Lifters need a slight crown to rotate. I used to chuck them up in my old Sioux valve machine. Many years ago I wrote this on my manual for my old Sioux valve machine. The edge of an approximately 7/8 lifter needs to be 0.0021-inch lower than the centered reading. I'm not a machinist and I would have difficulty setting up a dial indicator to run across the face of the lifter to be able to measure 0.0021 from the center of the lifter. I usually just chucked them up and refaced them from the center to the edge. The crown can be felt by putting a straight edge across the face and I could feel that the center was higher than the edge. But they have to be crowned in order to spin. I always made sure I put the lifter in the hole from which it came. If I used a used lifter I gave it a little crown and watched it when it was running. Lifters aren't that hard to pull out and put back in on a GMC. With Y-Blocks it is a different story. They need to be right the first time. I'm not sure what the tolerance is on an old 645 Sioux valve grinder but I just kinda guessed at things like the crown. Y-Block lifters chucked up just like a valve. GMC lifters were too big so I did those on the back side where I ground the tips of the stems. It has a V-Block cradle against a flat cupped stone. I laid the lifters in the V-Block and the stone and then spun them with the middle of the lifter at the edge of the stone. Nothing scientific, but I never had a cam go flat. But I'm an old fashioned country fixer. I thought I was really uptown by having a glycerine filled made in Germany vacuum gauge. The vacuum at the fuel pump on a Cummins N14 or ISM like we run needs be about 6". Otherwise there is a restriction somewhere. I recently saw a guy that had a $600 adapter for his Fluke digital meter. He discovered something like 12.325 Inches of vacuum. That might impress him, but If I read over 7" I start looking for restrictions.