Just got off the phone to NZ Cylinders and they are quoting $645 NZD to weld and replate the cylinder, ouch! It's my birthday soon, anyone want to give a poor student a cylinder?

Back to reality, what can cause an exhaust bridge to crack? Running lean > too much heat > too much stress > crack? Something like that? I know for a fact that the top cylinder has been running slightly leaner than the bottom.