There are two common ways to shrink a head from another toy line to mego scale.
The first was discovered by accident. In an attempt to remove the paint from an original mego head, it was dropped in a glass of acetone. The head expanded as it absorbed the acetone. When it was removed and the head dried out, it not only shrunk back to original size, but kept shrinking.
This is a technique many of us now use.
It really only works on certain types of vinyl heads, and will only shrink so much, so something in 9 or even 10 inch scale can be shrunk this way, but larger scale heads wont shrink enough to get to mego scale.
Basically, you pour some acetone in a glass jar. Enough to cover the head you want to shrink, plus some. Put the head in, and make sure it doesn't float. I use an old spoon and roll it around until the acetone has a chance to get in the neck through the neckhole. Once the head is at the bottom of the glass, keep an eye on it. Some heads will explode in only 5 or 10 minutes. It's a risk you have to take. If you see it getting too large or distorted, remove it and rinse it in water. Our own Bill Stevenson gave me this technique a few years ago. He said to use multiple dunks. A short first dunk, a longer 2nd dunk, and an even longer third dunk. I tend to let the first dunk go about 10 minutes. When you remove the head, be careful. It can tear easy. Base your 2nd dunk on how soft the head got on the first dunk. If it feels like it's going to rip apart easily, keep the 2nd and 3rd dunks short as well. If it still feels pretty firm, the 2nd and third dunks can go longer. The head expands each time, and as it dries it shrinks some. Wait a couple days between dunks and see what it's going to do. It may shrink enough after one or two dunks. It's not an exact science. Trial and error.
Check the head as it's drying. If it starts to get rock hard, there isn't any more shrink left in it.
When you get the head to size, cast it, paint it and use it to make cool customs of characters mego never made!
Note- This method actually shrinks the orginal head and ruins the paint job. Don't expect to put the original head back on the original figure when done! Also note, this method will ruin a head with rooted hair. All the little holes in the head for the hair unually make it tear apart in the acetone.
You'll also want to avoid contact with skin, use in a well ventilated area, keep away from flames, and use good common sense when using the acetone. It is highly flammable.
Another cleaner method is using a product from Industrial Polymeres (http://www.industrialpolymers.com/products.html) called Hydrospan 400.
Hydrospan 400 can shrink just about anything ALOT, without damage to the original part. In the mego world, this is most commonly used to shrink 12" scale heads to 8" size.
You make a mold of your original head in the usual way. Once made, you mix the hydrospan with water. COLD water. Hydrospan 400 begins to set in about 30 seconds, and will be unpourable within a minute. You want to mix it, stir it and pour it in 30 seconds or so. Refridgerated water helps slow the process. The colder the better.
The ratio of hydrospan to water depends on how much you want it to shrink. On average, 50%/50% will bring a 12" head to megoscale. Often though 60% water to 40% hydrospan 400 does the ticket. Again, it's trial and error.
Mix the two together and mix well, but quickly. Pour the mold and leave it alone for a few minutes. You can then remove the head from the mold. Hydrospan makes a spongy head when it's done shrinking. It's weird stuff to be sure. When you first remove it from the mold it will be wet feeling and milky white. As it dries, is turns yellow. It may distort as it dries out, but no worries, it will even out when it's done. This is because certain parts dry faster than others sometimes.
It may take a week or two to finish shrinking. As the water evaporates, the head shrinks. This takes some time, although use of a food dehydrator speeds up the process.
If you don't get the proper size, try again, changing the mix ratio (more water for a smaller shrink, less for a bigger shrink) until you get it where you want it.
Once you have it shrunk, you will need to recast it and make a version in plastic/resin. Often, modern heads have no neck attached to the head, so the first plastic cast of the shrunk head can be used to sculpt a proper neck on, fix any bubbles that may have resulted from your quick hydrospan pour, etc...
Once you have the new plastic 8" head how you want it, use it as a master and make a new mold and cast. This new cast is ready to paint and because a new mego custom.
Additional tips for using Hydrospan 400 from Austin Hough
Hydrospan 400 sets quickly. Three ways to delay the chemical reaction of this mixture and give you more time to pour the cast carefully and get it into a pressure pot (if you use one) is to
1.)Keep the Hydrospan 400 refrigerated or in the freezer (at least 24 hours before mixing with water)
2.) Keep the mold you are working with in the freezer for 24 hours prior to using with the Hydrospan. You should pour water in the mold first, freezing to ice a perfect cast of the head and keeping the inside of the mold nice and cold. Pop this "ice cube" head out before casting of course
3.) Use ice water when mixing with the Hydrospan 400. You now should have cold Hydrospan 400, a cold mold and cold water.
By following these three steps, you will delay the chemical reaction of the Hydrospan 400 and water mixture and allow yourself more time to carefully pour the mixture into the mold AND (if you are using a pressure pot) more time to get the mold into the pressure pot for the perfect cast.
I like to let my heads sit in the mold for at least 30 minutes, just to make sure they are firmed up. Then, to speed up the shrink process use a heat lamp or toast-r-oven. These can, when used properly, and safely, reduce shrink time by days. Use the toast-r-oven on a very low setting like 225 degrees or less and keep an eye on it. Keep the heat lamp at least 8 inches away from the head if you decide to you us that as well.