If you do not already own any core plug extractors then you will need to make 5 of them. On the left is the largest one and above it is shown located in an exhaust port plug. 3 blank tools were made with the aim of having them double-ended.
Attempts to remove one of the smaller plugs ended when the high tensile steel pins sheared off. The large plug, shown here, managed about 1/4 turn and it was decided that the risk of wrecking the head exceeded the desire to inspect the waterways - so it was tightened up again. Inspection with a torch and lengths of wire will confirm whether inspection really need to go further - plus the end plates are easier to remove. Remember that the plugs are steel and are screwed into cast aluminium where they have sat, surrounded in water, for the last 7 decades!
The following table gives the dimensions and quantities of core plugs in each head. It should be notes that there are two further plug holes: one is used for the temperature sensor and the other is situated in the water jacket. All plugs have 4 socket holes.