I have tried installing 4 prowin CNC hop up chambers into 4 GHK AK's. Three of them were a drop in fit and didn't need filing/modifications, one of them had to be filed down at the wings were it slides down the gun. After the filing, it fitted perfectly.
On another note, Ive installed the Hephaetus steel valve knocker on both of my GHK AK's. Ive noticed that on the stock valve knocker, it was all polished and silver, might be due to the wear when the magazine was inserted in the magwell and the blowoff valve hits the bottom of the valve knocker on impact. After installing the Hephaetus valve knocker, it seems to be doing great. The only issue was the burs around the cylinders of the valve knocker (these cylinders move along the curved part of the hammer). The burs I noted causes the valve knocker to raise up (as opposed to being pulled back) when the hammer cocks. As such, the burs around the cylinders need to be filed/removed. After that, it glides freely back and forth in relation with the hammer. Ever Hephaetus valve knocker may have burs in different location around the cylinders, just grind the ones that interfere with the hammer and you're good to go.
Here is the Hephaetus valve knocker:
Here is a picture comparing the stock vs Hephaetus valve knocker:
Closer picture of the burs causes the valve knocker to be raised up when the hammer cocks:
Going to be trying the Action cylinder floating valves to see if they make a difference. Too bad, I can't figure out how to pull the nozzle apart after removing those two damn pins:
