Just as the title says. I'm looking to test the pressure in the girlfriends Envoy today. The majority of my tools are not here since I tore down my garage, and I'll need to go grab one, but can't find anything anywhere about what size it is.
The LS oil galley plugs are 8mm, so that kinda makes sense that GM would 'standardize', as it were.
If you ever remove the large coolant plug, try a 17mm first; that's what the size is on the 5.3L. If you have one or more smaller coolant plugs, the 8mm will be the one for those, same as the oil plugs (use a little teflon sealant on the reinstall). 38 ft-lb, IIRC.