We'll start with robot mode.
I decided for this custom, I would use Skullgrin's body and the head from the Deluxe Classics Megatron.
Here is the original Classics Megatron figure. He does transform into an earth tank, but the figure is quite low quality and, I think, looks rather ridiculous. Although the head sculpt is one of the best out there. Photo from www.seibertron.com.
A close up of his head. Photo from www.seibertron.com.
And here's Skullgrin before I took him apart. Photo from www.seibertron.com.
OK, so the customizing begins.
This is the dremel saw that I used.
And a few of the other bits used to drill the hole.
Just for your reference, here's the whole dremel.
So this metal pin is what held Megatron's head onto the body. I figured I'd use it for this custom since it's already made to fit the head. To remove Megatron's head, I used a soldering iron to heat the bottom of the metal pin. Then, as the pin warmed, I slowly pulled his head upward until the plastic loosened enough from the heated pin and it finally came off.
I then used Aves Fixit Sculpt (a form of epoxy) to sculpt up the neck area and inserted and superglued the pin in the center. Aves Fixit Sculpt is avaliable here http://www.avesstudio.com/.
The figure was mostly the correct color to begin with so painting was more of a detailing sort of thing. The original Generation 1 Megatron design, from 1984, was grey with red detailing so I decided to keep him that way. All dirt/grime was added via a wash of brown acrylic. The dirt in the wheel and tread crevasses is a wash of light brown "Mississippi Mud" Acrylic. I also repainted his chest, lower arms, and feet to be the same grey as the rest of him. The white fingers and pelvis where changed to a metallic black.
And he even has red on the back and inside of his legs! Perfect!
I also repainted his head to match the lighter grey of his body plus adding all the brown dirt outlining. The Decepticon symbol on his chest was replaced with a decal from Reprolabels.com.
And the head can be easily removed, which is a good thing because the head is a bit too large to fit in tank mode.
The removed head.
Vehicle mode.
Now, most of these parts you can spot on the robot mode with the exception of the canon. That is removable and turns into an axe, which I don't want to use with the Megatron character. So I just remove it when the figure is transformed.
Right side.
Back side.
Left side view. Much more visible detail after giving the figure a wash of brown paint.
A close up of the new Decepticon decal from Reprolabels.com. I painted some grime on top of the decal to detail and blend it in a bit.
Top view.