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Ah, just when you think youre good to go, Murphys Law strikes in full force. For reasons unbeknownst to us, GM used square-head pipe plugs to block the oil pressure port on our ZZ502 motor. The inset square head is 5/16x5/16-inch in sizewhat to do? We headed to a good industrial supply shop and bought an 8-point 5/16 socket and a small length of 5/16-inch square key stock. Insert the stock in the socket, and the plug comes out flawlessly with a ratchet handle. We hope this saves you the 2-plus hours of scouring the city for a 5/16-inch square drive socket. With the plug removed, we screwed the sending units in this port (located above the oil filter on a Chevy big-block).