straight up.
On top of the shaft is a hinge with a bar hooked to it sticking out toward you.
Attached to that bar is another bar going straight down to the disk with a wooden block between the end of the shaft and the disk.

You grab the bar in front of you and start to pull down.
The wooden block begins to crush.
You pull so hard that you completely crush the block, and your feet would come off the disk if they were not strapped down.

Does the disk you are standing on settle any deeper in the water?

No, because you are not pushing the disk deeper, you are just increasing the pressure the lever, attached to the disk, is putting on the surface of the disk.

And in just the same way, a weight mounted on the flywheel, acting through a pivot mounted on the flywheel, clamping a disk harder to the same flywheel, cannot move that flywheel axially.

BUT, and here is the part I cannot tell from your writing, the key is that the weights DO NOT move axially.

They PIVOT.
In other words, they swing up away from the flywheel when the rpm increases.

THAT is an axial motion!!

Ok I see what you are saying, but when you think about it, the force of the weights is radial acting on the pivioting of the pressure plate acting axially on the clutch disk to hold it harder to the flywheel which is self contained between the pressure plate and the flywheel since they are bolted firmly together. But what might happen is that if the clutch fork is setup too tight (not enough excess play) when the weights move in the florks will also tend to move out and may contact the TO bearing and since its bottomed out the thrust has no where to go but act of the thrust bearing.

If my theory is correct, then maybe its a good thing that my clutch linkage is real sloppy so that the fork can travel backwards due to the outward force of the clutch fingers.

 

[4X4HIGH]

You're making this harder than it has to be. The weights on the centerforce pressure plate move outward which then redirects pressure to a forward motion to clamp the disk harder.

 

[Fordum]

Like 4X4HIGH said, if the throw-out bearing is touching the clutch fingers without your pushing on the pedal, you will burn out the bearing and possibly the clutch depending on how hard its touching.
I doubt that the fingers are going to move much. If they did, lots of people would have problems.

Check out the picture and notice how the weights move up and out.

And, reread my last post. I mistyped and have corrected it.