(not sure if that's the correct term/name for a cartridge style fork rod) when I recently did mine, I bought the new standard 39mm tubes, measured them, then measured my original tubes, and subtracted the difference. depending on how much longer your new desired fork tubes are, is how much you will extend the 'damper' rod. Extending the 1200s (sportster sport) and fxdx 39mm forks can be done.
