What the hell size are the bolts holding the water line on the back of the heads?????

I've tried 14mm 15mm 5/8 9/16 and they are two small or big.

It seems like it should be between 14mm and 15mm but I can't tell and don't want to run the risk of rounding them off.

 

I used 15mm, but I did round one off, had to order a new one from GM parts direct....

 

anyone else?

 

It is 15mm, but you need to use a closed end wrench on it. They're a pain, but not impossible... A whole new system came be bought from the dealer for about $30 that replaces the two bolts and lines all the way to the radiator cap.

 

Use a 15mm, 6 point tool.