I got out the tools and new parts this week and finished the upgrades on my 1377 "Ultra Carbine". The installation of the new aluminum flat top piston and valve that I got from A.C. Custom Parts went smoothly. Alejandro has very good instructions on his site on how to install his pistons and valves. If you just follow the directions, you can't really mess it up. I hear a lot of guys complaining about cut o-rings when installing them, but if you just debur the tube really well with some sand paper wrapped around a 1/2" dowel, you can't go wrong.
I haven't run any shots across my chrony yet, but judging by the pellet penetration in wood and by the increased noise I would say that my gun is now shooting ~50% faster than it was with a stock piston/valve. I'll get you guys some numbers soon to show the results. It dumps 15 pumps no problem. I haven't tried any more than that but I will just for fun when I break out the chronograph. I did notice that the first 5 or so pumps are much easier to do than stock. Once you get over 10 pumps the force required to pump the gun increases quite a bit as I expected it to.
I'm definitely going to have some fun testing this gun and my other 2 pumpers once I get the pistons/valves installed in them too. Thanks go out again to Alejandro for all his help and his excellent parts. You've taken my gun to the next level!
Here's a few shots of the gun now that it's finished. Have a great time shooting and go build yourself a custom Crosman, they are loads of fun!