The SBC and BBC use a flat lifter that is a bit smaller in diameter than those of Ford or Chrysler. There are mushroom shaped lifters available for Chevy engines, but the seemingly quick and cheap solution was to bore the chevy lifter bosses to accept the larger FoMoCo or MoPar lifters.

Within reason the larger diameter flat tappet reduces the loading between the lifter and the lobe; and extends max lift open time for the valve by virtue that the lobe tip must traverse a longer distance from one side of the tappet face to the other.

For a roller there is less to no effect since there is adaquate space in the Chevy lifter for sufficient roller diameter to minimize loading between the roller and lobe. Also, a roller doesn't have the effect of the one end of the face to the other wiping effect of a flat tappet. The roller makes instantainous contact rather than wiping contact, hence the big difference in lobe design. The flat tappet lobe is peaked, the roller lobe is quite broard across the top in comparison in order to get the open time comparable to that of a flat tappet. And yes all those magazine pictures that show a flat tappet and roller tappet cam together and tell you how that board lobe of the roller compared to the flat tappet results in more valve action and timing, simply ain't so.

Bogie