accessory holes B1Ba heads
The coolant passage from the head deck surface is right next to that accessory bolt boss area and it is not as thick there, but you should not be drilling there. Here is a picture with my caliper pointing to the thinner area. It is thinner inside the coolant passage here, not just thinner on the outside. The coolant passage is seen below the caliper on the deck surface, kind of D-shaped.
The coolant passage from the head deck surface is right next to that accessory bolt boss area and it is not as thick there, but you should not be drilling there. Here is a picture with my caliper pointing to the thinner area. It is thinner inside the coolant passage here, not just thinner on the outside. The coolant passage is seen below the caliper on the deck surface, kind of D-shaped.