Most common causes of cylinder cracking?

[PeterNSteinmetz]

So I did a top overhaul of my Lycoming O-360 3 years and 250 hours ago. Now I have cylinder #3 with a crack at the exhaust port seal requiring a total overhaul of the cylinder.

In people's experience, what is the most common cause of cylinder cracking?

 

[rotorwrench]

what is the most common cause of cylinder cracking?

Short of a material defect like pitting, its usually excessive thermal cycling or abusive ops.

 

[PeterNSteinmetz]

I am trying to think if I need to change my operations somehow. No engine monitor on this plane just an EGT which is the cylinder that cracked. I pretty much always run it 75 degrees rich of peak.