Originally Posted by cobalt327
First, the best way to do this w/the head on is w/the internal chaser, as you know.
The cheaper chasers can be used from the outside. The threads of the chaser should be tapered to allow it to start even w/buggered threads. The rubber O-ring is there to hold the chaser in a socket w/o falling out. If you're going to go in from the outside, the tapered threads are important because you generally only get one shot at it- either it'll work or it'll cross thread, and if you use a chaser w/an abrupt start to the threads (see below), starting the tool w/o cross threading is hard. If you're careful you can add the taper yourself.
You might find the double ended chasers are too bulky. I have a long single thread chaser here but I cannot seem to find it online. There are single ended thread chasers (like K-D p/n 3379) that can be used w/a 3/8" extension and ratchet or a 5/8" plug socket. It is shown below w/the abrupt threads.
Right words there are if you start the thread chaser wrong you will trash the threads. This is the reason that the head needs to come off so you can start the tap properly from the back side. You can force any tap down the hole and hope it hits the right spot but its going to cross and mess up the head. i would run the plug in from the back side and see if it cleans things up a little first.