[While] working on Grimlock [I was able to fix] that chest gap. [Below is] a pictorial "how to", in order. To do it [this] way, you have to disassemble the head and knock out the jaw pin. It is well worth it because [this modification] makes it part of the transformation rather than a removable plate.

First, trace [the] chest cavity with paper:










Next, cut pieces out of styrene. [Then] overlay parts [that] are thinner gauge[d]:






Test [the] fitted-cut styrene:








[Then, fit the] collar pieces to [the] main piece with [a] small strip of styrene, bending to match angle of collar to chest:




[Next take] the head apart to install [a] pivot bar. (Make [the] pivot bar approx 2.5mm - 2.8 mm):










Glue [the] bar to chest piece you made - be careful not to get glue on your figure!

(beast mode with new piece):




[Here is the] bot mode with new piece (also it can be laid towards the back of the head):






. . . . and viola chest gap is gone. I still need to add lower tabs and clean up the bottom edge, but hopefully you get the idea. On mine, I want the red band to go around the front of the chest. [To be more animation] correct, omit [the] large overlay piece. Also remember when looking at this, I have not sanded it or added any putty to blend it together.


[Here] is something you can do to make the pin not so loose and to keep the chest plate flush with the sides of the chest and neck. [Using] epoxy sculpt, (or any type of putty that [hardens]), [make a hole]. To make this hole, wet the post and stick it in the putty before it cures, then level the top off. After the putty cures, [drill] out the hole with a Dremel and a drill bit. [Do] this slowly and test fit often:






Finish [the] chest plate before paint[ing]. Notice the red putty [used] to square up edges as well as the last piece of styrene on the bottom (I beveled the outside edges of the bottom strip so it would blend into figure well):









[Coat the] chest plate with [a] primer coat [first] to show [the] flaws most of this coat will be sanded away. Notice my line under [the] neck plate is off-kilter slightly, [but] it is rather minor:

 

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