Re: Lifter bore
The larger the diameter of the bore the less contact it makes with the cam allowing the tappet to ride on top of the ramp without digging in to the face of the cam with a more aggressive cam profile.
Think of the planet Earth as being flat compared to a sphere. We know it is a globe but with a large diameter and a tiny person making contact with only their feet it acts as though it were flat. Change the scale back to parts that fit back in an engine and you can see where a larger lifter has less chance of flattening a flat tappet cam.
In essence you are regaining the some of the advantage a roller cam has. Rhodes sells a mushroom tappet that has a large flat disk on the bottom of the lifter to accomplish a similar function. They are illegal according to all sanctioning bodies (NHRA, NASCAR, USAC, etc.) as they offered an unfair advantage because a stock ground cam with mushroom tappets acts as though the duration is actually longer and by lofting the lifter over the nose of the cam you can gain additional lift.
You gain that duration advantage (not the lofting part), while a bigger diameter lifter acts like a mushroom tappet, and they are legal in COMP classes just not stock.
I bush my big blocks tappet gallery for Chrysler roller tappets to gain an even bigger foot print and I am also moving the lifter a bit off center as well as straightening out any mess up in machining (not as common today with CNC machining as it was in the old days with humans machining the blocks). I run half inch push rods and triple wound springs with 0.800+ inches of valve lift to compensate for the tiny 2.30 inch intake and 1.98 inch exhaust valves in my head. The valves may sound big but consider the valve area per cube for a 302 with a 2.02 compared to a small 632 cube BBC with little 2.30 inch valves. You can easily see why I go to extremes with valves and port work.