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Discussion Starter #1
Im tearing down this 406 and found All the bearings so far look great but for 2.
The front main and the rear main. The motor spins very nice by hand so I dont think it is bent. This is a 2 bolt with ARP bolts and a cast crank.

Here are the bottoms which look more of a bronzy color than ive seen before. But dont seem burnt.


Here is the rear main off the cap. Notice the Blue ish hue and copper on a weird part And BOTH thrush surfaces are copper.


Here is the front main (cap side)
All looks good but for the copper near the keyed section


As I said all others looks good besides these two. Im usually good at reading bearing but Ive not seen these marks before.
The car sat for 3 years after blowing up the third trans he had bolted to it. But he did fire it up everry few months.
2 things I found is he had the converter shimmed away from the flywheel and tight to the trans pump which could have caused the rear thrust surface to copper but dont know why the front surface would be copper also.
Also he had 1 seeping head bolt so there was a small amount of coolant in the oil but not enough to see visually and he had heavy oil maybe 20/50 in it.

These are Clevite "H" bearings I will be replacing them but dont want it to happen again
Thanks
 

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Well, I'd say it needs to be align honed and the crank turned or straightened. My brand new thrust bearing has bronze sides. You have to check the clearance and also look to see if the thrust on the crank is blue or worn. The way that one bearing is wore, I'd say the crank needs to be turned. Not enough clearance. Dave
 

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An excessive heat build-up wiped those bearings. :(

On that last pic, you can see the heated and displaced bearing mtl moving up and filling the small tab area as evidence.

Tolerances too tight at install and more than likely the mentioned improper torque convertor shimming which will raise hell not only with the tranny pump, but place an excessive axial load on the main thrust bearing surfaces.

As mentioned, you need a full "sweep" of the block's bearing bores to check for correct crank alignment, and the crank checked for run-out & diameter specs.

This is a punched out SB 400, right? They're some what notorius for over-heating related problems that may have contributed to the crank fit-up.
 

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How old is this motor and what do you do with it would be the first thing I'd ask you. Steel crank or cast crank? You should not have any copper showing on any bearings for any motor that is properly built and maintained. The crank is bent, the block needs aligh honed, the clearances are bad, you have an oiling problem and maybe all of the above. I run my motor for two racing seasons between freshen up and the bearings look like new with no apparent wear at all unless there has been some problem during it's service. In extreme situations we see some polishing of bearings on the round track motors after alot of service but they are really punishing those motors with excessive RPMs and excessive temps for various reasons. JMHO, RM
 

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Discussion Starter #6
Cast GM crank. 5.56 rods trw forged pistons. Crank is .010 .010 No apparent scaring on crank.
 

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I once pulled apart a externally balanced 383 that had a bad balance job. The main bearings looked alot like yours. I'm also not sure if you can use the "H" style bearings on a cast crank, something to do with the bearing shells being too hard for a non hardened crank. KW
 

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kilowatt said: not sure if you can use the "H" style bearings on a cast crank,
stock cranks should use the "p" style bearings. you probably lost some oil at the front and rear bearings causing the wear. the stock cranks dont have the generous radi that the aftermarket cranks have.........by the crank weights. this is where the oil went. and the bearing material.
my 0.02
eric
 

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I used a cast crank in a 383 and had no problems with it and it had the P bearings. Not sure what to say other than the only thing that can polish a bearing is contact with the crankshaft. The oil is suppose to keep them apart. So something is not right. Either the crank is bent, the block is not right, the bearings were not sized right or something. I'd be doing some serious checking to find what did it. If there was a problem oiling it could have got the front bearing but the rear is closest the the pump so what got it? I see you live in Kent. I lived in Aurora, Streetsboro and Mantua before moving south. Who did your machine work? Best of luck with it. RM
 

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possible over torqued on #1 ???

take the bearing shell to your local, trusted engine machinist and they should be able to give you the best answer.
 

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Hi,


I use H bearings on cast 400 cranks fairly often, and I have had no problems (knock on wood). Some have ran in circle track cars for several seasons, and look great. I just finished freshening a 412 today that had two seasons on it and the bearings (H) looked like new. I reused the mains.

On cast cranks it is very important that the crankshaft is polished properly. If not, the surface of the crank will become very abrasive. Kinda like microscopic "teeth" that will tear the bearing as the crank rotates. When polishing the crank must rotate the proper direction and the polisher must also go the correct way. You can check the surface by rotating a bearing on the surface and then check the bearing for marks. You can find more on this from the bearing companies.


I would look at balance as someone stated earlier. I would also look at the mating surface on the bock and cap to see if there is any scuffing. If so this may indicate that the cap is walking, and possibly letting the bore tighten at the parting line and wiping the oil film.

Just my opinion guys.
Good luck

Jeff
 

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Discussion Starter #12
I measured the main and rod journals today.

Mains All checked 2.6385 -2.639

Rods all measured 2.0895-2.090

Not much taper or egg shape to mention.
 

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Discussion Starter #13 (Edited)
I found both problems..

Rear main bearing has .0005 at most clearance.

And front main lost oil because of a spun front cam bearing

 
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