My 66 305E had a knock before we overhauled it. There were two pistons that had the skirts scored. I have a 70yr old friend that was a GMC mechanic back in the day of V6s. He said they were notorious for that, and had his own cure by lightly knurling the skirts for better oil retention.We knurled my new pistons and so far so good. I have a spare parts truck that did not knock until I changed the oil and it did the same thing as yours as soon as I did. Don't know why, but it sounds the same as the one with the scored pistons. Maybe a coincidence??
