Big Cam III - Burnt Exhaust valve

What would cause an exhaust valve to burn on a Cummins Big Cam III.
Its a stock engine. When we first looked at it I thought something metal had gone thru the cylinder. But the seat is pitted and the other valve looks like to the beginning of pits on it. This engine has 20K since an inframe.

Bad Injector? we put another head on and that cylinder is running 10 -15 degrees less than the others.

We reset all the valves and injectors and found some valve lash on the undisturbed heads to be a little tight. Could someone have set this one too tight?

Recently the accessory drive shaft had broken off on a trip. A local shop repaired it. Could the timing gear have been assembled wrong and this be the cause?


Would like to have a clearer picture of what went wrong before putting back to work.

Any suggestions would be great.
Thanks.

 

We put on a good used head with low miles on it. It smoked heavily and #2 cylinder was running cold. So we changed the injector in #2.
Replaceing the #2 injector ( same hole as burnt valve) cleared up the smoke.
Double checked the valve clearances again. Everything seems okay now.

The accessory drive we had fixed is now making a growling noise at idle that comes and goes. We called the shop that repaired it and they said they only put in a used shaft and did not replace the bearings!!. So I'll be pulling the accessory drive off tomorrow.

Thanks for help, This was the first burnt valve I had seen in a Diesel. At first I had thought something had gone thru the engine.