It's because I don't have the cobra right now. I am looking to buy it and if it fits then I'll take my friend along to check it out, if not ill see if anyone else I know has a tester. Thanks.
compression testers generally come w/ the standard end and then a fitting to make it bigger, so hopefully he has the fitting. You could know for sure if you to autozone and ask for a spark plug for a 98 cobra and compare.