[rotorwrench]

If you figure a 96-inch radius into a circle diameter of 192 inches, that is a pretty large circle indeed. 16 feet as a matter of prospective. If you imagine a circle that large and cut a pie section of 1-inch width at the circumference, you would see that that 1-inch wide end would look to be almost a straight line to the naked eye. If you put an accurate straight edge from a machinist's square or something at least as accurate across the middle of the tappet face, I'm sure you would see a bit of daylight in the center. If not, then maybe it is flat.

There was a reason for the radius there. It makes the tappet rotate a bit so as to not wear a divot in the face during 100K miles of operation. It also helps to reduce wear in the tappet bore due to a reduction of side forces caused by the pressure of the cam lobe forcing it to move. I read a statement from Barney Navarro about what happened when they tried to use smaller diameter mushroom tappets from a 6-banger in a flathead V8 back in the day. He mentioned that the wear on the sleeves they put in the tappet bores to reduce the diameter was too rapid so they went back to regular tappets. I think he tried locked in roller tappets too and they just caused even more wear.

The old Tobin ARP tappet facing machines could be set for the desired radius but most all manufacturers were near a 100-inch radius, give or take a small amount, back then.