The Bolt with the hole is only the first of two parts used when installing a 59 and later short block, using the 58 and earlier head and rockers. The bolt and plug were supplied with replacement fitted blocks that were cast as service replacements for the 59 and later models (but were service replacement for the earlier models also.) In addition to the bolt with the hole, there was also a plug that has to be used.
The 59 and later blocks have the oil orifice drilled larger than the 58 and earlier blocks and those engines use a different rocker assembly (off set groove inside each rocker arm). This was to accommodate the last oiling change GM made to these motors. They created a sort of pressurized system with a higher volume. If you were just to use the 59 block and a say 57 head and rockers (with the centered intersecting groove), you would get too much oil to the head. so, if you were purchasing a service replacement block for your 57 truck or car, and were purchasing the fitted block in say 1960, you would get the 59 and later style block, that had motor mount accommodations for the earlier motor, along with the bolt and plug. The plug plugged up the larger stock oil orifice in the new fitted block, the oil was redirected to the bolt with the hole, the hole being the correct size for oiling the earlier rockers.
Last edited by Dragsix; 01/22/2024 9:59 PM.
