I just did the bio helmet mod using Tankmans method. Very happy with the results!
For anyone still confused I'll do a step by step guide that will hopefully clear up alot of questions.
This is what worked for me. Took a total of 25 minutes or so.
1. pop off the predators head from his body (comes off pretty easily and goes back on easily as well)
2. I let the head soak in really hot water for about 5 minutes to kind of soften up the glue so that it will be easier to pull off the bio.
3. i started by inserting a flat head screw driver through the top rear center part of his helmet and gently lifting up. You'll hear the glue kind of making popping sounds as the bio is separating from his head. once the top is loose, just kind of work your way around the sides the best you can. the bottom mouth areas were glued pretty tight on mine. basically so tight, i couldn't wedge the flat head under it so I just peeled off the entire bio helmet from his head (pulling from the rear of the bio toward his face with quite alot of force) until the entire helmet lifted. be careful because the plastic used for the bio is VERY soft once heated up.
NOTE: it took a few different soakings in hot water to soften up all the way) some people were able to take just the mouth piece off leaving the top dome part on the head.. this wasn't the case for me as the mouth piece was glued pretty good to the face. You can try to get the mouth piece off first instead of taking the entire helmet off.
4. once you have the bio off, you may need to soak it once more in hot water in order to soften up the mouth piece so it can be pulled off easier.
5. once the bio is in two pieces, you will need to shave off the little "teeth" like molding that's on the inside of the MOUTH piece. I used an xacto knife for this. be sure to shave as much of it completely off or else this piece will cause you to still have the gap. It will take a few test fits to get right.
6. from here on out its mainly about testing and fitting until you can eliminate the gap. once you get the general idea of how the mask needs to be positioned go ahead and pop the preds head back onto his body if you havent already.
7. once you have the predators head back on, glue the bio helmet back together first. when the bio helmet is in one piece and the glue has dried, glue the helmet back onto the predators face.
now your predator doesn't look like hes wearing shades!
Again, this is what worked for me. others have had slightly different methods. It really depends how tight the mouth piece is glued down. if you can get away with just taking off the mouth piece, it will save you alot of time.