The 13/16 hex of the old LA head plug won't fit a magnum head. The thread and reach are the same but there is not enough room to fit the plug and socket down into the head recess. The magnum head plug has a 5/8" hex and is actually a great choice for LA motors with headers, the smaller 5/8" hex socket will fit between the tube and the head on cylinders 5 and 7.