you should not be getting any piston slap on these engines, they r to new and have close tolerances, its not a 350 v8. if its going away and coming back i would think maybe a hydrolic lifter is sticking because its just been re-built. just run like 20w50 oil in it and take it easy for awhile because it may go away. maybe check and see if your getting the correct oil pressure too. let us know what happens cause im curious too. goodluck
