I just disassembled what seems to be an original 1958 235 engine,date code D298.Unscrewing the plug near the block right side center head bolt hole shows a drilled restriction of about.062.This restriction and it's passage into the drilled rocker oil feed in the block next to the head bolt hole, is below the threaded area in the head bolt hole.GMC's also use a .062 restriction to the rocker oil feed,it's a brass fitting screwed into the head on the external oil feed used by GMC's.
The restriction orfice can be located anywhere in the rocker oil feed,it only matter that it's there.Removing it will overoil the rockers and possible lower oil pressure to the bearings.
