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Old 08-20-2004, 05:07 PM
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SB Chev Main Bearing questin

81-350 SB, K5 Blazer
The crankshaft on my 3 yr. old rebuild seems to be floating back and forth, by as much as 3/8 in, to the point where the flexplate was hitting the inspection plate.
I have been toild that the Thrust Washer is defective or never installed in the first place.
After googleing and searching about, I can find no info on the illusive thrust washers.
They are supposedly part of, or contained in, one of the main caps.
Can anyone give me the straight goods?
Can they be added or replaced in the vehicle?
Probable damage? (Runs good, the main seal is wet though)
Thanks for any help...Depressed Comet

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Old 08-20-2004, 05:20 PM
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That doesn't sound too good!

the "thrust washer" is not a good term for it. The rear main bearing controls front to back crank movement.






It should be a few thousands! Not 3/8s of an inch. Sounds like that bearing died big time. You need to pull the pan and see what all got damaged. If the crank is in fact moving that far, you will probably find some serious damage to the crank, block, rods, piston pins and possible cylinder bores from the crank forcing the rods to go foward and backward, which they were never intended to do. If the rods are doing that, that in turn will slant the pistons in the bores and cause piston/bore damage.

Keep us informed on what you find, and some pictures would help too.

Mark

Last edited by Jmark; 08-20-2004 at 05:34 PM.
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Old 08-20-2004, 08:48 PM
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You can bet with about 100 % accuracy that the thrust surface on the crank is toast!!

On a Chevy the thrust bearing is designed into the rear main bearing. As pictured. SOME engines have a Thrust bearing that is seperate from the main bearings. Many industrial and Heavy Duty engines have seperate type thrust bearings. These are available in various thickness to allow for crank regrinding and wear. Some of these can even be shimmed to correct specs.

To get thicker thrust bearings for a Chevy you have to order about .040 undersize with semi-finished thrust. Depending on the bearing co..

Last edited by BOBCRMAN@aol.com; 08-20-2004 at 08:57 PM.
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Old 08-20-2004, 08:53 PM
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I had a truck brought to me with the exact same symptom. It was said to have only a few thousand miles on a rebuild. On tear down the thrust surface on the thrust bearing was worn to the steel shell, 1/2 of the shell was worn away and the crank was junk. The timing chain worn. The pistons looked OK but after cleaning, rinsing with water and allowing to dry rust spots on the pistons showed that a lot of iron particles were embeded into the the piston skirts so they were junk.
I wish I could say for sure what caused it. I guessing either the torque converter wasn't fully seated when the engine was started or the thrust surfaces were damaged before assemblely. Perhaps a misaligned V-belt?
Anyway, that was 4 years ago that I rebuilt it and it's still running now. Anybody else have any ideas on what might have caused this?

Last edited by tresi; 08-20-2004 at 09:00 PM.
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Old 08-20-2004, 09:44 PM
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Thanks for the info. If this crank is that bad, should I not be hearing noises, other than the inspection plate? Perhaps the mechanic was exagerating a wee bit. The engine sounds and runs very strong, burns no visible oil, no pings or rattles. I have been pulling a big trailer with no probs I've seen and it wizzed through aircare couple months ago.
I can't see any shiny metal behind the pulley and I'll have to throw a bar behind there in the daytime and see if it moves.
If the rear bearing is toast, is there any point changing thge crank in the truck or will I have to have the block machined?
This sucks bigtime...
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Old 08-20-2004, 10:02 PM
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I have had them come in with wear patterns on the main bearing webs that were made by the counterweights rubbing directly on them. The owners say they never heard a noise from the motor! Most are brought in for oil leaks. If you can get a bar in there and visually move the crank.IT IS JUNK!. The block is USUALLY ok. just the crank flange and the bearing take all the destruction. A new crank and bearings will usually save it.
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Old 08-21-2004, 10:38 AM
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Quote:
Originally posted by rustyredcomet
Thanks for the info. If this crank is that bad, should I not be hearing noises, other than the inspection plate? Perhaps the mechanic was exagerating a wee bit. The engine sounds and runs very strong, burns no visible oil, no pings or rattles. I have been pulling a big trailer with no probs I've seen and it wizzed through aircare couple months ago.
I can't see any shiny metal behind the pulley and I'll have to throw a bar behind there in the daytime and see if it moves.
If the rear bearing is toast, is there any point changing thge crank in the truck or will I have to have the block machined?
This sucks bigtime...
A new crank and bearing will probably get it going but I'd pull the motor and inspect everything. With the crank slinging off little bits of iron it will embed iron into the piston skirts. The oil pump gears are probably scored. All the bearings including the cam bearings will have iron embeded in them. During cold start up most of the oil bypasses the filter so all the iron junk gets circulated in the oil until the oil is warm enough to flow freely through the filter.

So how many people are seeing this type of problem showing up?It used to be that you never seen any thrust bearing problems. I know that the engine I seen it in was built with all new parts including the crank. It has me wondering if there's a quality control problem with bearings or cranks.

Last edited by tresi; 08-21-2004 at 10:38 AM.
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Old 08-23-2004, 09:46 AM
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Latest Info...hmmm

So I did some checking, what I could do from outside. There is no unusual wear on the belts, no obvious wobble in the pulley and no play that I can detect using a bar on the front pulley.
Whem asked, the shop guy says he saw no metal flakes when he pulled the filter and changed the oil.
I asked where he saw the 1/4-3/8 movement and he says when he pryed with a bar on the preasure plate. Is this a valid test or is there movement in the output shaft of the trans that he is seeing?
There was some oil behind the rear seal but I think that has been like that since the tranny was rebuilt.
The initial problem was the flexplate sometimes touching the inspection plate, a problem solved with a dye grinder.
I am going to have the pan taken off and inspected, but wondering if a different mechanic may be in order....Thanks for your info...Comet
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Old 08-23-2004, 11:59 AM
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Personally, I"d try prying it myself to see for sure. Just don't pry in the wrong spot and dink the pan! LOL
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Old 08-23-2004, 06:10 PM
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Re: Latest Info...hmmm

Quote:
Originally posted by rustyredcomet
So I did some checking, what I could do from outside. There is no unusual wear on the belts, no obvious wobble in the pulley and no play that I can detect using a bar on the front pulley.
Whem asked, the shop guy says he saw no metal flakes when he pulled the filter and changed the oil.
I asked where he saw the 1/4-3/8 movement and he says when he pryed with a bar on the preasure plate. Is this a valid test or is there movement in the output shaft of the trans that he is seeing?
There was some oil behind the rear seal but I think that has been like that since the tranny was rebuilt.
The initial problem was the flexplate sometimes touching the inspection plate, a problem solved with a dye grinder.
I am going to have the pan taken off and inspected, but wondering if a different mechanic may be in order....Thanks for your info...Comet
Something's not right here. If you have a flexplate how could the mechanic have pried on the pressure plate? Ok, there are some circle track clutches that use a flexplate but I doubt you;re running one. It's easy to bow a flexplate 1/4-3/8 inch with a pry bar with a perfect thrust bearing. The mechanic should have known that the thrust bearing is part of the main bearing on a chevy rather than saying a thrust washer might not have been installed. If he could get 1/4-3/8 play at the rear you should have been able to get the same play on the front. Forget the pry bar any that I have seen that worn you could just grab the pulley and push/pull it back and forth by hand.
In my opinion either your mechanic is a complete hack or worse he's trying to sell you a rebuilt motor because of a couple of bad seals. If that's the case he'd probably pull your motor, throw in a couple of seals, paint it blue and call it new.
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