I finally finished rebuilding my spark plug tester. I was worried that something wasn't going to work. I had to substitute some different tubing in a couple of cases. In the end, I fired it up and everything worked just fine. The machine serves two functions. First, it has a sand blaster to clean up the plugs. It is hard to do that without a blaster. Second, it provides a spark to the plugs under pressure simulating the compression of the engine. There are mirrors so you can observe the spark on the electrodes and see if they are functioning. I tried the plug we had problems with and sure enough, it didn't spark. This is a very handy gadget.