BBC Rear Main bearing damage mystery.

[BILL-C]

I have run into similar problems. The issue was that the crank fillet radius was very large and the edge of the bearing was contacting the edge of the radius the same time as the thrust flange of bearing touched thrust flange of crank. The hot oil could only escape in one direction causing bearing to overheat on one side of the wide #5 and start the destruction. We finally found issue by doing just 1 run and pulling bearing before severe damage would hide the initial witness marks. The solution was to chamfer the bearings much more. This has been a problem on rod bearings also. Crank people are making radii bigger than ever and bearing mfg are not keeping up. We almost always have to chamfer the bearings a little extra on good cranks.

[Dave Koehler]

Bill
You may be on to something there. He sent me a photo. 1 pull. This is the first time I have seen this.

mainbearing.jpg

[BILL-C]

Bingo!

[BILL-C]

It's safer to force feed more oil to thrust flange cavity at the parting line instead of the middle of bearing so you don't lose any load bearing area.

[1972ho]

Just curious was the groove in the lower bearing done by the bearings manufacturer are builder and what was the purpose of it.

[blykins]

Just curious was the groove in the lower bearing done by the bearings manufacturer are builder and what was the purpose of it.

We do that to help oil the thrust bearing. Truck pullers ride the clutch awfully hard sometimes.....

[Dave Koehler]

About that groove.
I have done that. I also have drilled through the thrust directly to the oil channel for the convertor balloon reason.
This current issue has me re-wondering about this "fix".
The groove method dumps out to the radius and we assume it gets to the thrust. In normal operation that is logical.
Bills comment about doing the same at the parting line also makes some sense and it would be easier to do with just a touch of a file.
The bearing is thinner there and has clearance that should never get touched.
I have to go look but some thrusts are relieved at the parting line to allow oil escape.
My question now is:
Is 60-100 psi enough to overcome the forces of a clutch or ballooning convertor?

Note that this racer is not killing the thrust, just the bearing. I think Bill's lack of oil escape makes the most sense.
He has checked and measured the bejesus out of this and is convinced at the moment there is clearance.
Since this leaves us with a "believe your lying eyes" situation.........
I have advised him to get the extra clearance bearings and replace the rear main.

Riding the clutch:
This is a centrifugal Crower style 3 disc clutch and he is not killing the thrust. I don't think "riding" it comes into play does it?

The TF folks killed the rear bearing that does not have a thrust and maybe still do. As I recall after trying all kinds of things including complicated outboard support they opted for crazy loose rear bearing clearance. Fortunately they have 70w and the opportunity to check it every pass.

The maddening thing here is that he has built this exact combo for others with no such issue. It's just this particular (his own) engine causing this grief.

[allencr267]

Also a groove in the upper towards the front to help the rear main seal from flooding.

He got back to me with the following:
The thrust bearing has been grooved much in the same manner you describe...

I take it that groove is at the front, the side that's been kissed hard. Journal filet on a main & thrust is pretty far away from the bearing surface's edge, not anywhere close like a rod or the mains without the thrust.

[Warp Speed]

A groove that large is a bad idea!
Big leak there, that is where I would start!

[Dan Timberlake]

I wonder what a full length strip of .001-.003" Plasti-gage would show on that main bearing.

I'm kind of expecting it to show real tight at the edges where the bearing is shiny/wiping.

If so, the trick will be determining if the crank or block is machined badly. Or both half badly.

[roadrunner]

I wonder what a full length strip of .001-.003" Plasti-gage would show on that main bearing.

I'm kind of expecting it to show real tight at the edges where the bearing is shiny/wiping.

Maybe the clearance at the edge of the bearings could change with the force of the engaged clutch deforming the housing/block.

[xanadu]

Is there any possibility of correlating timing points on the crank under load, front and rear / could crank twist be a possibility causing the edge of the bearing to show signs of wear like that?

[Dave Koehler]

I proposed in the beginning of this interesting saga that perhaps the trans is tied down too hard and the twisting and bouncing of the truck was the putting things in a bind. At the moment he has one eye on it but doesn't think that is the issue.
In lieu of sensors it would be neat to hang a couple of goPros under there.

I don't think he has a sensor on the output shaft so we don't know when the centrifugal clutch begins to engage and when it's more or less 1:1.

Bill's radius / oil flow restriction idea has the most merit at the moment.

I will pass along his next move but we won't know anything for a few weeks when the next pull occurs.

He is engine dyno equipped and it is not happening under load there.

I wonder if a truck like this can be tied to a a chassis dyno.

[Dan Timberlake]

Do pulling trucks rigid mount the engine trans (transfer case) combo?

"Maybe the clearance at the edge of the bearings could change with the force of the engaged clutch deforming the housing/block."

If the The static Plastigage clearance check shows .001 (or worse!) tighter and barrel/bell shaped at the edges it ain't likely it will get better in the heat of battle.

As suggested by others, simply adding an extra .001 or .002" clearance on that bearong might perform miracles.

[BILL-C]

Notice how the witness marks on upper shell go right to parting line. That is classic insufficient clearance. Either shells have taper, housing bores are tapered, or that crank radius is reaching over farther than people want to believe. A true .0038 should have been enough clearance, but something doesn't measure up here. Unless crank is under spec, it usually takes an X shell to get to .0038 clearance. If that really was a std H set of bearings then some X shells will definately help. If worried about being too loose then get them coated and they will conform to your components.