Originally Posted by Infinite Monkeys
Can the chaser be used from the outside.
If not I'll pull the head and first try chasing then the cutter from the inside.
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 slightly 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.