This is something that I tried when I created my Classics Omega Supreme custom. I'd never done anything like it, but it seemed worth a shot, and thankfully, I was correct.!

The problem that put me in this situation is that for the tank treads, I had to cut them and make them able to fold and rotate to accomodate the transformation. The model's treads, as an unfortunate bit of accuracy, were actually slightly stretched over the tank's wheels in order to be nice and taut. So, having cut them, when he was in his tank mode, there was a gap between the halves, about a 1/4 on the top portion and also on the bottom portion. Well, this would've looked rather bad had I left it that way.

This is a VERY easy trick. take a saucepan (doesn't matter the size, I know of no Transformer parts made of rubber that are very big) and fill it about halfway full of water. Bring the water to a boil. now, grab your rubber part with some tongues, and dip the piece in the water, holding it safely off the bottom of the pan and keep it in there for about 30 seconds. you'll visibily see the part go limp if you swish it around in the water while you're waiting.

Now, remove the part from the water, and with your fingers pull on it. If you're stretching tires to fit a larger wheel, hook your index fingers in the center. If you're doing a flatter piece, just grab each end and pull. Now, this is NOT exact, so you should be conservative as you can always stretch more, but you can't stretch it down. Just pull hard enough you feel it go taut, and hold it like this for a minute or two as the rubber cools back down to room temperature. At this point, just check it out to see if its to where you want it, and if not, just repeat the process.

The first time I did it, I experimented with a spare tire from the now junker model kit that was used for making REDLINE, and it indeed kept its new size. I then did this to all 4 tread halves, each time placing the part where it goes to see how much of a gap I still had left, and then just repeated the process until I had no gap at all.