Bas,

There isn't any difference in the 6.4 between the blue and polished case engines other than the color. I use the 10mm (CM6) plug and CH ignition on mine with no heat issues whatsoever. I've heard that the 1/4" plugs don't last as long before they need changed, but that's to be expected. A few years ago CH was provided with some parts (not the fault of CH) that would not tolerate heat well, but they resolved that issue immediately after it was discovered. A better source for an answer to your question about any heat problems and ignitions would be Terry at CH Ignition.