Hi Adam,

This is the ESC in question, right?
http://www.swanghobby.com/hobbywing-...1-pr-1670.html

If you haven't yet unblocked it by poking, you'll have to take the shrinkwrap off and use a fitting drill (2-2,5mm) to clear the heatsink holes.

I added a picture of how these holes are drilled, what I don't know, is if the connecting hole at the bottom is closed by a grubscrew, or by a plug, that's glued in with epoxy.
If the latter is the case, there may be epoxy, blocking the flow and you'll have to re-drill the holes to free them up and close up the bottom end hole again.

Let us know how things develop.

Regards, Jan.