spun bearing? I think, little play SBC 5.7

marty79

marty79

Well-Known Member
Joined
Nov 24, 2013
Location
Newton, NC
Hey guys, my 97 Tahoe Crawler has had a 2600rpm knock for little while now, sounds good at idle and past that sounds pretty decent so took oil pan out, found cylinder 8 rod with play in it (what I expected). I haven't taken the rod cap off but I know I've always heard people saying you can "fix" a rod bearing/spun bearing if it's caught early enough...so any guidance on the "better way"???
It's just a crawler so I'm not concerned about making it "perfect" if that's even a thing lol, I just want to fix it like I've been reading but there's a couple different ways of doing it and looking for a little help with maybe step by step instructions since my reading sucks to understand. thank you
(motor ran good, plenty of power, so I just want to get it "good enough" if that's even a thing, otherwise I'd just swap engines but seems like it's fixable from how good it runs and only sounded at certain rpm?!)
Happy Blessed Thanksgiving to All!!
 
Jody Treadway said:
It's no different from all of the 4.0s you've rebuilt over the years
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but I've never had a "Spun bearing" though...I've just rebuilt/refreshened the 4.0s with standard parts to reseal refresh things but this is little different from what I'm understanding. Is it true I can't "know" anything until I pull the cap and see if the crank is ok? From what I'm reading right now, if the crank journal is ok, smooth still, it can be repaired using plastigauge and proper size bearings. If the rod and/or crank is too scored up, obviously throw it away and replace motor...sound about right?
 
UTfball68 said:
Plastigauge will be your friend for ‘good enough’…

HOW TO: Check Main & Rod Bearing Clearances with Plastigage - CorvetteForum - Chevrolet Corvette Forum Discussion

C7 Tech/Performance - HOW TO: Check Main & Rod Bearing Clearances with Plastigage - NOTE: This How-To guide is neither endorsed by or property of Corvette Forum or myself in any way/shape/form. All liability stemming from any actions taken in relation to this guide is solely placed upon the end...
www.corvetteforum.com
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I'll do this this weekend and post up numbers with gauges and stuff and get yalls help on the right size bearings to use...I just want it to run decent, don't care if it only lasts 5kmiles(like I've been reading about), cause it crawls around in double low 95% of it's time and won't see 1k miles forever lol, thank you
 
I'm just too busy with work to change motors and just want to play on sundays till later on get a whole engine built or new to drop in once and done
 
If that's all you're looking for, measure the crank journal diameter and just buy a bearing the correct size. If it's a stock motor that's never had the crank out, just buy a stock bearing.
 
Couple of things to look for. Has the bearing spun? If so, Crank may be damaged. Is inside of bearing a silver color or is a copper color, if copper, very worn. Are the bearings still in the lock position? If crank is OK, you may be able to get by with just a new bearing. If I had to replace one bearing, I'd go ahead and replace them all. Also look and see if the bearings are standard or has a .01, .02 and so on been replaced in it. The loose may be in the bearing, it may be in the rod. Which you'd need to have the rod sized.
 
jeepinmatt said:
If that's all you're looking for, measure the crank journal diameter and just buy a bearing the correct size. If it's a stock motor that's never had the crank out, just buy a stock bearing.
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so plastigaguge isn't needed, just mic the journal of the crank and get that size bearing? that's it...
upnover said:
Couple of things to look for. Has the bearing spun? If so, Crank may be damaged. Is inside of bearing a silver color or is a copper color, if copper, very worn. Are the bearings still in the lock position? If crank is OK, you may be able to get by with just a new bearing. If I had to replace one bearing, I'd go ahead and replace them all. Also look and see if the bearings are standard or has a .01, .02 and so on been replaced in it. The loose may be in the bearing, it may be in the rod. Which you'd need to have the rod sized.
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havnen't gotten that far yet, just pulled pan and finally found the #8 rod with little bit of side to side play in the rod bearing...not endplay, but side to side if that makes sense. Tomorrow I'm pulling it apart and will post pictures and measurements here to see what I'm working with.
 
jeepinmatt said:
If that's all you're looking for, measure the crank journal diameter and just buy a bearing the correct size. If it's a stock motor that's never had the crank out, just buy a stock bearing.
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from my knowledge it's stock motor, with long tube headers and the ECU has a race tune on it.
I believe the "race tune" is what caused this from "spark knock???" maybe/probably...I can't imagine race tune being good on this rig putting around crawling in Double Low for 5months rather than being on the open road...I'm just assuming here but that's my guess. I got another computer, stock tune, and it ran so much better and sounded better already right before pulling the pan.
 
marty79 said:
so plastigaguge isn't needed, just mic the journal of the crank and get that size bearing? that's it...

havnen't gotten that far yet, just pulled pan and finally found the #8 rod with little bit of side to side play in the rod bearing...not endplay, but side to side if that makes sense. Tomorrow I'm pulling it apart and will post pictures and measurements here to see what I'm working with.
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Have you ever replaced bearings in an engine? Seems like you haven't
If not, you need a good mic, plastigauge and some patience.
 
Jody Treadway said:
Have you ever replaced bearings in an engine? Seems like you haven't
If not, you need a good mic, plastigauge and some patience.
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i have but I guess this is my first time dealing with "spun bearing" and I'm probably overthinking it? this is what happens when I read up on something too much at first, I psych myself out I guess and overthink stuff. But in my defense i guess, replacing engine bearings in a good running engine for maintenance is different compared to when a bearing takes a crap and knocks pretty bad but I'll wait and see what it looks like tomorrow
 
Honestly and I know it's what you want to hear, you'd be wasting time with plastigage or a measuring especially with a mic. I've never seen a wiped rod bearing not also groove the crank, undersized bearings will be too tight and destroy themselves quickly. I don't know why you didnt just pull the cap and look if you were already there, you really need to pull them all and look. If the crank is chewed up nothing is going to last, if it looks alright you can clean it up a little and put a factory bearing back in then just expect to do it again before long. It's a 350 chevy, just buy a new crank and bearings for cheap, clean the engine out the best you can and put it back together.
 
braxton357 said:
I don't know why you didnt just pull the cap and look if you were already there,
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yeah I know, I was in a hurry and had 2 customers pull up to deal with then got sidetracked.etc etc. lol. I'm gonna replace all of them unless like you said, crank is chewed up...
 
honestly John, all you have said at this point is speculation. Your question can be answered many ways. But untll you pull the cap, pull the plug for that cylinder and knock the piston upward a smidge so you can remover the bearing. There isn't a valid answer until proof is shown that it is indeed spun. You do understand? A spun bearing actually spins around within the cap and rod. If it has done that then yes, I'd say the crank is toast. If not, then you at least have that going for you. It may not be the rod where it connects to the crank. It shouldn't be piston slap, as it's a higher mileage engine. Could howed be a cracked piston skirt. The sound is very similar to a rod knock.
 
upnover said:
It may not be the rod where it connects to the crank.
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it's def at the rod where it connects to the crank, it has about 1/16" play side to side play (not the "endplay" that's allowed) so it's there for sure. I apparently jumped the gun on asking for help without actually pulling it all apart...my bad
 
skyhighZJ said:
I’m not saying that I’ve never taken a long strip of very fine Emory tape and had someone crank the motor over while I laid under the truck and “polished” the crank.







But I have.
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Dang thats a heck of an idea
 
If the bearing has actually spun, where one bearing half has overlapped part of the other half, the crankshaft end of the piston rod will likely be out of round. May or may not be enough to matter for what you're trying to do. You can use plastigauge several places around the journal check this.
 
skyhighZJ said:
I’m not saying that I’ve never taken a long strip of very fine Emory tape and had someone crank the motor over while I laid under the truck and “polished” the crank.
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Done that, exceeppt I used a leather boot string wrapped aroiund and just pull it around like I was doing like I was using a string saw to cut down or cut up a tree.
skyhighZJ said:
But I have.
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Ok it spun for sure. Crank journal is perfect shape, rod is smooth also with black markings like underside of these bearings. I'm assuming I'm lucky enough to put new bearings in and send it for my uses?
Thanks guys
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