You breach the headgasket when you vibrate bore, purging coolant into the catch can/reservior. At that point, while everything still seems fine, it slowly degrades. By the time it manifests, you think it's a recent problem, when the problem's been brewing for some time.

 

gotcha. ill look into the timing then

 

Respect the above JD statement. I personally take it as fact. I also can atest to this fact first hand. Arg...