No holes need to be drilled to add a factory style bypass filter to the 216/235/early 261. The holes are already there from the factory. The feed line is taken from the same port that feed the pressure gauge, using a T fitting. The return is located just forward of the engine centerline above the oil pan rail, it should have a 1/8NPT plug in it. That hole drains pack to the pan.
The modification that requires drilling the block is for adding a modern full flow style filtration system, and really should only be attempted on an engine that is already apart for overhaul. I considered it, but the machinist I took my engine to said the bypass would be more than adequate.
As noted in my earlier post, the late 261, '59-'62, from the 2 ton truck does require a filter in order to oil [roperly, as it has a full flow filtration oiling system. All the oil has to go through the filter before it goes through the engine. If a filter is not present, a "jumper line" has to be installed to allow for oil flow.
I do find it odd that Patrick's would say "not to add a filter", considering that they sell a Beehive style bypass filter for the Chevy 6. That seems counterproductive to sales.
