after making a few durhams molds I have a few tips. use 1/4" carriage head bolts for index, if you push the head down just to the edge of the bolt head, leave until lower half of mold sets up the bolts snap out of the durhams easily and make a very nicely rounded locator. pop your mold halves apart while still warm, this gives you a little time to work on the cavaties ,clean up and cut your vents prior to full set up. I used a piece of plastisol for the sprue, if you use something ridged it better be exactly half way in the mold, the plastisol has a lot of give or stretch easily pulls out when mold has set weather half way in or not without deforming mold. I also used high temp engine paint to seal the mold, let it cure completely or it will stick to the other mold half and peel away. purchase a 25lb tub of durhams it will save money in the long run. 8 lb will yield 2-3 small molds. definitely make the mold thicker, too much is better than a broken mold. all in all durhams is easy to work with. thanks to all that have helped with my endeavor in mold making, I think I will stick with durhams rather than silicone or vac 50, it is cheaper.