I am not so sure about this drilled bolt business. The drilled bolt is one part of a two part assembly. Its actually a drilled bolt and a plug or cap plug that came with fitted block assemblies post 58. So if you had a shot lower end on a 55-58 motor, you could purchase a fitted block that was service replacement for 59 and up, but in order to use the 55-58 head and rocker assembly on that later block, you had to use the drilled bolt and plug. The reason was that in 59 GM changed the oiling system and made it a more pressurized system that permitted more flow. The 59 and up motors metered this flow different then the earlier blocks and the 59 and up rockers had an offset grove in the rocker to further limit oil flow (55-58 had the grove directly intersecting the dribble hole). The plug stopped the stock oil flow to the rockers, and redirected it to the special drilled bolt that metered the oil to the rockers consistent with oil flow and pressure to the 55-58 motors as opposed to the 59 and up motors. Otherwise, without using the bolt and plug, you would flood the valve train with oil, and possibly have the rockers spit oil over the rocker arms and springs instead of dribbling resulting in a lack of lubrication.
Conversely, using the drilled bolt and plug on a 59 and up motor, with 59 and up rockers, you run the risk of not having enough oil flow to the rockers. This is vitally important because new rocker assemblies are miserably difficult to find these days and a proper rebuilding by Rocker Arm Unlimited in California is not exactly on the inexpensive side.
So what year is your block and what year is your head, and what style rockers are you using?
