Dip the whole back end of the tube in a bucket of hot water. I clamp the flat end of the trigger block in a padded vice & after releasing the cocking lever from the slot, break the barrel & use that as a lever to turn the cylinder. Be aware that there is a lot of pre-load on the spring when at the end of the thread & its best to remove the barrel before this stage & support the end of cylinder with a square flat bit of half inch ply or similar on your torso for the last few turns. you should replace the buffer washer behind the piston head before re-assembly as they are prone to breaking up with age(just done mine recently) When you get the end block off you should support the bottom half of the cylinder end on a wooden block on the floor & use a hardwood block to hit the front end of the maxigrip with a hammer to start it moving. A squirt of wd40 will help free it & get it sliding, it needs to come back at least half an inch before the keyways will disengage to remove. You can't remove the piston before you remove the maxigrip.Don't forget to carefully de-burr the inside of the maxigrip slots & the cocking slot before re-assembly with a new o-ring seal or they can cut the o-ring causing low power.