I am not a professional repair guy, but I have worked on some of my Sheaffers and even in a similar Oversize Vacuum fill pen. I may be incorrect in some of the things I am about to write, but I have tried them all and they worked for me.
Probably you have done some of what I am about to say, I am putting some alternative from the easiest to the hardest:
1) Are you sure the packing unit is useless, some times after soaking them for a while and putting them in the US they start working again because the felt inside goes to "his place" again. I have read that in this excellent book I have, tried it in a Sheaffer and worked, the pen is working for the last 3 months...
2) Have you tried soaking it for long and applying heat to see if the packing unit comes off? Leave it inside a water and ammonia solution for 2 or 3 days and then apply heat ( I use a hair dryer not to risk excessive heat that can deform the celluloid). Increase the heating time in 10s leaps starting in 30s. Apply gentle force to pull the paking unit at the end of each allowing the pen to cool for 10 s before pulling or unscrewing.
If it does not come out (Solution 3 and 4 will need a lathe):
3) using a lathe would be easier: Hold the barrel in the lathe chuck, bore just the external wall of the packing unit (rear knob side on your 2nd picture), install a new oring or Xring, machine a new external wall in hard rubber or black celluloid if you have, glue it back in place an its done. (cosmetically its not the ideal as the glued border will be visible, but is easier than the next solutions)
Here is a picture of the end result. This particular repair was not done by me but used this solution.
4) Also with a lathe: Hold the barrel in the lathe chuck, cut the threaded part and bore the remaining of the packing unit from the outside of the pen side (rear knob side). Make a new packing unit install the oring, close the unit and glue the unit back on the barrel. It is radical, but is not hard to do and does not take a lot of time. For the oring / X-ring I just go to an Oring specialized store and choose the correct one for the unit/rod I just machined.
If I am not mistaken the thread is a single lead 30TPI
5) You can always try to drill from the inside part of the pen and only take the external wall of the packing unit and do the repair from inside. I don't like doing this because is harder to glue and I always get the felling I am about to break the barrel while drilling it.
Choosing the solution will depend on skills, equipment, and on Trial/failure of the first solutions.
Hope it helps
Best regards
Vasco
Edited by piscov, 17 December 2013 - 11:35 AM.