Yes it is, And you have probably have overtightened it, you only need to apply enough pressure to seal the end cap on the O ring not clamp it up, but it IS a common problem.. the instructions tell you to soft solder the parts together....BAD ADVICE, as I always reccomend that the parts are SILVER soldered together then pegged through so it doesn't come apart.
ALL IS NOT LOST as you can make the rod quite easily out of 4mm threaded bar or better still if you have access to a lathe machine one up from stock rod then screw cut the thread on one end and fit a lock screw to it by silver solder, I posted the dimensions on here once so they should be still on somewhere try looking under a common thread.
The other method is to do away with the rod and fit a Bayonet ring available from Norbert Bruggen in Germany see his web site
www.modelluboot.declick on the English flag then bayonett ring in seals then on the long German word in the centre of the page starting with bajonett for the sizes and prices.
Hope its of use.