main crank bearing replacement question

[mudhog]

my son has a 88 suburban and the thrust bearing went out. so we pulled the pan to change the bearings and they are stamped with .0006 so what do we replace them with? the motor is still in the truck so we cant just take the crank out and have the crank ground

 

[diesel4me]

Both halves are stamped with that undersize??...

I've heard of GM using bearings that are .001 undesrsized or even just one half of the bearing .001 under and the other hald standard sized ,to make it come out half of one thousandth or a whole thousandth undersize when a crank comes out slightly below standard size after the final polishing.--
So they dont have to scrap an otherwise "good" crank...just as they
sometimes bore a lifter bore .010 over and install a lifter thats .010 over in that hole..(and they stamp the undesized noting on the oil pan rail,or the lifter galley ,where no one ever really thinks to look.so they dont have to scrap a good block..!..bet some mechanics were pissed when they put 16 standard size lifters in an engine in a hurry,only to find later on some were .010 too sloppy in the bores and the oil pressure loss wiped everything else out!).

You cant really measure the mains with a micrometer while its installed,plasigauge is the only way and that might be not so spot on --only thing I'd suggest is to get the closest estimate of what size the journals are and how much undserize they are and choose the closest bearings available,and pray for the best..personally I've had no luck replacing a bearing or two in an engine except once on my 235 straight six when it burned the #1 bearing out and the crank was not scored up,just the bearing was showing copper..it had an external oil filter line fail and it got ran almost out of oil--was lucky thats all the damage it did!..

Several of my friends have tried replacing bearings "in the vehicle",and they had them fail shortly afterwards--my older brother has done a few with success though..its a gamble for sure..

 

[mudhog]

all the bearings look good it just took out the thrust bearing so the crank would move.

 

[wage92]

I would get some plastigauge and check the main journal with the old bearing...see what clearance you have currently and go from there. You can get one under or one over bearings that might work fine.

As for the clearance you are looking for .0015-.003" is usually fine. Closer to the middle of that range is best.

 

[4X4HIGH]

my son has a 88 suburban and the thrust bearing went out. so we pulled the pan to change the bearings and they are stamped with .0006 so what do we replace them with? the motor is still in the truck so we cant just take the crank out and have the crank ground

Those are called selective fit bearings that ONLY the GM factory have access to. In this case you will need to pull the crank and have it machined then use the appropiate sized bearings.

 

[mudhog]

well with your guys and others advice he is going to pull the motor and have the crank ground down to .010 with all new bearings

 

[CGT80]

I just posted to your other thread. I had to pull my 406 due to thrust failure. I had a cast scat 9000 crank. I now have a forged crank going into my engine. The forged crank was trued and re-heat treated, so it is now 10/10. The new bearings will be "H" bearings vs. the cheaper "p" bearings that the last builder used. The H bearings are supposed to be a better material and have the proper radius for an aftermarket crank.

My engine builder has all his cranks checked, even when new, for proper tolerances. That is the only step he has done outside of his shop. He said that even some new forged cranks have inconsistent heat treating, which is evident when he drills the crank for balancing. He checks the rods himself, before installing.

It seems that many manufacturers have quality control issues now days.

People tried to tell me that my trans caused the thrust bearing to go out. It is possible, but my uncle owns a trans shop and does any work I don't do. He checks for those issues and also use an anti balloon converter on my trans. The engine guy is best friends with my uncles right hand employee, so his word is as good as any trusted person. He has seen quite a few crank issues from bad machining to bad heat treating to the wrong bearings used, and some transmissions that were setup wrong. His crank guy said he has seen many cast scat cranks with issues right from the factory as well as many other brands.

My trans was built with many upgrades and was set to shift hard (my request, but most from my uncles shop are set for a firm shift.). It is possible that my trans put an acceptable but high end of the scale amount of force and the crank and combined with the wrong bearings and a soft crank, it may have created the perfect storm. My trans was toast. The crank movement sheared some teeth where the TC and trans pump meet, which sent metal through the trans and killed it, wiping out the case and drum in the process.

The engine and trans were pretty messed up and it was hard to tell where the problems started and where they ended. Now, we are trying to address any possible points of future failure.

OP, you guys made the right call to pull the engine. Just make sure that you have the crank checked for proper geometry and you use the right bearings, then make sure the trans has the proper clearance from the crank.