FX Impact Hammer Adjuster

I have a very odd issue. I am unable to turn the hammer adjuster screw.

A 2.5 mm allen wrench is too loose. A 3 mm allen wrench will not fit into the screw. I even found one allen that measures 2.93 mm. It is to be too large. I also tried a 2.73 mm and it is too loose.

This is happening after having a power plenum installed. I was able to make adjustments prior to that.

The action is not cocked. The power wheel moves easily.

Prior to the power plenum installation the adjuster was 14 mm. Now it is 20 mm.

Any idea what is going on?

It is 1.5mm. I'm not sure why I thought 2.5 or 3. The 3 mm won't even fit in the hole. The 2.5 fits in the hole but not the screw. 

Thanks for setting me straight.
 
Hi, I have stipped the hammer adjuster screw (C9), maybe also the threads on the adjuster(C10) are damaged aswell. So I want to disassemble it. First the power wheel came off. After that removed the C9. Underneath C9 is a Allen screw (C11). I believe it's a 2,5mm. I guess when loosing it the C10 should come off. But it's very tight and I don't wanna destroy anything. Is there a certain way to remove it? Thanks y'all!