This is usually the highest point in the engine block cooling system jacket
and it must be vented back to the highest point in the overall cooling
system. Here is were the steam and air bubbles wind up if any. Be careful when
tapping it out for a 1/4 pipe AN-4 vent line. The material around
the hole is not much and the rear housing can be easily cracked
if you put too much torque on the tap.

Back the tap out frequently and clean the tap each time. If it cracks you are out several hundred dollars for a new end housing not to mention the work taking the engine apart and putting it back together. Nothing we tried on another cracked end housing worked to seal the crack.

Use a magnet to get the small chips out of the hole.

Filling the engine with coolant is tricky. It must be done over and
over to make sure the block is full. This vent line should be opened up
when filling the engine.

Paul Lamar