In this months "Gears" magazine (for automatic trans people). There is an article on crank thrust failure. It describes the lube problems a thrust bearing has (surface finish, poor lube etc.). Tips to help avoid the problem. One thing mentioned is even a trans "shift kit" can cause crank thrust failure..

Most of the failures I have seen were from improper assembly, Cap mis-alignment (poor line bore/hone with non-stock caps), trans/flywheel problems and super heavy clutch/pressure plate assemblys.

Once the thrust is worn the only cure is replacement of the crank or have the crank thrust welded and re-ground.