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Club NA-T has a great thread on this. I'm not sure if I'm allowed to post the link or not, but check out the NA-T General Discussion area and you'll see the top sticky called "All about oil lines..."
If you are talking about the holes on the sides, then its either an m8 or m10 tap, i want to say m8 but its been a while. if you have both just see which one slips in better.