I am buying a compression test tool, I thought that they were 10mm thread? I remember buying a 14mm socket to remove them and that was too big.

 

they are a standard size, there are 2 for cars and yours are the smaller of the two, 10mm and 14mm is measured accross the flats of the plug and that is the size. I'm not sure of the thread size though. If you get a screw in compression tester, they ususlly come with the smaller size, and an adapter for the bigger thread. Hope this helps

Adam