Thread: O2 sensor bung threading damaged

Good day, can anyone help me out here. When i removed the old O2 sensor (after using a torch and some effort) i noticed the threading looked worn. When i installed the new one after about 2 months it ran slack. I then re-tightened and it keeps running slack.
My question is; Does anyone know what size nut & bolt (just for the nut) do i have to buy to use as a replacement bung? Is the thread pitch special or standard?