Chunk of brass arrived!
First order of business; thread that looooong hole M12x1.5....this is a VERY deep hole to thread. My tap got progressively harder and harder to turn until I was gritting my teeth and knew I was right on the edge of snapping the tap off and breaking it inside the hole (a very bad thing); this is as far as it would go in before it got too dangerous to turn anymore, about half of the tap length:
The tap threads were causing friction as it dug deeper and deeper into the hole. So, I ground off the upper threads of the tap to lessen resistance of the cutting threads. Normally that's a bad idea, because the threads help to align the tap and keep it running straight. But since I was threading it on the lathe (by turning the chuck by hand), it was fixed on axis and wasn't in danger of going crooked. It was ground down from this:
piston taking shape. The seals are 7mm and the slots cut in the stock piston were about 8mm... (.312"). I decided to tighten up the slot to a touch over 7mm (.285"), this is the top slot being cut:
The piston is then flipped over so I can cut the lower 2 slots:
finished, almost, but couldn't resist the urge to do a quick side by side with a seal. Still have to drill some holes. Note that the 3rd (topmost) seal is in a different location. I decided not to install a 4th seal, so all that space on top seemed like it was going to waste... so I located the topmost seal a bit higher, so it would have a larger moment arm to act against the shaft. In rifle shooting we call this 'sight radius' and is why rifle shooting is inherently more accurate than handgun shooting. You can aim better because your sighting plane is longer. Also, the seal depth is shallower... they are closer to the walls of the group head. The stock seals lands in the piston have a diameter of 1.613", mine has a land diameter of 1.640". I'm not sure if this is too high... if it is, I will have to put it back in the lathe and adjust it slightly lower.