Do you have to drill the oil passage on a 79 shovelhead

[jvall1201]

Is it absolutely necessary to drill the relief passage on a 1979 engine when installing a newer style oil pump. What will happen if you don't drill it?

[Ohio HD]

Never tried it, I always drilled the case. I imagine the oil pressure may be erratic without it.

[Burnout]

I think you are talking about the vent hole for the pressure relief valve.
I always drill this because if/when the passage fills with oil it will hydrolock the pressure relief valve.
Bad things happen to pumps with a malfunctioning pressure relief valve.
I could rattle off a list, but ask yourself how long you want your motor to run with no oil pressure.
And how good are you at replacing sheared keys beside the road?

[jvall1201]

Thank You for the reply. Really appreciate it.