Up until 1957 235/261 used an oil line that fed oil from the rear cam bearing and up around the head bolt. Keep in mind only Power Glide cars used and were drilled for hydraulic lifters. Starting in 1958 all 235/261 engines were drilled for hydraulic lifters whether they used hydraulic or solid lifters. At this time Chev got rid of the tube that ran from the rear cam bearing and now brought the rocker oil straight up the center from the lifter galley.

The plug on 1958-62 engines at the center under the side cover is used to block the hole necessary for drilling the oil path and is also where the restrictor orifice is on 1958 and 59 engines.(the restrictor orifice is the fitting for the tube on the 1953-57 engines that screws in the block at the rear)

The restrictor orifice was 1/16" up till 1958. In 1959 it was increased to 11/32". At this time because of the increased oil supply the rocker connector tube was pinched off and the groove inside of each rocker was offset 1/4" so it no longer lined up with the supply hole in the shaft to pressurize the system.

If you use 1958 or earlier rockers on a 1959-62 engine which have the groove inside each rocker in the center the oil has to easy of a path and will over oil the valves.

Because of this pressurized set up I do not like the aftermarket shafts that have a flat spot ground the full length of the shafts. This flat spot gives the oil an easy path to the groove and out the vent hole and also to just dump out the edge of each rocker. This can lead to oil use as the valve seals can't keep up with so much oil (I have a easy fix for this if anyone is interested).

The plug Chev sold to go with the drilled head bolt goes in the block on the top/deck next to the right center/drilled head bolt. This blocks the original oil path and sends it through the drilled bolt.

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