Thrust bearings locate and keep the crankshaft from moving foreword and backward in the engine block. They provide for a specified amount of crankshaft end play. Most fit into grooves formed in the webbing of the engine block. Some engine manufacturers have these incorporated into the sides of the main bearing.
Measure crankshaft end play with a dial indicator placed against the harmonic balancer or flywheel. Prying the balancer or flywheel forward and backward indicates crankshaft end play.
Worn thrust bearings occur in older vehicles with manual transmissions. Overloading and misalignment cause damage to these bearings. Excessive end play is evident once the oil pan is removed. It damages and wears the crankshaft, main caps, and engine block where the crankshaft rides.