I made sure to sculpt the head about 3% to 4% larger, which is about the amount of shrinkage you will get, for a part this small and hollow. It's hardly noticeable when comparing it with the original hard copy, but something worth considering when we're talking about fitting pieces together.
The hole itself, was cast from the factory piece and this is the only area, I had to work around with each piece, to make sure the part would fit on the ball joint. Because even at 3 to 4 percent shrinkage, it would become a very tight fit.
For some of the casts, I had to take a small piece of high grit sand paper and sand around the hole for about a minute. Usually that's all it takes to get it to widen enough for a perfect fit.
Also, another way around, as I directed in the manual (enclosed with every piece) is simply heating the plastic cast with either hot water (*only if unpainted), or a hair dryer set on high heat, for a few seconds. The resin becomes soft (almost like hard rubber) and can then easily be applied over the ball joint.
After reading your post, I went ahead pulled out my remaining 7 casts, which were pulled a little over a month ago, to see how well they fit today, and 3 were very tight, while 4 others had a nice fit. So with the 3, I'll have to gently sand the holes a little, to make sure it's not a problem for anyone.
With my next turtle, I will experiment a little and see if I can get that solved so as to not have to go back and work on every piece I pull. But If I can't, then I'll just continue to properly sand each piece.
*if you run scalding hot water on a painted piece, you may face clouding issues on the surface, due to the sealer separating. Only use the hot water solution, with unpainted parts. If you have used hot water and see clouding on the surface, you may need to lightly spray a coat of paint sealer (acrylic based) for it to fill the separation and make the white cloudy problem, go away.