Thrust bearings locate and keep the crankshaft from moving foreword and backward in the engine block. They provide for a specified amount of crankshaft endplay. They’re placed into grooves formed in the webbing of the engine block. Some engine manufacturers have these incorporated into the sides of the main bearing.
Crankshaft endplay is measured with a dial indicator placed against the harmonic balancer or flywheel. Prying the balancer or flywheel forward and backward indicates crankshaft endplay.
Worn thrust bearings are more common in older vehicles with manual transmissions. Overloading and misalignment causes damage to these bearings. The damage caused by this excessive endplay is evident once the oil pan is removed. It wears the crankshaft, main caps, and engine block where the crankshaft rides.