2000 echo oxygen sensor thread size

[snowshoe]

I just replaced the oxygen sensors on my 2000 echo. The problem is the threads in the catalitic converter for the front sensor are factory defective a bit and it takes extra force to run the sensor in plus the threads do get a bit damaged. I had a tough time removing the original. My question is: what is the thread size for the sensor so I can buy a machine tap and recut the threads for easy operation.
Just in case you are doing this the check engine light came on steady and after the replacement of both sensors(they were both bad) I had to disconnect the battery for 15 min to get it to reset the computer and it works fine now.
Thanks for any helpful advice.