273 Rocker Arm to Shaft Clearance

What Rat said.
We are dealing with a steel shaft that is less than 1" diam & everything is in proportion when dealing with thermal expansion. I would venture that the smaller gap will improve oiling of the two surfaces. If you have a large gap the oil will just run out either side at the bottom of the shaft [ gravity ] without being cycled around the shaft. Like the stamped rockers with the sloppy clearance; only the stamped rockers do not have a 'top' so no oil reqd there.