We need to know which drain plug you are referring to, there are several.
There is no drain plug, remove the lower radiator hose to drain coolant.
It seems these 2007-2013 GM lambar do not have drain plug.
I have a 2007 GMC Sierra 5.3L V8...and I just changed my oil and found the drain plug on the opposite side of where the Oil Filter is. The plug is a 13MM.
The 2004 GMC radiator drain is located on the bottom of the radiator. The radiator drain plug can be removed with a half inch ratchet.
The spark plug gap for a 1990 GMC with a 7.4 liter engine is .035 inches. This is equal to .9 mm.
Wear is drain plug on 1997 GMC 7.4
There is no plug for the radiator, you have to take out the lower hose from under the car and let the radiator drain.
There is a turn plug on the rightside of the bottom of a radiator turn it and it will drain.
Bottom of the radiator.
On a 2001 GMC Savana 5.7 V8. There is no drain plug. You actually have to remove the big hose that joins the engine to the radiator. This will drain out all the fluids.
The oil drain plug on a 2003 GMC Yukon is underneath the motor. It is on the same side of the engine compartment that the battery is on.
Coolant drain plug is on the bottom of the radiator. You turn the knob and watch out. You need a wide drain pan not an oil change pan.