The road draft tube hole is about 1 1/4" and on top of the block just behind the distributer. This hole is about 1/2" on the LH head surface of the block, at the upper rt corner as you look at the head. It was mostly covered by the head but there is no corresponding hole in the head. I've been told it is an oil galley passage that should be plugged, but don't ever recall seeing it in other blocks.