Modifying the Heatsink
Each power transistor is rated up to 100 watts of power. With 16 of them that is 1600 watts potentially! Realistically, each transistor is dissipating around 80 watts at the voltage (50V) and current (~1.6A) that I like to run at. But, that is still about 1280 watts. This amount of heat dissipation requires a very large heat sink. The link above, to the heat sink that I use, seems to work pretty well but I still have a muffin fan underneath the TinyCNC-II heat sink to help move the heat away when I am using the system for any length of time.
Start laying out the holes to be drilled by positioning the assembled PC board in the center of the heat-sink as accurately as possible. Use a small center punch to mark the center of the mounting hole of each transistor and the 5 volt regulator.
Drill and tap every hole for a 4-40 screw. Cut down the screws to no longer the 1/4" in length if necessary. If you cut the screws to length, be sure to file the ends of the screws edges to be slightly beveled to help starting them into the heats-sink easier.
NOTE: Make sure that each newly drilled hole have been properly de-burred so as to not cause a short through the transistor isolation film between the transistors metal mounting plate and heat-sink. Use a twist drill bit, several sizes larger then the hole and spin the larger drill by hand in each hole. This is usually enough to de-burr the holes inside edge and create a slight bevel around the inside top of each screw hole.