There are 20 studs for the head.  Sixteen are 5 3/8".  Four are longer for the water return flanges but two are 5 7/8 and two are 6 3/8.  I can't remember which holes longer ones go into.  Can anyone check theirs and post back?  IIRC the 6 3/8 went to the rear flange and the 5 7/8 went under the thermostat housing.  But I could be wrong.