Mtu 4000 series cylinder head problem.

Just did a lot of Googling as that seemed odd to me and I can't find anywhere that has them made of cast iron..lots of high tech alloys....... very odd. And disappeared?

Are you sure they were in there to begin with?

Can you clarify if this was a complete new installation or if the new engines 2 years ago replaced existing engines of the same make/model?
You say that the plant is a little tight and that there is no solution to that, so I wonder how intake air and exhaust are accommodated.
Are the engines turbo or naturally aspirated?
How is the use of each engine cycled into the group? For example, are there three groups of six that have a rotating duty, with some of the six coming on-line as demand increases? It's been 18,000 hours since the engines were installed, more or less, so how are they used?
How is the failure discovered? As Capt J suggests; is it during maintenance inspections, or is there a sudden operability issue?
How many heads of any engine are affected by the failure, and what position on the engines are they?
Can all 16 on any engines all deteriorate at the same rate?
How many engine hours before the failure?

Generally valve seats are a much harder material pressed into the head.

Usually 50 hertz generators run at 1500 rpms and 60 hertz run at 1800 rpms...…..perhaps the engines don't like the 1500 rpms......and would prefer more rpms?

I seem to have misunderstood. I took it from the above that the engines were replaced 2 years ago. Pictures and a detailed time-line would be a great help.

The seats are machined on the head not a separate piece. And what do you mean disappeared?