I just put together a new engine (350 small block) also for reference the trans is a 4L60. After a couple weekends of driving on the street and tuning we dumped the oil and found it was had fine metal flakes in it and there were chips in the filter. We took the engine out and tore it apart and found the thrust surface on the rear bearing was completely worn (down to the steel shell in one spot). We read some posts on here and saw a couple people with the same problem found it was caused by torque converter ballooning or the crank thrust surface was too rough. We ordered a new Coan converter and got the thrust surface polished, as it was ground down during the rebuild process and was a bit rough. Got the engine back in and ran it for about 20 miles plus some idle time, dumped the oil and again it was metallic. Checked the endplay on the crank and it was at 24 thousands started at 8, so we ate through another bearing. The only odd thing I noticed before the last failure was a shutter in the higher rpms, somewhere above 5000, on the one full throttle test I did. Any suggestions about the cause would be appreciated.

 

Maybe a bent crank. Pull the caps and look at all the bearings.

 

Checked all the bearings after the first failure. They were fine although we replaced them all when we rebuilt it again to be on the safe side. Rod bearings wer fine but replaced as well.

 

What you would look for is the bearings at one end to be shiny opposite the bearings on the other end. But since checking that is out, you'll need a magnetic-base dial indicatot, to mount to the block, put the tip on the crank, then turn the crank to check if the thrust flanges turn true. Alternately, check yout thrust spec, the .008" you mentioned, on the left and right, both front and back, then turn the crank 90 degrees, repeat, turn another 90 degrees, repeat, etc. If you're finding it varies from like .004-.012, for example, you've found your problem. I hope you know to not put washers between the flexplate and the converter, the converter is not supposed to be as far back as possible, it's supposed to be 1/16-1/8" forward of bottomed.

 

For the crank I'll do some measurements and see what I find. For the converter I know there is supposed to be clearance there and we actually checked that. There's around 3/16 clearance when the converter is pushed all the way back.

 

had this same prob with a bbc and a th400 with a brake
turned out the pump was creating way too much psi and was forcing the converter out of the tranny and in turn wiped out the crank.

 

Checked cooler line pressure. 50 psi at trans. outlet pipe and 30 psi after radiator cooler fitting. Both pressures are idle in park. When TV cable is pulled out both pressures rise 40 psi. Not sure what "normal" is. Suspect restricted cooler.
Will measure pressure at WOT through 1st and 2nd if road is dry tonight.

 

not sure myself a friend at the time owned a tranny shop "FB PERFORMANCE" and he knew the prob right away