Originally Posted by TK383 @ RSLC

I'll show you some later today man, they almost all have them, but you won't see them unless you know what you're looking for.

I work on a limit of 3-4 threads down into the plug hole, more than that & I reckon it's a problem, less than that & they're good to go.

I always knock the lumps off around the plug holes - those ridges cause hot sopts which cause the cracks, remove them & polish over the area & the problem goes away. Removing the ridges usually gets rid of about 50% of the crack & the rest won't go anywhere once the hot spot is removed.