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BBC Rear Main bearing damage mystery.

General engine tech -- Drag Racing to Circle Track

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Dave Koehler
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BBC Rear Main bearing damage mystery.

Post by Dave Koehler »

Anyone have a problem child that hurts the rear main bearing only? Not the thrust, just the bearing

pulling truck. 632 dart block, bryant crank, injected NA alky. .0038 rear main clearance., 3 disc clutch, drop down box. Peterson dry sump, 20w50 oil.
Things checked and triple checked: Main saddle size and alignment, cranks size, straightness w flange runout checked. Balance of clutch checked. bellhousing dialed in.

About 5 pulls in the rear main gives up. Repeats. Crank grabs the bearing and shows some heat. Thrust is a non issue.

Trying to help the man chase down this anomaly.
This combination should never have this problem as countless 632s out there attest.
I tend to lean for increasing the rear main clearance if all else fails.
We think that we and other shops involved have covered every possibility but...... Just looking for more "what ifs".
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Re: BBC Rear Main bearing damage mystery.

Post by miniv8 »

Where does the engine get the oil pressure from the dry sump pump?
Any chance the oil is directed in such a way that it streams past the rear main passage, or the plumbing fitting is partly covering the passage?
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Re: BBC Rear Main bearing damage mystery.

Post by bill jones »

---what has been done to seal off the wetsump cavity where the original oil pump bolted on?
---that surface and the bolt hole for a cover plate could be a potential serious oil leak
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Re: BBC Rear Main bearing damage mystery.

Post by Abbottracingheads »

Give the mains some more main bearing clearance. Go to at least .0045 on that rear. Truck pulling is murder on mains compared to drag racing. .0045 on all the main bearings is fine.
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Re: BBC Rear Main bearing damage mystery.

Post by mag2555 »

Is the crank a Billet or a forging?
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Re: BBC Rear Main bearing damage mystery.

Post by bentvalves »

what does it have for thrust clearance?

let's compare this with vertical clearance on the rear main.
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Re: BBC Rear Main bearing damage mystery.

Post by In-Tech »

Hi Dave,
In those applications I generally use at least an extra .0005 on the rear and at least .010" thrust to keep the oil flowing through that large bearing area. Also a groove in the upper towards the front to help the rear main seal from flooding. 8 counterweight cranks too.
Not necessarily like Marine endurance but it would seem those pullers do sit at a constant full throttle RPM a while and those cranks wind and unload in a cycle at the constant RPM like Marine. If using a split set, I use the thicker bearing in the upper. Hth's
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Re: BBC Rear Main bearing damage mystery.

Post by golden_commando »

+1 on bigger clearance on the rear main cap
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Re: BBC Rear Main bearing damage mystery.

Post by Dave Koehler »

He got back to me with the following:

Bryant center counterweighted crank.

Rear main cap is solid with no provisions for an oil pump.  Oil is fed through a fitting in the side of the block where the oil filter would be on a stock block. This block has no oil filter pad cast on it. We use an inline filter. I verified all passages are open. Thrust is .0085 we normally run .007 to .010 without issue. The thrust bearing has been grooved much in the same manner you describe and we have used the drilled hole approach in the past however not on this one. All mains are.0038 pretty much in line with past engines we have built. Might look at .004-.0045 if this problem persists.   I also made sure there was adequate clearance between the release bearing and the fingers on the clutch. .625 is minimum recommend and mine is .675   I’ll know more after this weekend if it’s fixed.   Thanks for the info
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Re: BBC Rear Main bearing damage mystery.

Post by swampbuggy »

Dave, it sounds like the Crank now in use is ( 6 ) counterweights ?? Mark H.
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Re: BBC Rear Main bearing damage mystery.

Post by Dave Koehler »

swampbuggy wrote: Fri Aug 05, 2022 8:39 pm Dave, it sounds like the Crank now in use is ( 6 ) counterweights ?? Mark H.
eight
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Re: BBC Rear Main bearing damage mystery.

Post by Straight8 »

I’ve saw this issue before on a light weight crank with a rather large counter bore in that rear main. It was distorting under load and did exactly the same thing. A new Callie’s crank fixed it.
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