Parts for nelson based pumps built by Earon Carter of Carter Machine. These include parts for the Buzzard, the comp, the tri-car / tricar and more.

And articles on Carter Machine at:
http://www.baccipaintball.com/oldnews/tag/carter-machine

Many Carter Parts are totally propriety and some are not compatible between generations.

Carter Valve Retaining Screws. The Valve retaining screw on Carter nelsons varies according to generation.

Old pre 90 Buzzards, Comps, Ducks and Box Guns use standard 9/16 inch by 18 threads per inch valve retaining screw and the powertube is hole is standard ID.

Carter pumps of the 90s use 9/16 inch by 18 tpi valve retaining screws but the interior dimension is larger. Not sure when this change occurred?

Newer Carter pumps (maybe 2005 onwards?) use a larger outer diameter valve retaining screw.
The valve retaining size on these powertubes are 5/8 inches x 18 threads per inch (tpi). Some are 20 tpi though

And checkout this thread on mcb for more information on Carter Power Tube Valve Retaining Screws