The timing cover has been in the alum solution for hours now and I haven't noticed any difference to the hammer screws. It has covered the cover with black stuff though. I've altered my method slightly, I've allowed the level of the water to drop and I'm pretty sure it's saturated. In fact from the milky look to the solution and crystals forming on the cover I suspect its past saturation. To keep the hammer screws submerged I've turned the cover over as well. Apart from the gas its not going to cost me anything to keep the experiment going so I'll see how it goes.
While I've been trying to get the alum to do its stuff I've cut out one of the brass badges I etched for the timing cover. It would have been a lot easier if I had done it in the workshop where I have a vice but improvisation won the day.
This is where its at now.
All I need to do now is clean the resist off, spray paint it black and then rub on some 1200 grit to reveal the unetched areas in brass.