SO I took Friday off and did the LT header install everything ok but last o2 sensor went in hard and I know when it comes out in the future will need to chase the y-pipe threads what size are they.
Have little floor banging issue I need to deal with but a little tweaking and retightening should fix that. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

 

I believe they are 18mmX1.5, its the same size used for 18mm spark plugs.

 

Excerpts from the service manual -

INSTALLATION PROCEDURE

IMPORTANT: A special anti-seize compound is used on the heated oxygen sensor threads. The compound consists of graphite suspended in fluid and glass beads. The graphite will burn away, but the glass beads will remain, making the sensor easier to remove. New or service sensors wilt already have the compound applied to the threads. If a sensor is removed from an engine and if for any reason is to be reinstalled, the threads must have anti-seize compound applied before reinstallation.

Coat the threads of the heated oxygen sensor with anti-seize compound (GM) P/N 5613695, or equivalent if necessary.

*Install the heated oxygen sensor.
*Tighten the HO2S to 41 N.m (30 lb ft) .
*Connect the HO2S electrical connector.
*Lower the vehicle.

I think it's M18 X 1.4