by vulrath » Tue Mar 09, 2010 1:21 pm
I don't remember what HD stands for (I think "high density" but don't quote me on that). VC stands for "vacuum cylinder".
Quite frankly, just replace the piston, spring and guide, and call it a day. The stock cylinder is unexpectedly sturdy (mine has been going strong since I got the gun in the summer of '08 (when I started upgrading), and it worked fine stock for the previous owner, who purchased it in the summer of '07), and should last quite a while. I had problems with my first aftermarket piston, spring guide, hop up chamber, and scope rail, but not with my stock cylinder.
That said, if you want to replace the cylinder with a PDI, you're going to have to replace the piston with the appropriate one (you need to match the vacuum piston with the vacuum cylinder, and so on, if I remember right). Its honestly cheaper and much less confusing to replace the piston and head with Laylax parts, then get a higher-power spring, and get a metal spring guide (really, the spring guide is optional; the original one is pretty awesome).