It has got to be tapered to get in a fresh hole. Then you can finish it up with a square one to tap out the bottom of the hole.
acquire correct tap from hardware or automotive supplier. Drill smaller hole through EGR port boss on head. Drill larger hole , size stated on tap to same depth as on intake port. then tap. If metric thread tap cannot be acquired a tapered pipe thread tap will also work. Threads are very close and will seal because of taper.
In general a female thread is cut with a tap. A male thread is cut with a die.
Cutting a thread on a shaft
tap root
tap root
National Pipe Tap, the angled self sealing thread of a pipe fitting
A thread chaser is used to clean and restore damaged or dirty threads vs. a tap or, die which cuts threads.
A tap is a tool, a tap can cut or clean the threads of a bolt hole, there are differant diameters and thread pitch, a dye is the opposite of a tap it cuts or cleans the threads of a bolt.
Tap water is only slightly acidic
STI stands for "Screw Thread Insert." When tapping a thread with an STI tap, you're typically preparing the hole to receive a threaded insert, commonly known as a helicoil or thread insert, which helps reinforce or repair damaged threads. My Recommendation 𝐡𝐭𝐭𝐩𝐬://𝐰𝐰𝐰.𝐝𝐢𝐠𝐢𝐬𝐭𝐨𝐫𝐞𝟐𝟒.𝐜𝐨𝐦/
When we make the male thread, generally we use a die tool. When we make the female thread, we use a tap tool. If we do not have the suitable tools, we can also make the thread using a lathe. When we make the threads using the tap or the die, care should be taken in respect for the following. (1) Start the thread with a perpendicular positioning of the tap or the die. (2) Turn the tap or die in quarter turns and "back off" quarter turns to remove metal chips so that they don't clog the tool. (3) Always use a cutting oil.