Hello, I hope the following assists you.
Metal filling in the injectors would indicate a high pressure fuel pump failure. When the pump rollers start to break up the metallic particles go with the diesel through the fuel system and contaminate the injectors and pipework.
Unfortunately the only way to stop this damaging any new or reconditioned injectors is to replace all the pipe work, the high pressure fuel pump, fuel filter and empty and clean the fuel tank.
This is quite a common concern on this engine and is expensive to repair but without repairing the fuel system any money spent on injectors will be wasted as the replacement injectors will fail quite quickly.
Let me know if you have further questions regarding this and I'll reply to you as soon as I'm able to.