Abstract: A method for sampling oil from operating electrical distribution transformers includes the steps of determining the thickness of the container, forming a bore partially through the container wall, placing a self-tapping screw in the bore, and advancing the plug to penetrate the inner surface of the wall. The screw includes means for sealing the bore after the inner wall surface has been penetrated. The screw is counter-rotated to allow dielectric fluid to exit the bore in a controlled manner. The screw is then re-rotated to seal the bore. Samples may be obtained without removing the transformer from service.