The heads/block were milled .029, partially to true them (just slightly warped), partially to raise the compression to 9.3, which should run fine on hi-test pump gas. I also run flat-top hypereutectic pistons so they could be running a bit hotter than stock. Around here, it's hard to find pure gas, we end up with a lot of 10% ethanol stuff, which I suspect could be a problem as well. It's been warm around here, in the high 80's, so not really hot, and by the temp gauge (running 180 degree thermostat) I'm not running hot.
I don't have AC, by the way. What would be the point of stopping it in gear?
The main question is: why now after 2k miles? No mods or adjustment to the motor since it was installed with timing & carb adjustments made, with the exception that I raised the idle about 75rpm because it was set too low initially. I believe (I'll have to check) it's timed at 10 degrees BTDC, which works for this altitude (around 6,000 feet). It runs fine otherwise, and has been operated in higher temps than this. I'll check the timing tomorrow if I can, and I'll also check the intake bolts (although I'm not showing any of the classic signs of a vacuum leak caused by a leaking intake gasket).