Truckernix,

Back in the 50's and 60's when these old 6 cylinders were still used hard as work trucks it was very common to have a head to be cracked and only leak at operating temperature which of course means the crack opens up wider when they are hot. We had a company here up until a few years ago that repaired heads. He put them in an oven and heated them until they were almost red hot before he "stitched" them. He showed me how much more visible the cracks became as they became hotter. He repaired a couple of GMC heads for me, but they failed. I think repair techniques have improved much over the last 50 years, but fact still remains that cracks open up with heat.