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Old 03-04-2008, 05:49 PM
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400 sbc mains

Need advice- ok heres the low down Ive got a 408 with 30cc D cup KB pistons 5.7 rods Edelbrock RPM aluminum heads 64cc, Comp cams 292H Team G intake Holley 700, 3500stall, 700r, and 410 eaton posi. Everything seemed to be fine for about three months then oil pressure droped. I pulled engine to check the oil pump and found numder two main has spun and all the main bearing are shot. Machine shop says its due to detination. The motor never made any noises valves never ratteled I have no Idea. any advice I have the block being align bored but dont want the same problem to happen.

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Old 03-04-2008, 06:35 PM
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IMO signs of detonation would have took out the pistons and the rod bearings first, not the mains, the mains are usually last to go after everything else has been beefed beyond belief. my guess is the clearence was off on the number 2 journal or it was out of round but thats just a guess. I'm sure Machine Shop Tom and others with machine experience can help you more.
Years ago we had a 400 spin the number 1 main bearing, the clearence checked out at .001, the machinist who turned the crank said that was fine, we learned different after 20 minutes of break in.
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Old 03-04-2008, 06:50 PM
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I agree I think its the machine shop.. The pistons and spark plugs would show massive amounts of detonation signs. You would hear it detonate I would think. I think they ****ed up and said detonation to try and cover there *** if they put your motor together. If motor was put together right, you didn't get the oil full of gas and it was broke in anywhere near to correct or something else extreme.. it wasn't put together right IMHO. SBC's bottom ends take one hell of a beating in my experience? Maybe improper block cleaning and the mains weren't getting enough oil, or got some metal shavings??? I would bring a few friends or 20 to go talk to the guys at the shop and let them know they need to fix the F-up if they put your bottom end together right...
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Old 03-04-2008, 06:59 PM
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400 sbc mains

how was your oil ,was filter by passing oil
letting trash by ?
if you still have filter ,open it up .
had a bud put a truck motor in and left oil cooler mount on and plugged the line holes .
this let oil by pass through small bypass hole .
mains were trashed .
the filter mount you have , was the bypass hung opened .
there held shut with small spring , get rid of it and tap it 1/4 inch plug and use a allen plug like in freeze plug kits , or buy a non by- pass mount
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Old 03-04-2008, 07:06 PM
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400 mains

well I built the motor, and made sure everything was kept clean, used molly based lube plus oil and went easy on it through break in period which it only got about three hundred and fifty miles on it before i pulled it back out. Ive built a number of 350s over the years and never had this happen. but I also think it was a problem with the crank.
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Old 03-04-2008, 07:17 PM
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I could be more than one thing.. But probably not detonation. I would have your new crank checked out by a different shop and your block..
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Old 03-04-2008, 07:34 PM
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400 mains

I brought it to a different shop this time, Ive used that guy for two motors before this one and a few of my buddys that run big stroker motors use him and have never had any problems, but I just dont think it was a detination problem so I brought it to a different shop.
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Old 03-04-2008, 07:39 PM
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Was the block align-bored to start with? What kind of crank was used? Internal or external balance? Was the rotating assembly balanced? What kind of oil pump? What were the clearances? Was the crank checked for run-out? 2 or 4 bolt main? What kind of rpms were you running?

It it's a 2 bolt main, now would be a good time to put a good set (not Eagle or other cheapie) splayed 4 bolt caps on it.

tom
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Old 03-04-2008, 07:56 PM
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400 mains

Crank was stock cast turned to -.20 on both rods mains eagle rods, and no balancing was done on internals, external stock balancer and flywheel block was not alin-bored the fist time. went cheap there i guess, and I guess i made a rookie mistake by not checking bearing tolerances, I did have top steam holes pined and water ports reinforced deck plate bored, #7 sleeved, but did not have it alin-bored, until now. Oil pump was a jegs high volume. and never turned it over 5500, I would have this past weekend at the track but never made it there. and its a two bolt block.

Last edited by Johnny's 67 Camaro; 03-04-2008 at 08:20 PM.
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Old 03-04-2008, 09:14 PM
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Tom is vieweing this and likely about to reply. Not checking the clearences, you just found the problem. Live and learn.
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Old 03-04-2008, 09:32 PM
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400 mains

DoubleVision, I agree live and learn. I will check this time around, motor seems that it will run stong if I can keep the mains in it next time. Ive looked up the tolerances for the bearings and it shows .002-.003 is acceptable on mains .005-.007 travel front to back & .002-.025 on rods with .010-.020 side to side, yet it was mentioned that .001 was the problem with your crank. what would you say is acceptable? thanks.

Last edited by Johnny's 67 Camaro; 03-04-2008 at 09:45 PM.
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Old 03-04-2008, 09:54 PM
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Most of the time when my customer bring a crank in to our shop with all center mains worn out so bad and the end ones are fine is sign of crank is broke.
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Old 03-04-2008, 10:13 PM
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400 mains

racecar all of the main bearings were shot, they all looked like a new penny, # 2 was the only one that was spun, yet rod bearings looked fine except they felt a little rough from all the deposits in the oil from the mains.
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Old 03-04-2008, 10:31 PM
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what does the thrust face's look like when you took apart? are they wiped out, or are they good?

sam-missle
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Old 03-04-2008, 10:38 PM
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Internal or external balance? Was the rotating assembly balanced?
What Tom said.

I've seen 400's lose the mains from improper balance. They're prolly all beat about the same, uppers and lowers alike.
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