Odd main bearing marks

[Jim Comet]

I took the bottom end of my 390 apart tonight and noticed odd "chatter" marks on the main bearings. The rod bearings all looked great. This is a 390 with main studs and had the mains aligned honed on the last build. This block does not have the extra webbing in the mains. I am adding a set of Blair's pro ports and a roller cam and this wear pattern has me wondering if I may need to go with a different block with the extra webbing? Any thoughts or comments are appreciated. Thanks, Jim

[CaptCobrajet]

Hey Jim, if I am seeing that right, the center main isn't marked that way.......if that is true, check the crank for straightness.  Also, sometimes journals are not round.  Lastly, check the balance.  My thought is that one of those three things could be wrong.  Just things that come to mind.....

[Barry_R]

Broach bounce in the manufacturing process.

Worn bearing in the machinery leaves artifacts in the parts so small you cannot measure them with normal handheld shop equipment.  But they show up as polished striations in the used parts,  Somebody at the plant should be on top of that...

FWIW I am not a fan of that big ol' hole in the middle of the load bearing area on those bearings.

[CaptCobrajet]

That's interesting.  I have seen those same kinds of marks in crankshafts when super-polishing the journals.  I could see how worn bearings in a crank grinder might do the same thing to a crank journal.  Worn spindle bearings in a milling machine do that also.  I'd say that is a good call, and interesting.  The stuff that makes stuff has to be in good shape......

[Jim Comet]

More info and opinions wanted. All the rod bearings looked very good. This engine always ran 5W20 Mobil 1 after initial break in. It used a high volume, high pressure pump. The main saddle oil feeds have been enlarged to 5/16. The oil pressure was always good. So with does everyone think about using this block for a 500hp build? I have another block with the extra web supports that is standard bore I could have machined. I just hate to put this block to pasture as it has align honed mains (maybe faulty), zero decked parallel to the mains, torque plate honed for my pistons and the trans mounting face was trued to the crank as well. Thanks, Jim

[blykins]

I'm not too keen on the main feeds being drilled out, that's a no-no in my book, but otherwise, I don't see any reason why you couldn't reuse the block, especially since it's got a lot of the good machine work completed.

[machoneman]

More info and opinions wanted. All the rod bearings looked very good. This engine always ran 5W20 Mobil 1 after initial break in. It used a high volume, high pressure pump. The main saddle oil feeds have been enlarged to 5/16. The oil pressure was always good. So with does everyone think about using this block for a 500hp build? I have another block with the extra web supports that is standard bore I could have machined. I just hate to put this block to pasture as it has align honed mains (maybe faulty), zero decked parallel to the mains, torque plate honed for my pistons and the trans mounting face was trued to the crank as well. Thanks, Jim

It sounds like your real question here, beyond all the rest is simply: will this or another block support 500hp? Sure as many here have proven over the years.

Btw, I agree with the diagnosis on those odd main bearing chatter marks (bad tooling). Bet merely slapping in new bearings is the fix.     

[CaptCobrajet]

I think that block would be fine to 500 hp.  I think that cam and your new heads are going to send you closer to 600 than 500.  I did a 390 with like that with the last version of the Street Pro Ports that made 580 with a worked Street Dom on it.  Your heads are 15 to 20 cfm better than those at most lift points that matter. The three webs would help, but the 3.78 stroke will not be as hard on the mains as a long stroker would.  I have seen those marks in the bearings like that on cross-bolted 427s, so I don't think that situation is caused by the block giving up.  If the parting lines are not showing metal transfer from cap walk, your clamping was sufficient.  The extra web is better for the long term, no doubt.

[rockhouse66]

I am taking a risk posting after Blair and Barry but doesn't the application also play into this?  For example, 4-speed car racing on slicks will test the integrity of this block more than an automatic on the street.  I have had 428 blocks cracked through that main bearing oil feed passage which was thought to be caused by hard launches.

[Jim Comet]

I am hoping that me being a foot brake bracket racer will allow this setup to live until I can step up to a better short block. Jim

[Jim Comet]

Any thought on these main caps. Does it look like there was some metal transfer on the face around the stud hole?

[Barry_R]

Yep
Caps are fretting - moving around.
More clamp load or dowels might help some

[machoneman]

That is a lot of fretting. Just how much power did the engine have?

Between the main bearings and the cap fretting, I wonder if you've encounter severe detonation. How do the rings, piston tops and ring lands look?

 

[Jim Comet]

I'm guessing 400-450hp as my 3800lb car (w/me in it) ran 110-112 through the traps. I used ARP main studs. The rod bearings looked great and I cannot see any piston/ring issues. Is there anything that will help a regular 390 no cross bolted block live? Should I start over with a block with the extra webbing? Does that help stop the cap movement? Jim

[CaptCobrajet]

I'd be curious how much register the caps have.  Do they move around if the bolts are loose?  Do you have to tap them into place?  Can you pull them out of their register easily by hand?  Detonation is a likely cause of what you have, but loose register can also do that.

A three web block with loose register could still suffer from movement at the parting line.   Crossbolts would also be something to consider if the rest of the block is cherry.  We sometimes machine a key slot and put 3/16 key stock in the registers and machine for .002 to .003 press if they come in "loose".