FWIW Another source of knocks that often go away on warm-up is piston slap, though it's a sharper sound than a main bearing but increases with load like a main bearing. That sound will definitely go away if you short out that plug. I don't have any experience with cast iron pistons to predict whether collapsed skirts are common but that's usually the cause of slap. Chevrolet went to aluminum pistons about 1950 or so.
Slap can also occur if pistons are installed backwards, that is, with the offset to the wrong side. If someone wasn't careful when they replaced a piston and put it on the rod wrong, they might not have noticed it on final assembly. There's typically a mark on the piston crown indicating "Front," an arrow or a notch.
Last edited by 1Ton_tommy; 06/04/2023 4:01 PM.
