I just unplugged one the other day. I started out by wallering a small phillips screwdriver back in there. Then I used compressed air to finally unplug it. Once I got it to drain it kept clogging back up, so while I was continuing to flush I kept blowing compressed air back into the hole until I eventually got all the trash flushed out. I had both the freeze plugs on that side of the block knocked out at the same time. It helped aid the flushing. My block was almost half full of mud and scale. I got it all cleaned out.
