if it is steel it could be the thrust washers on the crankshaft wearing out. in a 22r i had the thrust washers put grooves so deep in the crankshaft that eventually the washers fell out into the bottom of the oil pan. at that point i could grab ahold of the crank pulley and slide my crank in and out at least an 1/8th of an inch! it was in pretty rough shape. i would check if you have any excessive endplay on your crankshaft to start with.
