Recently lost the crank thrust bearing in my 496.I was told that the trans convertor was probably the culprit.There does'nt seem to be anything externally wrong with the convertor.How can I tell if it is bad? Trans was working fine when I discovered the crank would push back and forth about an 1/8th of an inch.

Been there done that --- I couldn't find any answer on how to test the converter short of cutting it open.

You can pull your transmission oil pump and look at the cavity where the gears are. If the gears show signs of cutting into the rear half of the pump case then it probably is the converter. In theory, a ballooning converter would push both forward and backward so if it causes damage on the main bearing thrust surface there should also be damage in the transmission.

Main cap alignment could be another cause. It's unlikely, but if the main cap were too far forward or too far back the bearing shells wouldn't line up properly on the thrust surface and could cut into the crank.

There is also a modification that can be done to the thrust bearing to improve lubrication to the thrust surface. I forget exactly where and how much, but you can grind a very slight camfer on the edge of one of the bearing halves to allow more oil to the thrust surface. Maybe someone else that has done this can comment . . .

My trans pump was also gone so I put in a new pump and converter in mine; haven't had any problems since.

PS. mine's a small block, should be the same though.

http://www.atra.com/crankshaft/