It's off an electric Linde forklift. I bought a rebuild kit for the cylinder but am having a **** of a time getting the piston off. It has an internal Hex casted in the end of the piston no nut keeping the piston on. So how I see it is the piston is threaded into the rod. Is there a trick to un-threading this so I can replace the o-rings within and remove the head. I welded a nut in the end of a bolt and put the head of the bolt in the hex on the piston and was able to get a 3/4" impact on it but had no luck trying to back it out. Maybe heat?? Any information would be greatly appreciated.

Thanks
Kirk
Bregar machine and industrial maintenance