The following illustration demonstrates
why you must install new lifters:
• A new lifter has a slight "crown." A new cam has a slightly tapered face that causes the lifter to rotate. Note the smooth, wide contact patern in Figure 1.
• Installing a new cam with used lifters creates a concentrated contact pattern on the edge of the cam lobe (Figure 2).
• Eventually, the contact wears out the edge of the lobe and lifter. Soon, the cam lobe wears down and the lifter stops rotating (Figure 3).
– Tech Tip courtesy of Lunati 
