Why does the head gasket keep blowing on a Komatsu WB140-2N with a Yanmar S4D106 engine?

[Robert M.]

I'm working on a Komatsu WB140-2N with a Yanmar S4D106-1FH engine, and I'm completely stuck on a head gasket problem. The machine runs and drives great, but after a few hours of use, the head gasket blows again - always on the same 3 cylinders. Has anyone here dealt with repeated head gasket failure on a Yanmar S4D106 engine before? I'm trying to figure out what could keep causing the same cylinders to fail even after all the machine work.