Are you using $15 Amazon coils that have uneven bottoms to crack when tightened down? Try also not tightening so much, they really don't have to be that tight. Plastic, you know. People around here love to crack the stat housings too, again, plastic.
Why did you change the coil mount? If not dead flat the out of plane (say one corner high or low) will crack coil when tightened down.
The longterm cracking on these that normally occurs after long periods of time is from temperature.