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Old 10-14-2002, 09:22 PM
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Red face Spun a bearing-350..Why?

Broke in the cam, changed the oil, started driving it around. Ran it up and down the various speeds. It ran good, started easy, ideled fine. 2 days into it, tonight I pulled up to check the mailbox and heard it squeking with a mild knocking.
Darn!!
Is there a major reason? I honestly would assemble it the same again. We were careful with the whole assembly. At this point I won't rebuild it, as I don't have a clue as to what I'd do different. This was my 5th engine rebuild and first failure. <img src="confused.gif" border="0">

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Old 10-15-2002, 03:07 AM
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The most common cause of spun bearings is not enough clamping force, usually from the rods or mains becoming oval shaped over time, that's why it's always a good idea to have the big ends as well as the mains checked to see if they need to be re-sized.
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Old 10-15-2002, 05:23 AM
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Besides the obvious oil pressure related questions I have here is another question, was the crank polished in the correct direction if it was polished? Doing this finishing operation incorrectly will make the surface asperities "lay" in the wrong direction, this will cause the journal to act as tiny file on the bearing and will shave off a little everytime it is started. Eventually the clearances open up and then the bearing will knock or worse...spin. You can find this kind of problem by examining the bearing shell for surface wiping, if it is even across the face and down to the copper then you have a good idea of what happened (journals embedded with bearing material is another sign). Don't be confused with radius ride where only the edge of the bearing is wiped, this is caused by a tapered journal.

Of course there are a million things that can cause your problem and finding the right answer means taking them all into account. Can you post a picture of the spun bearing and one that did not spin? Particles in the oil that doesn't goes away after the first oil change is a bad omen too.
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Old 10-15-2002, 08:18 AM
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The oil pres. was between 40-60 depending on temperature and RPMs. As the crank miked out std by the machinist, it was cleaded and reinstalled. No polish. A study of the old bearings showed normal operation wear.
I have a digi camra. When & if I tear it down, I'll post some pics.
This has been a kick in the nuts. I want to wimper and lick my wounds a day or two.
snifle, snifle, boo-hoo, I want my mommy, waaah!
Stupid is, as stupid does. I'm changing my name to Forest!! :o
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Old 10-15-2002, 08:32 AM
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Red face

I meant to spell "cleaned"
Since I need a new crank, I'm looking at a SCAT 80-60-06 cast steel. I'd like an opinion. The car is strictly a chrome queen for the street-daily driver.
The add says "check clearence, may require clearence work". Is this major machine work or minor grinding? What might not "clear"?
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Old 10-15-2002, 08:36 AM
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I am not an expert in engine rebuilding but my brother once built an engine in a Simca that ended up with a spun bearing. We were told that you need to tap the end of the bearing cap to seat the bearing before torquing it down. Since you have built several engines this is probably not the case but it is something I have always noted when installing bearings. I also recently read a good article on why you should have the crank ends of the rod checked for round and have them corrected before installing new bearings.

I'd pout for a while and then check it out. Hope the crank is ok.
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Old 10-15-2002, 09:54 AM
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Your positive it's the bearing? Have you drained the oil and filtered it to check for particles? I know it sounds dumb but sometimes other problems can masquerade as bottom end trouble, things like completely collapsed lifters, broken baffle in the oil pan, loose piston pins or cracked piston. Giving the crank snout a quick rotate both ways can help determine which cylinder is the culprit. Have you examined the plugs? Detonation could have contributed? Look for little aluminum balls attached to the porcelin center electrode. Maybe pull the heads first and find out which cylinder is affected.
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Old 10-15-2002, 01:16 PM
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here is a sad story. my pontiac 350 had askip in it...I replaced the lifters.. still skipping.. I replaced the cam.. still skipping.. I took off the heads and had them rebuilt. scip gone.. 5 min later it spun a rod bearing. so much compression on the old motor.. now when i hear a skip. I rebuild the whole motor. with new rods
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Old 10-15-2002, 05:35 PM
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No, I'm not positive about anything. It starts and runs, we move it out of the way once in awhile. Sqark, sqark, with a faint rattling sound down below. I'll do as you suggest tomorrow. Thanks.
Anyone know about SCAT cranks?
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Old 10-15-2002, 05:40 PM
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Scat is a very reputable company, and they build a great product, but beware of cheap imitations bearing the Scat name. They are out there.
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Old 10-15-2002, 05:52 PM
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Is something rubbing? Flywheel? Does the sound change with rpm.

I always describe a spun main bearing like a thunk that gets worse with rpm and load equally and evenly.

A spun rod bearing sounds like a thunk that changes with rpm and load also but also changes pitch when free revved. Spun mains typically do not change pitch when free revved.

You own a stethoscope? Buy one.
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Old 10-15-2002, 07:06 PM
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yes the scat cranks are good but beware there is a company called cat ( something like Cal Auto Trans, don't remember the full name) that is marketing a cast iron crank and many people get these confused. most of these aftermarket cranks are made in china or spain and a lot of the actual quality depends on who the final grinder of the core is, also remember the aftermarket crank has to be ballanced to your assembly,a must!
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Old 10-16-2002, 05:31 AM
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I'm sure these are Scat cranks. Northern Automotive is selling them. They haven't sold me anything but quality so far.
I feel like I'm a chick on Car Talk here, trying to describe the sound. It is loud and is a Squark, not a squeek. Like a shoe on a basketball court. It's coming from the crankcase. Strange thing is..it's holding 40# steady pressure at idle. Fuel pump is new, converter bolts are tight Total miles on motor is 90. <img src="confused.gif" border="0">
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Old 10-16-2002, 06:28 AM
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If you peek under a valve cover do you see any bearing material in the oil that accumulates in the heads?
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Old 10-16-2002, 02:27 PM
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I have a 67 GMC Suburban with a chevy 327 stock engine in it. It made an awful sound and spun a bearing. Does any one know of a great manual or book on rebuilding this engine?
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