It looks like BSPP ( British Standard Pipe Parallel ) but could be a DIN / ISO / German equivalent. Go to Discount Hydraulic Hose and look at their thread ID PDF.

On the plug, where the threads hit the under head hex, there should be a slight taper from the hex to the threads. ( this is a reverse of a tapered seat spark plug. ) The outer sharp edge bites into the soft aluminum cooler boss. This is typical for BSPP in soft metals. If used on steel, a sealing washer is used.

This should plug screw in easily until the under head hits the boss so you have a thread issue that needs addressed.