If you don't need to take too much off, you can cut off the threaded portion at the top of the barrel and VERY carefully use a crank arm removal tool to recut the threads in the plastic barrel. Generously wrap the portion you will be rethreading with duct tape or many layers of masking tape, the idea being to prevent the plastic barrel from expanding as you cut the new threads with the tool.
Jay Sexton
Sebastopol, CA