If you want to do a leak down test, keep the valves closed to do seals, or change the springs without removing the head, you'll need an adapter to connect your air line to.
An old spark plug is a good start. The ceramic in the plug is held in place by a swaged connection, and once you remove that, it practically falls right out.
I used the lathe because it was handy. A zip cut disc would do the job as well.
Once you turn that ring off, you can put in in the vice and knock the ceramic out.
The inside diameter on this plug was .600". About .030" larger than the recommended tap drill size for a 3/8" NPT pipe thread, so that's what I used. A few reducer bushings, and you can adapt just about anything to it. This is an NGK, but other manufacturers are very similar in size. Another option, if you don't have a pipe tap, is to put a 1/4" pipe nipple in the hole and weld it.
