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Old 05-10-2004, 12:22 PM
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crank problem

i am rebuilding a crate 350 motor i got out of a junkyard. i am in the rebuilding part of the project. After having engine cleaned, magnafluxed, prepped, etc, i installed the cam bearings, then the cam, the the main bearings and the crank. The crank was .20 under and the bearings are .20 over. they have all been miked, and miked and miked again, and are all spot on.
When i put the crank in and only on the bottom main bearings, without the caps, it turns nice and smooth by hand, and will even spin if prompted.
When i put the caps on(4-bolt), without any real tightening, it spins the same, with maybe slightly less freedom-almost imperceptible.
however, as i tightened the reart main cap, the crank would not budge. when i loosen that one and do the same to each cap individually and with the group, the crank will not biudge.

My machinist and engine mentor and myself cannot figure this out for the life of us, but something aint right.
Any experiences like this you can relay that may shed some light?

thx
Frank

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Old 05-10-2004, 12:35 PM
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when i was trial fitting my engine, the rear main would cause some bind. it would spin freely until you torqued the rear main.
i was torquing to the max listed in the book, i backed of ten pounds, and was still in the middle of the listed spec range, and it spun freely.
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Old 05-10-2004, 12:44 PM
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Surely sounds strange.

I was just about to say, the crankshaft line bore is not straight.

Now on larger diesels like I build, we have something called bearing nip.
Bearings are getting "squeezed" when installed at the parting line, as they press against one another
If the bearings are too large for the bore in the circumferential direction, the ends pressing towards each other, will be pressed inwards, scraping the oil of the crank journal.

Now if this inward squeeze is large enough to hold the crank that might be your problem.

Bearings too large for the bore that they are installed in.

Otherwise I would say that the measurements are wrong ( with the chance of you being upset...)

To check either:

Try another set of bearings/ brand if you have access to some.
If problem is gone, bearings where the cause.

Find another block, install the bearings there and measure again, or put in the crank, to test it.
If OK, block is the problem.

Hope you get it sorted...


edited for typo's
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Old 05-10-2004, 01:22 PM
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is the crank new to you... or is it the same one out of that motor?
i am assuming the bearings are new... and installed with the locks in the proper locations?
when you turn the crank does it put any marks on the crank or bearings?
when you say crank is under .020 and bearings over .020, what are you spec-ing that from? or i guess what book and specs are you getting the main size ( X - .020 = your miked size)
have you tried to plasti-gauge them to see what the clearances are? it will at least show you wich one(s) are the problem.

i had a shop turn down (undergrind) all but one before,

i also had someone tell me that a crank was .020 under and it was .030 under, which caused lower oil pressure and almost none when oil got hot. - not good on the track.
needless to say i had to pull and put new bearings in...
i got lucky and didn'tt spin any bearings. lesson learned... ALWAYS plasti-gauge it!!!!! it sucks to put it in, torque it down, take it off, and look at it, and spec it for each one ( i do rods too)

but if there is a problem, you will find it. it is better to find it on the stand than on the track!

31
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Old 05-10-2004, 02:23 PM
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thx for the speedy replies.
here is the conundrum.
the crank is a different one because my original was scored on the journals. so this is said to be .20 under. we miked the journals and they were all .20 under, so we know that's fine. the bearings are all new, but we miked them anyway, and they all measure .20 over, so we know that's fine.
when the crank is in the block, sitting only on the block bearings, it spins fine. when i put the caps on, and DO NOT TIGHTEN THEM AT ALL, it spins fine. As soon as i tighten the caps, even w/o torquing them, it simply will not move. I tried the caps one by one to eliminate any of them, but each one, by itself, tightened on, prevents the crank from budging. THere is some scoring, i noticed , on the lip of the rear main bearing, on the inside portion of the block, that faces the front of the engine. It appears there is some unusual friction there, but i can't tell why and it doesnt seem to make sense to have no friction unless tightened, because itis a side portion of the bearing.
i am as frustrated as you guys might think i am, but not in any way deterred. i think of it as a mystery of the project. what fun is it w/o mysteries and challenges?
i am learning ALOT as well!
but still, no solution.
btw, the guy that is mentoring me, has been rebuilding engines for 40 years, and he knows his stuff. he is confounded right now as well, especially since he did my machine work!
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Old 05-10-2004, 02:37 PM
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crank

Zartan take all the bearings out make sure you have the caps on wright and tq down with out bearings put a straight edge in across the mains and see if they are all even do this to the caps also you should not be able to get a feeler blade under the straight edge.If all checks out ok recheck the align bore and make sure it is some were in the spec.Look under the bearings and verify the are .020 under.Next put the front and rear main bearing in oil lightly and put the crank in with out rear main seal then put a dial indicator on the center main you want less then .001 bend in the crank.


mike rogers
Rogers Performance

ps call me if i can help out in any way i,am 15 min from you
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Old 05-10-2004, 03:07 PM
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did you inside mic the mains after they were torqued in place without bearings? i didn't see that listed anywhere, maybe the line bore isn't .020? mic and see. and make sure it's straight like Roger said.

when i did my motor, i mic'd everything, did the math, and it FIGURED right, but i plastigaged everything to be sure. like sudden psycho said, better to find out on the stand than on the track. or break in or actually break up......
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Old 05-10-2004, 03:36 PM
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Had this same exact problem. My problem was the main caps had some burrs on them. I filed them down a little, and put them on and it spun nice and easy.

oh and by the way, dont forget to plastigage after you do that just to make sure the clearences are ok. I'm not trying to insult you, just making sure
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Old 05-10-2004, 03:56 PM
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You also need to be very sure your mikes are accurate and that you take into account that you do need some clearance in there. I had a high dollar set of mikes re-calibrated and one was off .0015! Not good when miking cranks.

If you are measuring 20 under on the crank and 20 over bearings, there is NO clearance and the crank will bind. If you are measuring 20 under on the crank and 21 on the bearings, you're getting there.

In theory, mikes are the best way to measure, PROVIDING they are dead on! I like plastiguage in addition to mikes to be "double sure" everything is correct.
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Old 05-10-2004, 04:04 PM
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I ran into this case only once, the bearings ordered were for a standard crank, only they were .002 oversized, so when the mains were tightened, the crank wouldn`t turn, the fellow who tore the engine down read the back of the GM bearing, and it said .0005, which he mistook for .005, so the bearings he ordered were .002, so there wasn`t enough clearance for the crank to rotate. Remove your crank, and take a exact measurement, then clean the bearings of all lube, reinstall the main caps and torque them to spec and take there exact measurement, you`ll likely find your answers there.
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Old 05-11-2004, 07:57 AM
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couple questions, and thx for all the help. this is my first project, so noone is insulting me!

1.when i plastigage, i put it on the journal then torq the caps , w/ the bearings in? so it would be bearing to plastigage to journal?

2. if i torque on the caps w/o bearings, how do i protect the journals from scrapes, etc? can i try and turn it w/o bearings?

btw, this is all part of the check-double check process for me. i have done everything to eliminate the chance of problems later, but i realize i'm not perfect.
thx for all the help.
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Old 05-11-2004, 08:42 AM
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I think I know what the problem might be. I had the same problem a few weeks ago. On the crankshaft close to the rear seal is a raised area or ring, looks like a ring on the planet Saturn. If your not sure what im talking about look at this web site and goto the photo 2nd from the bottom, you will see the ring clearly im talking about. Although this is a ford crank, it looks as if the chevy cranks have this ring also

http://www.scatcrankshafts.com/smbig...dcranks02.html

I'm not sure why cranks have this ring, but I would guess it helps keep oil out of the rear seal by using centrifugal force.

Sometimes on new cranks the ring will be too big and contact with the block or cap. I just got a new crank and, it was hitting the block and the cap by about .001 or so, just enough to lock up the crank. You may want to check this and see if its the problem.

Ben

Last edited by brainsboy; 05-11-2004 at 08:47 AM.
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Old 05-11-2004, 08:58 AM
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i'm thinking along the lines of align boring myself too. when you plasti gauge you obviously need the bearings in home-slice! no lube would be a groove too. make SURE you lightly spray wD-40 on the bearing and journal BEFORE you put the plastic on or it is hell to scrape off. .001 is what you would LIKE to see, unlike SOME of us (ME) that have .002-.003!!! does mine bind up? HAHAHHAHA! i need to get new bearings, but that's another bed-time story. ps. you have to torque your bolts down all the way as well
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Old 05-11-2004, 09:14 AM
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when you plasti-gauge, you assemble just like you were going to leave it in.

on the crank, with uppers already in (this is the bottom of the motor if it is on the stand). put plasti-gauge on the crank, put caps on WITH bearings, no lube, torque to specs.
this is were clearances should be checked.
it gets quite confusing with miking the extreme decimal places.
when thousandths of an inch are important... torqued plasti-gauge is the only way.

Quote:
THere is some scoring, i noticed , on the lip of the rear main bearing, on the inside portion of the block, that faces the front of the engine. It appears there is some unusual friction there, but i can't tell why
this sounds like it needs to be addressed, i would take up the offer by rogers performance.

31
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Old 05-11-2004, 12:04 PM
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my crank has the ring that looks like it's from saturn. i was thinking that myself, but not knowing what to do about it, i am looking at other area's. how would i address that?
this is where i think the unusual friction on the lip of the bearing is coming from as well, so it all ties to gether. i guess i could grind it down a little, but sdince i dont know what the hell that ring is and what it does, i dont want to solve one problem and create a bigger one!

as for the plastiguage, i always thought you used the bearings in, but it sounded to me like gi=uys were reccomending putting in the crank w/o bearings to see if it spins freely...so i wanted to check to be sure.

thx
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