I just got in from the garage, i took the remaining valves out, then the stem seals, then got my dremmel out to polish up the ports. Fate would have it that i ended up working the ports of the "impact" zone last. There appeared to be a black steak that wouldn't polish off, so i washed the port through with carb cleaner and its a massive crack in the wall of the inlet tunnel. There is infact two cracks, down the drivers side of ports 3+4. The crack starts slightly after the valve seat and are about 2 inches long each, both finishing about an inch from the inlet port opening.