When you press new races into cases - the ID(inside diameter) shrinks. If you have +.0025 press fit, your race could possibly shrink .002 or more. When the races come from the manufacturer, they are usually undersize. If I press the race into the case as is - I may have to line lap up to .005 from the race to meet the correct size needed. That's a lot of lapping my friends. McFarland made me this fixture which will allow me to use our Sunnen Hone to take some material out of the race before-hand. Then we press it in . . . and lap .001 or .002 out of the race and save wear and tear on our lapper and give me more time to watch The Waltons reruns.