Take a look at a thread I started not so long ago which might help a little. I'd be super careful with what you are trying to do. It just unscrews counterclockwise and if it won't I'd be super careful heating it up, in fact I just wouldn't risk that on a 136.
The hard stuff is where the section is secured to the barrel but I've not heard it being used on the piston end. I have a couple of 136 pens, one in plastic, the other in BHR. I also have a 232 with telescopic piston - they all came out with no trace of this bitumen like resin I've heard about.
Soaking it in some WD-40 or other mineral lubricant is your best bet - seriously, just leave it there soaking for a week and it will loosen up eventually. This was a problem I had with a BHR MB20 also referenced in the thread (photo) which refused to unscrew until I soaked the end in a 5ml Pyrex beaker. This technique works with tops and Onto sections too though BHR to BHR is more forgiving with heat, plastics are not.
Edited by Stylomeister, Yesterday, 18:06.