How to messure spark plug size for compression tester.

[250X_project]

I have an XR80 that won't run. It has a 4194 Auto-lite (NGK is much better, I know) I can fit the open end and boxed end of a 10mm on this just about perfect (9mm won't fit). What is the correct way to measure these w/o calipers. Also where can I find the right fitting, I have access to a tester just every fitting is too big-used on cars...Thanks!

[underpowered]

prolly 10mm threads then. if i am thinkin right that takes a c-series NGK plug so it is 10mm threading.

[250X_project]

Oh, okay. cool. Thanks!

I have feeler gauges, but how would they help me?