the ones on my poly, which i think were stock, were 3/8 course thread on both sides. replacements i found were course in the block with fine thread nuts. 1.5inch sounds right, but i dont recall exactly. just measure the thickness of the manifold, they are easy enough to get to without taking it apart. be carfeull of the shoulder on the stud, i bought too long and had to add an extra washer under the lock washer.